Final Report on the Rail Squat Project R3-105

Transcription

1 Final Report on the Rail Squat Project R3-105

2 DOCUMENT CONTROL SHEET CRC for Rail Innovation Floor 23, HSBC Building Brisbane Qld 4000 GPO Box 1422 Brisbane Qld 4001 Tel: Fax: Document: Title: Final Report on the Rail Squat Project R3-105 Project Leader: Malcolm Kerr Authors: Dr W. Daniel Project No.: R Synopsis: Rail squats are cracks that grow below the surface of the head of a rail, depressing the surface due to plastic flow. They can be caused by excessive cyclic plastic deformation of the crack surface (shakedown or ratchetting), or by thermal damage to the rail surface that leads to martensite on the surface a white etching layer (WEL). In both cases, an initial flaw is needed to initiate the cracking. Work at The University of Queensland on this project has confirmed that WEL is associated with most squats. Work at Central Queensland University has demonstrated that WEL can be detected with eddy current measurements, using a higher frequency than that used for crack detection. Detecting squat cracks with eddy current is also possible. Substantial sliding of wheels on rail forms the martensite layer, assisted by the high contact pressure between a wheel and a rail. This project has found that more testing and modelling is being undertaken internationally to better understand this problem. More can be done in Australia as well. The exact conditions of temperature and pressure under which a WEL can form need to be clarified. An improved test also needs to be conducted to clarify if WEL detection by eddy current can be developed into a routine inspection tool. REVISION/CHECKING HISTORY REVISION DATE NUMBER 0 [insert date] ACADEMIC REVIEW (PROGRAM LEADER) INDUSTRY REVIEW (PROJECT CHAIR) APPROVAL (RESEARCH DIRECTOR) DISTRIBUTION DESTINATION Industry Participant for Review REVISION x Established and supported under the Australian Government s Cooperative Research Centres Programme Copyright 2013 This work is copyright. Apart from any use permitted under the Copyright Act 1968, no part may be reproduced by any process, nor may any other exclusive right be exercised, without the permission of The University of Queensland.

3 Final Report on the Rail Squat Project R3-105 Table of Contents Executive Summary... iii List of Figures and Tables... iv Abbreviations and Acronyms... Introduction Rail Squats Evidence from Australia and Overseas Traction control and squats Squats in Australia Squats in Europe Fatigue Versus Wear Crack Initiation White Etching Layer Residual Stress in a Rail Head Observations of Crack Initiation Predicting Where Crack Initiation is More Likely Conclusions About Crack Initiation Crack Growth Introduction Plasticity and Residual Stress The effect of water Thermal Influence on the Growth of Short Squat Cracks Stress intensity values at the tip of a squat-like crack Modelling Crack Growth by Extended Finite Element Method and by Element Removal Measured crack growth Growth rate of cracks in rail steel Fractal Dimension of a Crack Conclusions About Crack Growth Squat and WEL Detection Eddy Current Detection of White Etching Layers Eddy Current Detection of Squats Thermographic Detection of Squats Magnetic Barkhausen Noise Detection of WEL White Etching Layers Acceleration measurements Conclusions Open Questions and Findings CRC for Rail Innovation December 2011 Page 1

4 Final Report on the Rail Squat Project R Causes of Squat Initiation and Growth Squat Detection Maintenance Findings References CRC for Rail Innovation December 2011 Page 2

5 Final Report on the Rail Squat Project R3-105 Executive Summary Rail squats are cracks that grow below the surface of the head of a rail, depressing the surface of the rail due to plastic flow. Rail squats can be caused by excessive cyclic plastic deformation of the surface of the crack (shakedown or ratchetting), or by thermal damage to the rail surface that leads to martensite a white etching layer (WEL). In both cases, an initial flaw is needed to initiate the cracking. Work at The University of Queensland has confirmed that WEL is associated with most squats, and work at Central Queensland University has demonstrated that WEL can be detected with eddy current measurement, using a higher frequency than that used for crack detection. Detecting squat cracks is also possible with eddy currents. Marsenite layers are formed by substantial sliding of a driven wheel on a rail, assisted by the high contact pressure between a wheel and a rail. Internationally, the research states that modern traction control systems may make the problem of rail squats worse, by allowing operation closer to the limit of a wheel sliding on a rail. Water exacerbates the problem worse, by encouraging crack growth and leading to sudden changes in wheel rail friction. The occurrence of squats shows some patterns. Often, some feature of the track disturbs steady rolling and encourages wheel-slip. A range of the product traction force times sliding velocity also promotes cracking. With a lot of sliding, wear dominates, preventing crack development. Therefore, as a result of the trade-off between wear and fatigue, the high rail of a transition into a curve is often where squats occur or grow. A particular range of curve radii also promotes squat development head hardened and plain carbon rails both show evidence of squats, head hardened rail tends to develop more squats. Work at Monash University on this project measured the rate of growth of cracks in rail steel, formulating a crack growth law covering both very short and very long cracks. However, determining accurate stressing of a squat crack as it grows is difficult. Metallurgical studies at The University of Queensland confirm the thermal origin of squat cracks. The University of Queensland has also undertaken work involving the finite element and XFEM modelling of squat-like cracks, testing to measure residual stress in the head of a rail and testing to confirm that a WEL can develop at temperatures below 720 C. Work at Railcorp measured the growth of squats at several sites in Sydney. While growth is influenced by weather, the growth rates of cracks do not appear to accelerate with crack size, which would be expected for a constant loading cycle. A better picture of the tonnage after which these cracks start to appear is needed. From international experience, the tonnage at which cracks appear seems to have reduced in recent years, due to the prevalence of thermal damage to rail surfaces hastening the initiation of cracks. Many questions about rail squats remain unanswered and, internationally, no one has a simple solution to the problem. Reducing the use of head-hardened rail has been suggested, as has appropriate use of friction modifiers to limit tractions. Better automated track inspection is needed to detect squats. Processing of accelerometer signals is shows promising results for detecting squats. A more detailed analysis of ultrasonic rail testing data is proposed, using rail testing replay and GPS positioning facilities to obtain a more complete and accurate picture of squat distribution. Further comparisons can then be made against parameters such as curvature, rail type, grinding history, rail profile, traffic type and track grade. CRC for Rail Innovation December 2011 Page iii

6 Final Report on the Rail Squat Project R3-105 List of Figures and Tables Figure 1a: a squat growing from peak contact pressure on one side 1b: a squat growing from peak contact pressure on both sides... 3 Figure 2: Typical surface appearance of rail squats... 5 Figure 3a: Squat from Moss Vale, NSW with well-developed beach marks underside of broken-off top surface on the left... 5 Figure 3b: Zoomed in view of the region of initial growth... 6 Figure 3c: Cracks showing two phases of growth... 6 Figure 4a: Scanning electron microscopy picture of the surface of a squat near its initiation 4b: Surface of a squat from Yerrimbool, NSW. Traffic direction is to the left for both images Figure 5: Angles of squat cracks... 8 Figure 6: Flowchart of factors influencing development of a squat... 8 Figure 7: Damage to the high rail of a tight curve in Mayne Railway yard, Brisbane. a b Figure 8: Squats developing from RCF cracking in a 1500 m radius curve on the down North Shore line Figure 9: Proportions of squat-affected rail by curvature bands, obtained from ultrasonic rail testing surface condition reports Figure 10: Rail profiles on squat affected moderate radius curves Figure 11: Sequence of photos at yearly intervals, Down Shore down rail km, 31 January 2007 (before squat grinding), 13 May 2008, 4 June 2009, 6 May 2010 and 14 November Figure 12: (a) Squats at 1.9 km down-sub up rail 60 HH 2003 in trailing transition of 600 m radius curve. (b) Up-sub down rail approx km. Severe squat at weld. High rail in arc of 928 m radius curve. 60 HH rail, not recently ground Figure 13: Hardness profile of an aluminothermic weld and squat locations Figure 14: Occurrence of squats in flashbutt welds Figure 15: German railhead damage due to the presence of squats initiating additional cracking Figure 16: T model to predict crack initiation versus wear Figure 17: A baby squat Figure 18: Microhardness test on WEL and cracks in WEL Figure 19: Fragments of WEL in a squat crack Figure 20: Typical optical microscopy view of a transverse section of a portion of the running band with a WEL Figure 21: Regions of WEL detected by eddy current testing Figure 22: Deformed microstructure of the surface of a squat crack near the rail surface (Hunter Valley) CRC for Rail Innovation December 2011 Page iv

7 Final Report on the Rail Squat Project R3-105 Figure 23: Variation in distortion of the subsurface microstructure on a transverse section through the running band of a rail lateral traction to the left Figure 24: Deformation below a rail surface: a: a squat; b: a wheel slip Figure 25: Stressing due to volumetric expansion of the top row of eight elements shown in blue Figure 26: WEL test Figure 27: a: WEL from test at the lowest speed (10 m thick); b: Surface texture from lowest speed test Figure 28: Residual direct stress components (MPa) in a virgin rail and a service rail with WEL 30 Figure 29: Direct stress v depth over 5 mm below the rail head, xx = transverse, yy = axial 31 Figure 30: Shear stress versus depth over 5 mm below the surface of the rail. yz vertical/axial plane, xz transverse plane Figure 31: Flat 50 profile superimposed on an AS50 profile Figure 32: Crack in rail deformed (x 5 scaling), due to a wheel load. Von Mises stress (Pa). Traction acting to the left Figure 33: Contact pressure distributions disturbed by plastic deformation and by a crack 36 Figure 34: Plastic strain after a wheel loading transverse crack case traction to the left 37 Figure 35: Contact pressure distribution for the transverse crack Figure 36: Residual Von Mises stress (due to shear stress) at the crack tip Figure 37: Directions of principal stress in elements around the crack tip, showing tension normal to the crack in the circled elements Figure 38: Subsurface crack modelled Figure 39: Cycles of stress intensity at the ends of a subsurface crack due to a wheel passage 41 Figure 40: Cycles of stress intensity with the contact zone at 200 C Figure 41: Portion of mesh deformed Figure 42: Local coordinate system showing displacements of nodes, when a wheel approaches a crack Figure 43: Nodes at crack tip in general coordinate system. φ is the kinking angle Figure 44: Crack model matching Bogdanski s [11] model Figure 45: A slice model of a 60 kg/m rail laid on an elastic foundation: a: isometric view; b: the crack on a rail head surface, direction of travel is left to right45 Figure 46: a. Dimensionless stress intensity factors around the crack front from midpoint up to the right node, for a particular wheel position. b. Dimensionless K I and K II at the crack tip midpoint obtained from benchmark and compared with Bogdanski s results, varying as a wheel passes the crack. K III is zero due to symmetry Figure 47: Stress intensity along the crack tip when the crack s minor axis is aligned with the traffic direction (γ=0) or has angle of γ=45. Different longitudinal traction ratio values and lateral traction ratio values are considered CRC for Rail Innovation December 2011 Page v

8 Final Report on the Rail Squat Project R3-105 Figure 48: Stress intensity along the crack tip when the crack s minor axis is aligned with the traffic direction (γ=0) or has angle of γ=45. Different longitudinal traction ratio values and lateral traction ratio values are considered Figure 49: Predicted squat growth by element removal in 2D Figure 50: Half model of a squat crack showing elements removed. Z = direction of travel, TR = 0.1, Figure 51a: Crack growth at Erskineville. Shape of crack and crack depth Figure 51b Crack growth at Erskineville. Growth v time Figure 51c: Crack growth at Erskineville. Growth of equivalent radius time-shifting curves 52 Figure 52: Crack growth at Chatswood. a: December 2007 b: May Figure 53: Growth of cracks in head hardened rails steels fitted to a Hartman Schijve law 53 Figure 54: Squat growth data replotted, where y = squat size (mm), x = months Figure 55: Squat crack surface tested for fractal dimensions Figure 56: Typical eddy current readings from a rail with WEL. Dashed line is the WEL threshold 57 Figure 57: Eddy current readings for squats Figure 58: Lock-in thermographic image of two closely spaced squats Figure 59: Schematic Layout of a MBN Detector Figure 60: Wavelet spectrum of vertical axle box acceleration due to a squat Figure 61: Factors combining to enable squats to occur on track Figure 62: Squats initiating near the gauge corner of a high rail (Railcorp), Sydney. The arrows mark the positions of small squat cracks in synchronisation, but not aligned to the pitch of the most recent grinding (the white marks) Figure 63: Periodic squat damage associated with grinding marks in two rails from Banedanmark, Denmark CRC for Rail Innovation December 2011 Page vi

9 Abbreviations and Acronyms Final Report on the Rail Squat Project R3-105 ANSTO Australian Nuclear Science and Technology Organisation ARTC Australian Rail Track Corporation FE Finite Element FIM Friction-induced Martensite HAZ Heat-Affected Zone HH Head-Hardened LEFM Linear Elastic Fracture Mechanics LR Lateral Traction Ratio = Lateral Force Tangent to the Rail/Normal Contact Force MBN Magnetic Barkausen Noise RCF Rolling Contact Fatigue SC Standard Carbon TGT Tangent Track TR Traction Ratio = Longitudinal Force on Rail/Normal Contact Force WEL White Etching Layer XFEM Extended Finite Element Method a Crack Length da/dn Increment in Crack Length Per Cycle K Stress Intensity Factor Kmax Maximum Applied Stress Intensity Factor in a Cycle of Loading K Stress Intensity Range Kth Threshold Stress Intensity Range Kc Apparent Cyclic Fracture Toughness Kink Angle of a Crack Twist Angle of a Crack σ Remote Applied Stress σy Material Yield Stress Thermal diffusivity Thermal penetration depth CRC for Rail Innovation December 2011 Page

10 Final Report on the Rail Squat Project R3-105 Introduction Fatigue cracking due to rolling contact is a common problem in the rail industry, but it is still not fully understood. Cracks that grow in the sub-surface of the head of a rail, called squats or studs are a particular issue, despite extensive study of the problem. The name squat comes from the visual appearance of the crack as they form as a depression in the rail surface, which is caused by plastic flow of metal above the crack. The alternate term stud was coined by Grassie [1] for such depressed cracks initiated by heating due to wheel-slip. For these types of cracks, understanding both crack initiation and crack propagation are significant problems. The textbook initiation of a crack due to rolling contact is a severe cycle of shear stress peaking subsurface that occurs due to rolling. However, with significant tangential forces (tractions), as well as normal contact pressure, the worst stressing shifts back to the surface. Severe plastic deformation of the surface also occurs in a cyclic way called ratchetting or shakedown. Ratchetting can lead to exhaustion of ductility. Ratchetting is well understood, and was thought to be the origin of squats until recently, when increasing evidence of thermal damage to the surface has emerged. The thermal damage is typically a hard, brittle martensite layer, tens of microns thick. The layer initiates cracks at its edge or under it. At one atmosphere, rail steel would need to be heated to 720 C to form austensite, which transforms into martensite. Heating of rolling contact due to friction has been analysed and can now be easily predicted. To obtain this temperature by frictional heating requires high amounts of sliding (e.g., 10 per cent of the velocity lost to wheel-slip). While there is a lot of evidence of wheel-slip is associated with cracking problems, there is also evidence that sliding less than that required to achieve 720 C also causes cracks. The martensite layer is called a white etching layer (WEL); it does not etch with nitric acid. There are several reasons why WEL can occur at lower temperatures. One reason is that austenite can exist at lower temperatures with large hydrostatic pressure, as it does in a rolling contact situation. Another reason is that the sudden release of pressure when contact is lost can form martensite. A third reason is that heating is not only due to friction, but can be due to rapid plastic deformation. WEL is commonly found in ballistics, where it is caused by unstable plastic deformation producing heat faster than can be conducted away, forming an adiabatic shear layer. With WEL, damage due to plastic deformation is secondary. WEL can be detected on tracks using eddy current testing, as demonstrated by Central Queensland University as part of project. Growth of such squat cracks is complicated. Initially, cracks grow in shear, along the distorted plates of the heavily deformed pearlite microstructure of rail steel, helped by presence of WEL. Cracks are closed while compressed by a wheel passage, except for the redistribution of contact pressure caused by plastic flow above the crack, which unloads the material above the crack, causing peak contact stress before the crack mouth. Redistribution of contact pressure also causes material to yield above the tip of the crack as the contact loading is rerouted around the damaged section of rail surface. This process depresses the rail surface. A crack that is not completely closed can still grow in the shear modes II and III. This growth is accelerated by water in the crack. If the crack mouth can be sealed, high pressure can develop in the crack. However, there is debate about whether it is possible to seal the crack. Even without sealing the crack, fluid in the crack can still have a lubricating effect, while being squeezed out or sucked in, reducing the effective coefficient of friction between the crack face and encouraging the crack to grow. Squats have been observed to grow faster in wet weather. Under acceleration, the traction on rail from a driven wheel will open a surface crack before the wheel arrives. Under braking, a surface crack is opened after the wheel passes. These tractions, combined with lateral traction, which peaks on curves, can also potentially cause mode I (tensile) crack growth, although tensile stressing is less severe than the shearing from direct contact. Railcorp gathered experimental data from ultrasonic measurements of the extent and depth of cracks. This data shows the growth rate of squat cracks does not accelerate as the crack grows. This is due to the either the redistribution of loading or due to a change in the mode of growth as the crack grows and becomes larger than the wheel rail contact zone. Water may also be sealed in a small crack, but not in a larger crack, lubricating the smaller crack. Residual stresses in the head of a rail, partly due to the way it is manufactured and partly due to the cyclic loading from rolling contact complicated the issue of cracks. These stresses, comprising at least 300 MPa compression of CRC for Rail Innovation December 2011 Page 1

11 Final Report on the Rail Squat Project R3-105 the surface longitudinally and laterally, combined with lower amounts of vertical tension below the surface, influence the direction of crack growth, but not the amount of growth. This project has been helped to raise the level of understanding of rail squats. In particular, this project has led to: the successful demonstration of eddy current detection of martensite on the surface of rail insight into the conditions under which squat cracks form by using vehicle dynamics studies measurement of the rate of growth of cracks in rail steel, in the laboratory and on tracks formulation of a crack growth law covering both very short and very long cracks metallurgical studies confirming the thermal origin of many such squat cracks finite element and XFEM modelling of squat cracks testing to measure residual stress in the head of a rail testing to confirm that WEL can develop at temperatures below 720 C. No definitive solution to squat cracks has been found; however this project has highlighted possible ways to reduce creep by traction control systems and control of wear. Extensive use of head-hardened rail and modern traction control systems tend to make cracks worse by shifting the material failure trade-off between fatigue and wear in favour of fatigue. This report is organised by topic, but is a collation of work undertaken by Railcorp, The University of Queensland, Monash University and Central Queensland University. Each organisation took a different approach to the problem. More detail is available in three previous project reports, internal reports from Railcorp and the ARTC, and in a literature survey. A number of conference and journal papers have been written on this work (see references 4 to 10). CRC for Rail Innovation December 2011 Page 2

12 Final report of the rail squat project R3-105 Chapter 1 Evidence from Australia and Overseas 1. Rail Squats Evidence from Australia and Overseas A well-developed rail squat appears as a depression in the surface of a rail, formed by plastic deformation of material above a crack that is growing below the surface. Cracks can grow both in the direction of travel and in the opposite direction, as well as transversely across the rail. Figure 1 shows two types of squats, but there are many variants. The surface of the rail may show a well-defined transverse ridge of material in the middle with plastic flow on either side corresponding to cracks growing in each direction (Figure 2a and Figure 1b), or the surface may reflect a saddle-shaped crack surface, with just a V or U shaped surface crack from which the subsurface crack spreads out. (Figure 2b and Figure 2c and Figure 1a). Cracks can start anywhere from the centre of the head to the gauge corner, with existing head checks being a possible source of cracks (Figure 2d). The rail surface above the crack may be less polished by wheel contact, as shown Figure 2b, which are known as dark spots. There is evidence of loss of wheel rail contact with the material above a squat crack. For example, in Figure 2e grinding marks have not been worn off above the crack, but they have been worn off elsewhere. In Figure 2f, the rail surface appears to be rusting in the depressions above the squats, contrasting with the surface polished by contact. Surface-breaking cracks tend to be accompanied by a widening of the running band, with a polished surface indicating high contact pressures on one or both sides of the centre of the crack. In the case of a butterfly squat, with cracks growing in both directions, surface cracking may be on both sides of the running band, as visible in Figures 2g and 2h. Crack surfaces are often damaged by frictional wear and by corrosion; however, one example from Moss Vale near Sydney, showed little corrosion when it was broken open using liquid nitrogen. However the crack had well developed, near-circular beach marks clarifying its propagation (Figure 3a). This example has evidence of two phases of growth as shown in Figure 3b. Other examples show a crack surface that is smooth for the part of the crack growing in the direction of travel, with a rougher surface for the part of the crack growing against the direction of travel, as shown in Figure 4. This evidence suggests the growth is in a different mode. Growth of sub-surface crack 1a Region of high contact pressure Surface crack 1b Region of high contact pressure Surface cracks Figure 1a: a squat growing from peak contact pressure on one side 1b: a squat growing from peak contact pressure on both sides. Two phases of growth can also be seen in the cracks of Figure 3c, with initial growth from flaws on the gauge side of the running band changing to growth in the direction of travel, or against the direction of travel. CRC for Rail Innovation December 2011 Page 3

13 Final report of the rail squat project R3-105 Chapter 1 Evidence from Australia and Overseas a. Squats with a central ridge from Roma St, Brisbane b. V-shaped surface flaw on track at Chatswood, Sydney c. U-shaped surface flaw on track at Engadine, Sydney d. Squats growing from gauge corner head check cracks (Illawarra Line, NSW) e. Squat showing grinding marks f. Squat showing rust CRC for Rail Innovation December 2011 Page 4

14 Final report of the rail squat project R3-105 Chapter 1 Evidence from Australia and Overseas g. Squat with surface breaking cracks on each side h. Squat with surface breaking cracks on each side Figure 2: Typical surface appearance of rail squats Figure 3a: Squat from Moss Vale, NSW with well-developed beach marks underside of broken-off top surface on the left CRC for Rail Innovation December 2011 Page 5

15 Final report of the rail squat project R3-105 Chapter 1 Evidence from Australia and Overseas Figure 3b: Zoomed in view of the region of initial growth Figure 3c: Cracks showing two phases of growth CRC for Rail Innovation December 2011 Page 6

16 Final report of the rail squat project R3-105 Chapter 1 Evidence from Australia and Overseas Figure 4a: Scanning electron microscopy picture of the surface of a squat near its initiation 4b: Surface of a squat from Yerrimbool, NSW. Traffic direction is to the left for both images. Damage to the rail surface occurs on both on heavy haul lines, on suburban passenger lines, on lines with mixed traffic and on high-speed lines, although the cause may be different. Squats tend to occur in locations with increased wheel-slip, but where the sliding is not enough to wear away the flaws before they develop into cracks. These locations also seem to have momentary peaks in the frictional power the rate of frictional work done on the rail. Slipping wheels may not be visually evident, but they are obvious from the etched rail section. Often, squats are accompanied by severe plastic distortion of the microstructure of the surface layer or a phase change on the rail s surface into a hard, martensitic WEL. Areas where squats are most likely to occur include: a. Transitions into curves. The motion of a wheelset is disturbed as it enters a transition into a curve, promoting slipping on the high rail. Squat development also may be related to the range of curve radii that occur in transitions. b. Turnouts and crossings. The vertical dynamics of a wheelset is excited by crossings and turnouts, and increased slipping can occur on both rails. Also, high-gauge corner stresses occur in unground turnouts. c. Welds between sections of track are often dipped and also excite vertical dynamics of a wheelset. d. Other changes in track stiffness, for instance a sleeper that is unsupported by ballast, can excite the vertical dynamics of a wheelset. e. Regions of acceleration or braking, where the track experiences higher tractive or braking forces. The initial angle of a squat on the rail surface reflects the direction of the traction and is consistent in a particular location, as illustrated in Figure 5. Squats do not occur on low rails in curves or on the high rails of sharp curves. A number of factors may combine to cause a threshold to be exceeded, which will cause a crack to start. Figure 6 illustrates the relationship between these factors. CRC for Rail Innovation December 2011 Page 7

17 Final report of the rail squat project R3-105 Chapter 1 Evidence from Australia and Overseas Figure 5: Angles of squat cracks Figure 6: Flowchart of factors influencing development of a squat Squats do not occur in tunnels. The most obvious reason for this is a lack of water to pressurise or lubricate the crack. A more subtle possibility is that tunnels provide a controlled environment, with relatively uniform temperatures and without sudden changes in wheel rail friction that can set off wheel slip, or peak tractions. CRC for Rail Innovation December 2011 Page 8

18 Final report of the rail squat project R3-105 Chapter 1 Evidence from Australia and Overseas While rust is found in squats, scanning electron microscopy at The University of Queensland has not shown any intergranular or transgranular microscopic flaws that are characteristic of stress corrosion cracking, concluding that it is not part of the process of forming a squat Traction control and squats There is growing evidence from cities around the world, such as London and Helsinki [1], and also from Sydney and Brisbane, that squats start to appear in urban areas when AC traction control or DC thyristor traction control is introduced. Urban trains have more driven axles, and the new traction control systems permit trains to operate closer to adhesion limits, with an increased likelihood of exceeding these limits and inducing full sliding. However, while the newer traction control systems do react more quickly, more information is needed about how they react to traction problems, such as a sudden increase in available adhesion. Traction control systems can permit sustained operation at 10 per cent creep or more in low-friction conditions, rather than just momentary peaks of high creep. Railcorp has reported spikes above 10 per cent creep. The solution to the squat problem may be to modify how traction control systems operate Squats in Australia Squats in Australia are found anywhere the factors as discussed in Section 1.0 can be found. However squats have been increasing in urban areas, notably in Sydney. These squats appear to be mainly thermally initiated, as discussed in Chapter 2, although there is also an increasing number of rolling contact fatigue (RCF)-initiated squats in Sydney. Grassie et al., [1] suggested that thermally initiated squats should be called studs to highlight their thermal origin. On a very tight curve, squat-like cracks can accumulate substantial local plastic damage due to material flowing above the crack, such as those in a railway yard at Mayne in Brisbane, shown in Figure 7. a. b. Figure 7: Damage to the high rail of a tight curve in Mayne Railway yard, Brisbane. a b Experience in Sydney suggests that track near trackside lubricators has a similar number of squats to unlubricated track Squat Frequency versus Curve Radius Railcorp reports an increasing tendency for squats to develop from RCF in moderate radius curves ( m). This pattern is seen in the arcs of curves with above radii, as well as in the transitions of sharper curves where the instantaneous radius is in the above CRC for Rail Innovation December 2011 Page 9

19 Final report of the rail squat project R3-105 Chapter 1 Evidence from Australia and Overseas range. This pattern has been noted in numerous field observations and some early statistical data has been obtained from rail surface condition reports produced from ultrasonic rail testing. This data includes only squats that are sufficiently well developed to affect ultrasonic rail testing. The data contains minor errors of location, but provides a useful early indicator of the relationship between curve radius and squat development. Figure 8: Squats developing from RCF cracking in a 1500 m radius curve on the down North Shore line Figure 9 shows lengths of squat-affected rail as a proportion of the total length of rail in each curvature band, obtained from ultrasonic rail testing surface condition reports. Some notable features include: a peak of squat frequency in the curvature band m a secondary peak in the curvature band m, possibly associated with squats in transitions of sharper curves; however, the data does not differentiate between arcs and transitions a secondary peak of squat frequency for the curvature band m, possibly associated with standard carbon rail and a small amount of shows that squats occur in longer radius curves with standard carbon rail a relatively small proportion of squats in tangent track a small proportion of squats in curves <400 m almost all the squats in curves are in the high rail with only an extremely small proportion of squats in the low rail of any curves. CRC for Rail Innovation December 2011 Page 10

20 Final report of the rail squat project R3-105 Chapter 1 Evidence from Australia and Overseas Proportion of rail with squats 2.50% 2.00% Length 1.50% 1.00% 0.50% 0.00% Tangent all Low all curves Radius Range Figure 9: Proportions of squat-affected rail by curvature bands, obtained from ultrasonic rail testing surface condition reports A study of rail profiles at locations where squats occurred on moderate radius curves examined curves on a passenger only line (the North Shore, Sydney) and a mixed line (the down Illawarra). The study found no connection between existing rail profiles and squat development, noting that the squat development occurred over a number of grinding cycles, possibly with different profiles. An inverse connection between squat development and grinding frequency was slightly evident, although the sample size was too small to be meaningful. Other observations from this study included: Squats developing from RCF occurred on curves with radii in the range of m. In many cases, there were multiple, continuous multiple squats throughout the length of the curve. Some other curves with similar radii and operating conditions did not have squats. Squats occurred on both passenger-only and on mixed-freight lines. However, on the mixed-freight line, squat development was predominantly in the direction carrying loaded freight (down Illawarra). The empty direction freight line (up Illawarra) was relatively free of squats. The squats occurred on 60 kg/m head-hardened Head Hardened (HH) rail on concrete sleepers and on 107 lb/yd rail on timber sleepers. One squat-free curve consisted of relatively new (2009) 60 kg/m standard carbon (SC) rail on concrete sleepers. Squats occurred on curves that were ground with H2, H3, Tangent Track (TGT) and MT profiles. The curve at Artarmon, down shore km to km, which was ground heavily for squat removal in 2007, has developed new, early stage squats from RCF above the gauge corner. The squats have been visibly evident over the past two years. On the shorter radius curves (below 1000 m radius), wheel contact and RCF and squat development tended to be close to the gauge corner, noting that rails were CRC for Rail Innovation December 2011 Page 11

21 Final report of the rail squat project R3-105 Chapter 1 Evidence from Australia and Overseas mostly, but not always, ground with H2 and H3 profiles on these curves. On the longer radius curves (1000 m and above), wheel contact, RCF and squat development were closer to the centre of the rail. These rails were ground with TGT and MT templates. Significant evidence of squat development was seen for both longer and shorter radius curves. Profiles in the contact area tended to be similar for rails with and without squats. The only curve with squat development on standard carbon (107 lb/yd) rails had a 1500 m radius, at the longest end of the radius range considered. Squats on 60 HH rails were observed on curve radii down to 480 m. Figure 10 shows rail profiles taken from various locations with squats on moderate radius curves on the North Shore line. It also shows one location (8.202 km) where no squats are evident on a similar curve. There are no obvious differences between the different rail profiles, except that the squat-free rail has greater curve wear, causing a slightly larger effective gauge than three of the four rails affected by squats. Shore Rails Km Km Km Km Km Km Km 0 Figure 10: Rail profiles on squat affected moderate radius curves Figure 11 shows a sequence of photos from a site where squats were partially removed from the arc of a 1000 m radius curve by grinding in The rail was essentially clean (free of squats) after the grinding, except for the remnants of several old, deep squats. The old squats can be seen re-developing throughout the sequence. Very early gauge corner squat initiation can also be seen in 2009, which developed slightly by 2010 and developed even more by Some evidence of wheel-slip can also be seen before the squats were removed. Cyclic grinding was undertaken on 29 June 2008, just after the CRC for Rail Innovation December 2011 Page 12

22 Final report of the rail squat project R3-105 Chapter 1 Evidence from Australia and Overseas second photo was taken. Grinding was due to be undertaken again on 6 October 2010, but it was not completed until 5 August It is important to achieve consistency of grinding without grinding stone chatter producing deeper scratches. Deeper scratches allow martensite to form within the scratch reducing the benefits of grinding km 31/1/ km 13/5/ km 4/6/ km 6/5/ km 14/11/2011 Figure 11: Sequence of photos at yearly intervals, North Shore down rail km, 31 January 2007 (before squat grinding), 13 May 2008, 4 June 2009, 6 May 2010 and 14 November Head-Hardened versus Standard Carbon Rail An inspection between Redfern and Illawarra Junction on up and down suburban lines was conducted on 14 December 2011 to ascertain the differences in rail squat development for 60 kg/m SC compared to 60 kg/m HH rail. Many squats were found, such as those shown in Figure 12. CRC for Rail Innovation December 2011 Page 13

23 Final report of the rail squat project R3-105 Chapter 1 Evidence from Australia and Overseas traffic (a) (b) Figure 12: (a) Squats at 1.9 km down-sub up rail 60 HH 2003 in trailing transition of 600 m radius curve. (b) Up-sub down rail approx km. Severe squat at weld. High rail in arc of 928 m radius curve. 60 HH rail, not recently ground. Overall, the squats in 60 HH rail are worse than squats in 60 SC rail. The 60 SC is older rail from compared to the 60 HH from around Some significant contrasts in performance can be seen at the interfaces between the rails. In several cases, a heavily squatty rail then changes to rail with minor checking or small squats as the boundary is crossed. Additional instances of squats occur where 60 HH glued insulated joints are located in 60 SC rail sections. There is mildly squatty rail at Redfern departing from the platform on the down suburban. There are also squats on the Sydney-side of Redfern on the up and down suburbans. These squats are associated with areas of high traction with significant wheel-slip. This level of wheel-slip has the potential to cause an increase in rail hardness. There is a strong tendency for squats to occur at aluminothermic welds and also at flashbutt welds. These welds have hardness variations within them. This raises the question about whether the primary driver of squats is related to the dip typical of such welds (particularly aluminothermic welds); to the hardness variation; or to some other feature of the rail. A typical hardness profile for an aluminothermic weld has been super-imposed on a weld in Figure 13 (orange curve). The area between the new weld material and the parent metal is typically softer and is known as the heat-affected zone (HAZ). In this example, there is a large squat on the left-hand side that has caused the contact band to widen. In this case, the squat started the trailing side of the weld on the boundary with the parent rail, where the softer HAZ meets the harder parent rail. Overall, as shown in the table in Figure 13, the most common location for squats is in the harder metal, although some, like those shown, are in the transition to harder metal beyond the weld. Flashbutt welds show similar hardness features to aluminothermic welds, although the weld itself and the HAZ are narrower. With flashbutt welds, the area for squat development (at least initially) is in the zone where the weld is, not where the HAZ is. The start of the squat is towards the approach side of the weld, that is, at the transition from softer to harder steel, in the direction of traffic (Figure 14). CRC for Rail Innovation December 2011 Page 14

24 Final report of the rail squat project R3-105 Chapter 1 Evidence from Australia and Overseas Squat HAZ Tempered zone Austenitised zone Weld metal Austenitised zone Tempered zone HAZ harder traffic Tempered HAZ Austenitised HAZ Weld (melted material) Austenitised HAZ Tempered HAZ Figure 13: Hardness profile of an aluminothermic weld and squat locations harder Figure 14: Occurrence of squats in flashbutt welds 1.3. Squats in Europe Squats are a major problem on European railways. The occurrence of additional transverse cracks severely damaging the head of a rail has been observed in Europe although, to date, these additional transverse cracks have been very rare in Australia. An example of a transverse crack is CRC for Rail Innovation December 2011 Page 15

25 Final report of the rail squat project R3-105 Chapter 1 Evidence from Australia and Overseas shown in Figure 15 from a tangent track on a high-speed passenger line (160 km/hr) in Germany. A complete break of a rail in service has not yet been reported in Australia, which may be due to our lack of very cold winters that continuously put welded rail into tension. However, the situation in Australia will need to be monitored into the future. In France, softer R200 European grade rails are being trialled on lines with lighter traffic that do not experience excessive wear, but may show a greater resistance to crack initiation. Data is still being collected from these lines, but statistics are encouraging when comparing the R200 lines to the standard lines with R260 grade rail steel. Prorail in Holland are relying on plans to monitor the performance of friction modifiers, and to spray the track, as required, to maintain appropriate frictional characteristics to avoid crack initiation. Figure 15: German railhead damage due to the presence of squats initiating additional cracking 1.4. Fatigue Versus Wear Locations with high wear do not usually show as many squats. Traditionally, locomotives without traction control caused more wear, but this has not lead to cracking problems. There is a trade-off between crack initiation and wear. This trade-off has been captured for crack initiation by repeated plastic flow in a diagram of damage measured by traction times creep (T ) versus a damage parameter creep ( ) as shown in Figure 16. The diagram shows competing, increasing damage tending to either initiate a crack or to cause wear. Wear dominates at higher amounts of sliding, as the angle representing the wear rate is larger. CRC for Rail Innovation December 2011 Page 16

26 Final report of the rail squat project R3-105 Chapter 1 Evidence from Australia and Overseas Figure 16: T model to predict crack initiation versus wear Multi-body dynamics software, such as Gensys, and track design software, such as Track-Ex, includes the ability to estimate a fatigue index, like Burstow s [2], which expresses this trade-off. Squats that are thermally initiated cannot be predicted by this type of software, but the software can provide an insight into why squats occur in transitions, and into the effects of changing wheel or rail profiles. However, an examination of new and worn wheel and rail profiles from the Illawarra line from regions, with and without squats, does not show any correlation, indicating that the profiles are not themselves causing squats to develop. Simulations with the Vampire rail-vehicle simulation software, discussed in the June 2010 report from this project [3] show that on entering a transition, the balance between fatigue and wear favours fatigue on the high rail, as longitudinal and lateral tractions build up, causing a transient peak in frictional power, measured by T. A Gensys simulation was used to compare the AS60 and Flat 50 ANZR1 rail profiles, using a rail vehicle travelling at 80.3 km/hr entering a 400 m radius curve. The AS60, less conformal profile shows a larger T peak before wear takes over. CRC for Rail Innovation December 2011 Page 17

27 Chapter 2 Crack Initiation 2. Crack Initiation Initiation of cracks due to RCF is usually due to a process of exhaustion of ductility caused by cyclic plastic deformation, or ratchetting. The tonnages of traffic needed to cause head-check cracks in this way are well known [1], and form the basis of preventative rail grinding. A distinctive feature of squat cracks is that they can form at lower tonnages, and there is often evidence of a thermally induced crack associated with wheel slip, especially in urban areas. This evidence led Grassie et al., [1] to propose calling cracks formed thermally studs to distinguish them from the European experience of squats on high-speed passenger lines. Thermal initiation of cracks is discussed in Section 2.1. While train wheels remain relatively hot, rail cools rapidly after every wheel passes, creating a greater cyclic range of temperatures and making the effects of thermal stressing of a rail more severe than that caused by a wheel, especially when the wheel is driving [11]. There are significant residual stresses associated with the head of a rail, which affect crack initiation. Residual stresses and local measurements of residual stress made at the Australian Nuclear Science and Technology Organisation (ANSTO) are discussed in Section 2.2. The location of crack initiation is also of interest to this project. Why particular positions along the track show squat damage and not other locations in unknown. This issue is discussed in Section 2.4. By observing the surface of the rail, it is not always obvious whether the crack initiated on the surface. In other rolling contact situations, such as bearings and gears, crack initiation is sub-surface due to the most severe cycle of shear stress occurring sub-surface. However, with significant traction, the maximum shear stress predicted from an elastic contact analysis moves back to the surface, and crack initiation is expected. There is always some traction in rail wheel contact, due to the cant of the rail, even with non-driven wheels, but squats often occur where there is more lateral traction due to a curve, increasing the surface shear stress cycle. The section of a small squat shown in Figure 17 with the unbroken rail has beach marks highlighted that clearly show surface initiation. CRC for Rail Innovation December 2011 Page 18

28 Chapter 2 Crack Initiation Baby squat broken open to show growth (from dotted section on next Baby squat Squat initiation Figure 17: A baby squat 2.1. White Etching Layer When squat cracks are observed under a microscope by etching a cross-section, mostly, but not always, a white layer tens of microns thick that does not etch, is observed on the surface. This phenomenon occurs in squats in Australia and also in Britain and Europe. The WEL is hard (600 HV or more) and brittle and often contains cracks (e.g., Figure 18b). Fragments of WEL have also been found in squat cracks (Figure 19), even when there is no WEL on the surface. Interpretation of this layer raises issues about its origin. a. Is the WEL what caused crack initiation to occur, or is the WEL a result of wheel-slip caused by the squat? b. Is the WEL due to frictional heating causing friction-induced martensite (FIM) from wheelslip, or due to rapid plastic deformation (an adiabatic shear layer)? c. Are the WEL and the crack initiation both aspects of the same thermal overload due to wheel-slip? CRC for Rail Innovation December 2011 Page 19

29 Chapter 2 Crack Initiation a. Hardness (HV) profile top of rail on the left b. Cracks in WEL Figure 18: Microhardness test on WEL and cracks in WEL Figure 19: Fragments of WEL in a squat crack Support for the crack initiations being caused by WEL comes from the fact that WEL is also found along the running band where there is no crack. A recent observation of a rail from Moss Vale shows WEL of varying depth right across the running band on the head of the rail (Figure 20), and eddy current observations from Central Queensland University, reported in [12], show patches of it along the running band (Figure 21). Observations from the London Underground also support this view especially that the WEL can exist without distorting the underlying pearlite microstructure [1]. Cracks have been observed to initiate at the edge of the WEL, or under it [12]. However, most samples observed in this project show elongation of lamella in the pearlite indicating substantial plastic deformation near the surface of the rail (e.g., Figure 22). A lateral cut of the sample from Moss Vale, shows a variation across the running band of this surface distortion of the microstructure, as well showing the WEL (Figure 23), with the squat initiating on the side of the running band showing the most distortion of microstructure. This ratchetting damage contributes to the crack formation, especially as a crack starting on the surface follows the distortion of the CRC for Rail Innovation December 2011 Page 20

30 Chapter 2 Crack Initiation microstructure, initially growing along the lamellae of the sheared pearlite. This is true of headcheck cracks and may be associated with formation of a shear localisation zone on the head of the rail [13], like the shear banding that can occur in other situations of severe plastic deformation. Flaws initially growing in shear have been observed in Australia and also in Europe. Figure 20: Typical optical microscopy view of a transverse section of a portion of the running band with a WEL Figure 21: Regions of WEL detected by eddy current testing CRC for Rail Innovation December 2011 Page 21

31 Chapter 2 Crack Initiation Figure 22: Deformed microstructure of the surface of a squat crack near the rail surface (Hunter Valley) Figure 23: Variation in distortion of the subsurface microstructure on a transverse section through the running band of a rail lateral traction to the left The wheel-slip required to form martensite by frictional heating is discussed in Section 2.1.1, and the wheel-slip required to form an adiabatic shear layer is discussed in section Section discusses residual stress associated with a WEL. Section discusses tests to generate a WEL under controlled conditions. Section summarises our understanding of white etching layers Frictional Heating of the Rail Surface Due to Wheel-slip A transient rise in temperature in a rail and a wheel is due to frictional heating, as a result of slip between the rail and the wheel. This temperature increase is the major source of micro-structural changes on the surface of the rail, including the formation of a frictioninduced WEL. Although the temperatures on the rail due to frictional contact do not typically reach the 720 C needed to form austenite, which becomes martensite when cooled rapidly, the high compression of the rail surface from contact with a wheel is likely to enable this phase transformation at a slightly lower temperature [14, 15, 16]. A high cooling rate does occur due to the high thermal conductivity of the rail. Exactly what temperature is required is uncertain, with Wang et al., [17] indicating that a thermodynamic limit gives a minimum temperature of 470 C at which such a layer could possibly form under very high pressure. Shaban and Ghazani et al., [18] reports that a sudden release of a large hydrostatic pressure enabled martensite to form in a 3.3 per cent carbon plain carbon steel, when the cooling rate was otherwise too low to get the quenching effect normally used to produce martensite. Quenching requires a very high cooling rate (e.g., C/s). The reason the martensite developed is that under high pressure, austenite can exist at lower temperatures. When the pressure is suddenly CRC for Rail Innovation December 2011 Page 22

32 Chapter 2 Crack Initiation released, the austenite is no longer stable and the material is able to suddenly form martensite. This situation is analogous to the wheel rail scenario, indicating that both a lower temperature and a lower cooling rate than normal may be sufficient to form martensite. Tests of spring steel on a lathe at The University of Queensland, discussed in 2.1.3, indicate a WEL forming at a temperature that is significantly below 720 C. The basic work into temperature rises from frictional contact was carried out by Blok, Jaeger [19] and Archard. These earlier were not undertaken specifically for the rail wheel contact patch, but for other sources of frictional heating such as between gears or for engine parts. Also, the contact patches they took into account were either a circle (Blok and Archard), or a square (Jaeger) [19]. Harrison later used the work done by Archard to determine the temperature rise in rail wheel contact. A further detailed analysis was done by Tanvir by using the Laplace transforms in approximating the ellipsoidal pressure distribution [20]. Knothe and Liebelt provided an equation to determine the maximum temperature in Hertzian contact where the contact surface is an ellipse [21]. Gupta et al. used a simple analytical calculation to determine the heat generated from the wheel rail contact by combining it with finite element analysis of frictional heating [22]. Ertz and Knothe [11], gave equations that can be used to determine the heat generated and rise in temperature due to the frictional contact between the wheel and the rail. The equations that had been developed can be used to determine the temperature rise at various depths below the contact surface. These equations were used by Widiyarta et al. to determine the temperature rise in the rails due to the frictional contact [23]. In the calculations below, the equations mentioned above [11, 10] are used to determine the temperature rise in rail Solutions for Slip Required for Friction to Heat the Rail to 650 C For the calculations and plots in this section, it is assumed that to facilitate the formation of WEL, a temperature of around 650 C is required around the contact surface. The slip rate for the various loads and speeds are varied so the temperature at the contact surface reaches a minimum temperature of 650 C. Traffic direction gauge side The wheel loads (N) used in the calculations are 8, 10 and 15 tonnes respectively, and speeds of 5 and 10 m/sec are assumed with Rolling Direction the coefficient of friction assumed to be 0.3 and 0.4. From the calculations for the rise in temperature at the rail wheel contact interface, the slip rates required for the required rise in temperature is determined. The values obtained have been tabulated in Table 1. CRC for Rail Innovation December 2011 Page 23

33 Chapter 2 Crack Initiation Table 1: Value for slip rates required for the required rise in temperature Slip Rate ( ) Speed (m/sec) / Coefficient of Friction 5 at.3 5 at.4 10 at.3 10 at.4 Load (T) 8 18% 13% 13% 9% 10 16% 12% 11% 8.5% 15 13% 10% 9% 7% Plots of the temperature variations over the wheel rail contact zone that lead to the data in this table are given in [24]. Note it is the product of speed and slip rate that determines the frictional power causing heating Slip Required in Cornering Situations By considering the case of pressure exerted by a train while cornering, with the load from each wheel being 8 tonne, pressure exerted is taken as 2500 MPa. Two conditions are considered, one each with eccentricities of 4.1 and 5.0 as well as speeds of 5 and 10 m/sec respectively. Slip rates obtained are tabulated in Table 2. The value of the coefficient of friction this case is taken as 0.4. Table 2: Slip rates required in cornering situations Slip Rate ( ) Speed (m/sec) 5 10 Eccentricity % 5.3 % % 10 % Heating of a Rail Due to Adiabatic Shear If heat rise due to plastic work is much greater than the heat able to be dissipated by conduction, then an unstable thermo/visco/plastic phenomenon known as adiabatic shear can occur, associated with localisation of plastic deformation. This shear can create martensitic bands seen in objects subject to ballistic impacts or cutting. This mechanism may also contribute to generating WEL on the surface of rails. Reference [24] gives a formula to predict the temperature rise due to plastic deformation: CRC for Rail Innovation December 2011 Page 24

34 Chapter 2 Crack Initiation T 2 3a P 2 1 LD E Here = 0.86 = plastic deformation vertically as a fraction of total deformation locally below the wheel. L D = depth of the plastic surface layer. a = contact length between wheel and rail, P = contact pressure,µ = density, χ = thermal capacity = 500 J kg -1 K -1.This rise in temperature due to plastic work is of the order of 250 C for a = 10 mm and P = 1.5 x 109 Pa, indicating a significant addition to frictional heating. To consider this in a more specific fashion, a Johnson Cook model of metal plasticity is useful. This models yield stress associated with equivalent strain due to plastic deformation pl, as a product of three power law effects, including thermal softening and hardening due to high strain rates: n m T T0 p ( A B pl ) d p (1 ( )) (1) T T d p is plastic strain rate, T temperature, T 0 = room temperature, T m melting temperature. The thermal term causes a complete loss of strength on melting. The constants A, B and n can be found to fit stress-strain data. The values in Table 1 match available data for a head-hardened rail and a plain carbon rail. The powers m and p for the strain-rate and thermal effects are uncertain for rail steel, so published values from Bonnet-Lebouvier and Molinari [17] for CRS1018 steel were used. Table 3: Parameters used in the Johnson Cook model Parameter Plain carbon rail Head-hardened rail A (Pa) 385 x x 10 6 B (Pa) x x 10 8 n m p To form an adiabatic shear band, the thermal softening must overcome the strain hardening. When this occurs, plastic deformation becomes unstable because less stress is needed to continue the deformation. The literature reports two constraints on forming an adiabatic shear band. 1. The maximum shear strain must be high enough (corresponding to the stressstrain curve starting to level out). Bonnet-Lebouvier and Molinari et al., [25] reports that > 1 for CRS1018 steel. 2. The plastic strain rate must be high enough to achieve adiabatic conditions: that is there should be no time for heat to be conducted away, so that it all goes into causing a temperature rise. Bonnet-Lebouvier and Molinari et al., [25] report this as: d for pure shear. For adiabatic conditions, rate of heat pl pl p produced per volume is q ct CRC for Rail Innovation December 2011 Page 25 m 0. is density and c is specific heat. To cause strain softening to balance strain hardening, the change in stress in a short time t can be estimated using the Johnson Cook equation, and set to zero. That is, the new yield stress is:

35 Chapter 2 Crack Initiation n m T t ( A B( pl d p t) ) d p 1 Tm T0 The rates of temperature increase due to both friction and plastic deformation are needed, to find the rate of temperature change. For frictional heating, Ertz and Knothe [11] give an analytical prediction of temperature versus position across a contact patch of length. For this exercise, the maximum slope of this temperature variation, which occurs shortly after the start of contact at = x/a = -0.78, is used to estimate the frictional contribution to the rate of temperature change. That is, Ertz and Knothe provide: (dt/d ) max. dt v T d a where v is rolling velocity. p For the heating due to plastic deformation, 90 per cent of the adiabatic prediction of temperature rise is used. That is: q ct 0.9 d p The frictional heating effect depends on sliding velocity v S, and the plastic heating depends on strain rate. If the sliding velocity is set to a value typical of the onset of full sliding (v S = m/s, which corresponds to a 10-tonne wheel load and v = 10 m/s), and contact patch size for this load a = mm, then the critical shear strains causing local plastic flow of the surface found with strain rate pl 400 are 0.49 for the head hardened rail and 0.98 for the softer rail. As shear strain is the tangent of the angle of deformation, = 1 represents 45. Deformations of the microstructure of a rail near the surface are comparable to, or greater than, as shown in Figure 24. Unstable plastic flow can occur on the rail surface, if traction is sufficient to cause the yield stress corresponding to the critical deformed state of strain. As this stress is high at the critical strains (e.g., 1 GPa), unstable flow is only likely with high axle loads and high traction. The critical values of plastic strain are not very sensitive to strain rate, but do increase as the strain rate reduces. a. b. Figure 24: Deformation below a rail surface: a: a squat; b: a wheel slip A phase change is another matter. Unstable plastic flow may just lead to rapid wear, as noted by Mazzu [27]. Looking at the temperature increase that would occur if the rate of temperature increase can be maintained across the contact zone, with the head hardened rail pl 1550 is needed to get to 720 C from room temperature of 300 K to form martensite. This value reduces as sliding velocity increases. For instance, with v S = 0.2m/s, about four times as much, or 2 per cent sliding, a sufficient temperature rise is achieved with pl 1060, well above the minimum for adiabatic behaviour. This is still a high value of strain rate, especially as it must be an average over the depth of a WEL, not just a CRC for Rail Innovation December 2011 Page 26

36 Chapter 2 Crack Initiation surface value. At 10 m/s train speed, it corresponds to an average plastic shear strain of 1.73 occurring over the length of a wheel rail contact zone, during one wheel passage. However, 2 per cent sliding is a lot less than the 8.5 per cent predicted in Table 1 for a condition similar to this without accounting for heating from plastic flow. A further complication is the argument by Lojkowski et al., [24], that the presence of hydrostatic compression not only lowers the temperature at which a WEL can form, but enables it to form gradually from exposure to a more moderate elevated temperature with every wheel pass Residual Stress Due to a WEL There are significant residual stresses associated with WELs, that assist in cracks starting, in addition to the residual stresses that are present in the head of a rail due to their manufacture and due to traffic. Wang [28] has experimentally studied stresses due to WEL for intermittent WEL resulting from severe rail corrugation. Wang [28] found residual compression of 600 MPa associated with WEL. Seo et al., [29] found that finite element models show the stressing adjacent to a WEL is due to successive wheel passages. Seo et al., [29] demonstrates with 2D modelling that WEL encourages crack initiation at its edge. An effect Seo et al., do not take into account is the differing thermal expansion of martensite and pearlite. This situation has been investigated in this project by modelling the WEL that could occur due to a wheelburn with a model having simplified axisymmetric geometry. The high temperature caused by a wheel slipping and the resultant stresses are quite local to the contact zone. The results are presented in the December 2010 project report [32], and show that significant residual tension can develop at the edge of a WEL due to its differential expansion (Figure 10). Figure 25: Stressing due to volumetric expansion of the top row of eight elements shown in blue CRC for Rail Innovation December 2011 Page 27

37 Chapter 2 Crack Initiation Test to Generate a WEL Figure 26: WEL test A spring steel strip (with high carbon) was joined on surface of a wheel disk with a radius of 210 mm. A constant load was applied, using a lathe, by a polished nut with a spherical surface (radius 500 mm). The disk was rotated at constant speeds of 31, 50, 80, 100 and 125 RPM. The peak contact pressure P 0 was measured at around 500 MPa during each sliding test. The tests were conducted for 10 revolutions at different speeds. All five tests were conducted at 100 per cent slip. The test strip was removed from the disk and sectioned, moulded, polished and etched with 2 per cent Nital and observed under a microscope. Hardness measurements were carried out without polishing the top surface. Three measurements were taken on each test line at various locations. These measurements are for comparison purposes and not absolute values. Measured pressures were as follows. Test 1: Speed 31 RPM, 10 Revolutions, P0 = 490 MPa, sliding velocity = ms- 1 Test 2: Speed 50 RPM, 10 Revolutions, P0 = 506 MPa, sliding velocity = 1.11 ms-1 Test 3: Speed 80 RPM, 10 Revolutions, P0 = 522 MPa, sliding velocity = 1.78 ms-1 Test 4: Speed 100 RPM, 10 Revolutions, P0 = 490 MPa, sliding velocity = 2.22 ms-1 Test 5: Speed 125 RPM, 10 Revolutions, P0 = 516 MPa, sliding velocity =2.77 ms-1. The microstructure of all the strips was examined and showed the WEL. This suggests that WEL is created at all the tested speeds. Figure 27 shows the longitudinal and top surface of one of the test strips. The longitudinal section shows the presence of white etching layer. The top surface was etched with 2 per cent Nital and observed without polishing, grinding, as polishing and grinding would have removed the damaged surface. CRC for Rail Innovation December 2011 Page 28

38 Chapter 2 Crack Initiation a. b. Figure 27: a: WEL from test at the lowest speed (10 m thick); b: Surface texture from lowest speed test. According to Ertz and Knothe [11], the temperature rise predicted from the frictional heating only, setting rolling velocity equal to sliding velocity to simulate just sliding, should be significantly less than 720 C at all these speeds. With a high estimated friction coefficient of 0.6, the temperature rise is predicted at a peak of below 600 C within the contact patch at the highest sliding speed, and below 300 C at the slowest speed. This is below the minimum temperature thermodynamically possible [17]. The contact pressures used may cause some temperature rise from plastic work. A shortcoming of the tests was that the traction was not directly measured, so the friction is uncertain. Despite this uncertainty, 720 C is very unlikely to be reached at the lowest sliding speed, and the tests do indicate that the phase change causing the WEL can occur at temperatures lower than 720 C. The coefficient of friction may have changed during the tests, increasing due to the surface wear visible in Figure 27b. A further test conducted one revolution at a time, at the lowest speed, showed that the WEL did not develop until the third revolution, which was associated with increased contact noise, suggesting an increase in friction had occurred Summary on WEL Development A WEL on a rail is principally friction-induced from wheel-slip (friction-induced martensite). The predictions of the amount of sliding needed in Table 1 could easily occur in connection with variable normal contact forces and variable friction associated with turnouts, crossings and dipped welds; however, significantly less sliding may be needed due to the rapid plastic deformation of the rail surface contributing to the production of heat, and due to the compression of the rail caused by contact pressure. These factors appear to aid the phase change, allowing it to occur at a lower temperature, and at a lower rate of cooling. Tests undertaken at The University of Queensland support this theory Residual Stress in a Rail Head Residual stress in the head of a rail has been measured by a number of researchers, including Luzin et al., [31] who used neutron diffraction measurements of new and trafficked rails. Osterle [30] conducted x-ray measurements of residual compressive stresses in a WEL, and found significant compression both along the rail direction and transversely. Wang [28] made similar measurements on an intermittent WEL due to corrugation. CRC for Rail Innovation December 2011 Page 29

39 Chapter 2 Crack Initiation Results by Luzin et al., [31] show the stress due to manufacture of the rail, as well as stress due to two distinct running bands on the rail surface. As part of this (current) project, measurements of residual stress on an Australian rail known to have a WEL were conducted by neutron diffraction at ANSTO, consisting of tests of the whole cross-section of the head of a rail, using the coarse 3 x 3 mm grid of cells in Figure 28, and tests to resolve the stress variation near the surface of the rail versus depth over 5 mm below the surface on a finer grid (0.5 x 0.5 mm cells). Virgin Rail Mesh of Rail head Service Rail Transverse Direction Transverse Direction Vertical Direction Vertical Direction Figure 28: Residual direct stress components (MPa) in a virgin rail and a service rail with WEL CRC for Rail Innovation December 2011 Page 30

40 Chapter 2 Crack Initiation 200 Normal stress components\ 100 Stress( MPa) S_Rail, <T>=<Sxx> V_Rail, <T>=<Sxx> V-Rail, <L>=<Syy> -500 Distance from the surface (mm) S_Rail, <L>=<Syy> Figure 29: Direct stress v depth over 5 mm below the rail head, xx = transverse, yy = axial Shear stress components\ 20 Stress ( MPa) Sxz Sxz Syz Syz Distance from the surface (mm) Figure 30: Shear stress versus depth over 5 mm below the surface of the rail. yz vertical/axial plane, xz transverse plane Notable in the service rail results in Figure 28 is the extent to which the whole head of the rail is in residual transverse compression of 300 MPa, rather than just a narrow running band. Figure 29 indicates this transverse stress does not decay over the 5 mm of depth considered, although the longitudinal direct stress does drop to around 200 MPa. The vertical residual stress in Figure 28 becomes tensile fairly quickly subsurface, and shows a patch of compression on the gauge face, indicative of flange contact. The measured residual shear stresses (Figure 30) are small, reflecting the uniformity of the direct stress. The significance of these residual stresses is discussed further in Chapter 3. Steenbergen et al., [35] has proposed a theory of how a squat crack develops [27]. These observations conflict with the assumption of residual stresses including high residual shear stress existing in a narrow running band. The tested by ARTC was probably not ground frequently and would have had a broader running band. The origin of the residual stresses in the as-manufactured case is explained in detail by Zerbst et al., [36]. The vertical residual tension evident subsurface, will encourage crack growth parallel to the surface. The horizontal compressive residual stress does discourage crack growth down into the railhead. CRC for Rail Innovation December 2011 Page 31

41 Chapter 2 Crack Initiation 2.3. Observations of Crack Initiation Surface damage of a rail leading to development of a squat can be from an inclusion [32]; from existing head checks combined with thermal damage or from cracks that form in a WEL or at its edge, or even from grinding marks. Just the surface roughness of rail and wheel is enough to produce variations along a rail in the extent of plastic flow of the rail surface [33], making some locations more prone to crack initiation. Transient heating of the surface of a rail, not only induces a white etching layer, but can also cause significant thermal stresses if a flaw is present on, or just below, the surface. The layer that heats is quite thin, typically 100 m or less, unless a train is moving slowly. Thermal expansion of the surface induces vertical tension just below the surface, and the shear between the expanded layer and the layer below. Severe thermal expansion causing yielding will change the existing residual stress in the head of a rail, tending to reduce or remove the protective effect of residual compression. Where wheel-slip has occurred, damage to the rail surface is often visible on both rails Predicting Where Crack Initiation is More Likely The trade-off between wear and crack initiation due to the frictional power input from sliding, help to explain why squats tend to form on transitions into curves. A study by Simson [25] at Central Queensland University demonstrated that the balance between crack initiation and wear favours crack initiation on the high rail of a transition into a curve. Wheel and rail profiles can also influence the balance. A study by Spiryagin at Central Queensland University compared two ideal traction scenarios (curve of radius 400m, cant of track m) for the following profiles: 1. AS60/ANZR1 2. Flat50 new/anzr1. The Flat50 profile differs from a standard AS50 rail, as shown by the top curve in Figure 31. ANZR1 is a wheel profile with a 1:20 taper, which gives 2-point contact on a new rail. A surface fatigue index by Ekberg [26] was computed, as well as the T-gamma index. The Flat 50 profile scored better compared with the AS60 due to the lower value of lateral creepage, with a lower Ekberg fatigue index, although the index still goes positive indicating crack initiation. The traction/creep T-gamma product was also lower, implying a lower thermal input. In general, however, there is a lack of evidence of a benefit from changing wheel profiles. Figure 31: Flat 50 profile superimposed on an AS50 profile CRC for Rail Innovation December 2011 Page 32

42 Chapter 2 Crack Initiation 2.5. Conclusions About Crack Initiation While squat cracks can be produced from exhaustion of ductility of the rail surface, in situations where flaws are not worn off, they can be also produced much quicker from thermal damage to the head of a rail. Where the thermal damage is present, other surface flaws like such as head checks can initiate the growth of squats. The exact conditions of temperature and pressure at which a WEL can be produced need to be clarified. Further testing and simulation is justified. WEL is produced mainly from frictional heating, but with some heat generated by rapid plastic deformation contributing. Austenite can form in steels under high pressure, like a rail compressed by a wheel, at lower temperatures, allowing a transformation to martensite when the pressure is removed. The surface of a rail is in existing residual compression, and this has to be overcome to form a crack. Again, a thermal effect is considered to be involved. The frictional heating is enhanced by anything that causes wheel-slip, but not so much as to allow wear to dominate. Therefore, factors that promote oscillation of the contact force between a wheel and rail, causing moments of wheelslip, all contribute to the initiation of cracks. These issues are discussed further in Chapter 5. CRC for Rail Innovation December 2011 Page 33

43 Chapter 3 Crack Growth 3. Crack Growth 3.1. Introduction Understanding the growth of squat cracks is not a simple, due to: 1. the extent of plastic deformation and consequent residual stress that occurs in a rail head (Section 3.3) 2. the role of water in permitting crack growth in shear and in pressurising a crack (Section 3.4) 3. the crack closure that is expected to occur with every wheel passage 4. the contact pressure between a wheel and a rail, and the associated traction distribution, both change significantly as the surface of the rail above the crack subsides plastically to form the characteristic shape of a squat. While the crack is still very short, both shear localisation due to anisotropic plasticity, and thermal stressing influence its growth (Section 3.5). As a result, the computations in this chapter, such as the stress intensity variations in Section 3.6, should all be viewed as exploratory, giving insight into the physics of how a squat grows, rather than representing the process accurately. Linear elastic fracture mechanics (LEFM) is used in some of the models, but with reservations that the problem is really neither linear nor elastic. Cracking due to rolling contact is usually associated with the cycle of shear stress induced by contact. In the absence of traction, cracks typically initiate subsurface, as the shear stress due to contact peaks subsurface. Squats, on the other hand, show evidence of surface initiation, which is consistent with traction shifting the peak shear to the surface. The first peak of shear stress due to contact tends to make a tensile crack grow down, and the second peak of opposite sign makes a tensile crack grow up. This explains the spalling or shelling behaviour of bearings or railway wheels. Squats on the other hand, tend to grow straight ahead subsurface, not up or down, and show a reluctance to shell. Dubourg and Lamacq [37] explained this situation with a mixed-mode failure criterion due to Hourlier and Pineau, predicting dominant mode II growth in 2D. This criterion, however, depends on a crack-locking ratio in Hourlier s model, which if lowered will turn off mode II. A squat is a short crack for a significant portion of its fatigue life; therefore Paris Law is inadequate, and needs correction, as discussed in Section 3.7. The fatigue behaviour of rail steels has been measured, and fitted to a crack growth law applicable to all crack lengths, reported in Section 3.7. Growth of squats in the field has also been measured, and is outlined in Section 3.8. The fractal dimension of the surface of a squat crack has been measured, as reported in Section 3.9. This data suggests a lack of crack closure Plasticity and Residual Stress The reasons that a squat crack grows subsurface, rather than remaining a small surface crack like a head check, are subtle. Steenbergen et al., [35] proposed a theory of the growth of squat cracks that has some merit. The theory relies on transferring load from the material above the crack to material before the crack mouth, as the normal contact force is re-routed around the damaged section of rail surface. This encourages the crack to grow. A 2D elasto-plastic plane strain simulation of a railway CRC for Rail Innovation December 2011 Pag

44 Chapter 3 Crack Growth wheel simply pressed onto a rail, with an inclined surface crack, was created to help understand how the contact pressure changes with the crack. Contact pressure between a wheel and rail is classically a Hertzian ellipsoidal distribution, peaking in the centre of the contact patch. A plane strain model of a wheel contacting a rail was used to see how this is modified by a crack (5.1 mm in its horizontal dimension), growing at a small angle to the rail surface (8.4 degrees in this example). The wheel was treated as elastic, but the rail was treated as either being elastic, or elasto-plastic, with properties of a head-hardened rail. A traction was applied, using a coefficient of friction of 0.4 between the wheel and the rail. Friction within the crack is turned on (µ = 0.4), or turned off. Peak contact pressure between the wheel and rail without the crack is 1.29 GPa. With the elasto-plastic rail, the wheel was pressed onto the rail twice, by imposing deflections on the wheel of 420 mm radius. The first time the wheel was pressed to allow the rail surface to flow plastically, and the second time was to get a better estimate of the contact pressure distribution, taking account of a redistribution of pressure due to plastic flow. However, this test does not allow for plastic deformation from the previous growth of the crack. The shear stress peaks at the mouth of the crack and at the tip of the crack, as shown in Figure 32. The material on the rail surface ahead of the crack will become strain hardened. As the crack grows, at any particular wheel passage, the surface will be pressed down, as material is sheared plastically above the current crack tip, as the wheel load takes this path around the damaged material to the rail below (red area at the right in Figure 32). All the material above the crack will become depressed into the squat as it grows. Figure 32: Crack in rail deformed (x 5 scaling), due to a wheel load. Von Mises stress (Pa). Traction acting to the left A similar effect on the contact pressure distribution is obtained in all cases. The plastic deformation makes the contact more conformal, increasing the size of the contact zone predicted elastically. As a result, the peak pressures are lower when plastic deformation is permitted. There is a peak in contact pressure, just before the crack begins at the 6 mm position in Figure 33. The crack with a friction coefficient of CRC for Rail Innovation December 2011 Pag

45 Chapter 3 Crack Growth 0.4 between its faces shows a drop in contact pressure at its mouth. The contact pressure then builds up gradually until above the tip of the crack. With no friction between crack faces, there is also a dip in contact pressure associated with the crack tip, as well as the more pronounced one associated with the crack mouth. Figure 33: Contact pressure distributions disturbed by plastic deformation and by a crack These changes in contact pressure are of interest to judge if a crack can grow in mode II while the wheel is still over it and will be more likely to occur with any reduction in contact pressure over the crack. The reduction in pressure is limited in these examples because the material can only deform in-plane, and because a previous history of loading is not taken into account. The changes in contact pressure will affect the maximum shear stress at the crack tip and, therefore, a K II estimate. The ellipsoidal contact pressure distribution based on elastic analysis must exaggerate the real effective stress intensity. A similar plane strain model is used to represent a transverse crack 5.1 mm long. This time, the rail is curved with a 300 mm radius and the surface of the wheel contacting it is straight. The crack is frictionless. Just pressing the wheel on the rail with the centre of the contact patch above the mouth of the crack and removing it, gives the residual plastic strain in Figure 34. Again, the material above the crack tip is plastically deformed. CRC for Rail Innovation December 2011 Pag

46 Chapter 3 Crack Growth Figure 34: Plastic strain after a wheel loading transverse crack case traction to the left This model does not show a large concentration in the contact pressure at the start of the crack, but it does show dips in contact pressure at the start of the crack and above the crack tip (Figure 35). The peak contact pressure without a crack should be at the start of the crack (the 8 mm position on the x axis). The shape of the pressure distribution is affected by the irregular mesh. Figure 35: Contact pressure distribution for the transverse crack One detail of the plane strain models with a transverse crack is that there is significant residual shear stress left at the crack tip after the wheel load is removed. This is shown in Figure 36, which applies to a case where the contact patch is centred directly above the tip of the crack. CRC for Rail Innovation December 2011 Pag

47 Chapter 3 Crack Growth Figure 36: Residual Von Mises stress (due to shear stress) at the crack tip There is also residual compression at the crack tip, which will discourage crack growth. However, if a compressive residual stress of 300 MPa parallel to the top surface is introduced into all elements in the top 5 mm of the rail, the yielding at the crack tip increases when the wheel load is applied, with some yielding below the crack tip, not just above it, but less on the surface of the rail. This results in the crack tip springing back into some residual tension, when the wheel load is removed, as shown by the principal stress arrows in Figure 37. Figure 37: Directions of principal stress in elements around the crack tip, showing tension normal to the crack in the circled elements The residual compression in the surface of a rail does not necessarily stop a crack from growing, when the wheel is not directly over the crack. The opening of a squat crack due to residual stress crack thickness has been measured by Pyrzanowski [38], who attempted to model the degree to which a wheel closes a crack that was initially open. CRC for Rail Innovation December 2011 Pag

48 Chapter 3 Crack Growth If rolling contact is modelled in 2D, extending the model of Figures 32 to 36, there is a ratchetting process where the conditions at the crack tip settle down into a steady stress-strain cycle. If crack surface displacements are used to estimate effective stress intensity values, relative to the permanent displacements due to residual strain, then the cycles of effective K I and K II are similar to elastic estimates shown in Figure The effect of water Water potentially has three effects on crack growth: 1. Corrosion microscopic examination of squat cracks does not provide evidence of crack growth due to corrosion. 2. Lubrication of a crack, permitting crack surfaces to slide over each other, enabling growth in the shear modes II and III. This almost certainly occurs. 3. Pressurisation of a crack enabling growth in the tensile mode I. It is uncertain the degree to which the wheel rail contact patch can seal the mouth of a squat crack. A seal is more likely for smaller cracks than larger ones that extend beyond the running band. Bogdanski [39] has modelled the effect of water pressurising a crack in a 2D model. An improvement of his modelling, representing both flow of a fluid into and out of a crack is presented by Balcombe et al., [40]. This research shows a peak K I that is less than that obtained by simply placing the contact pressure inside a crack. Squats in Sydney have been observed to grow faster in wet weather. The LEFM analysis of a squat-like crack in Section 3.6 shows that pressure due to water in a crack leads to a more credible prediction of the direction of propagation. CRC for Rail Innovation December 2011 Pag

49 Chapter 3 Crack Growth 3.4. Thermal Influence on the Growth of Short Squat Cracks The presence of WELs on tracks indicates severe thermal heating of the surface. Fletcher [41] argues that WEL can influence the growth of cracks by considering a plane strain boundary element model of a longitudinal crack 0.5 mm below the rail surface and parallel to the surface. The cycles of stress intensity experienced at the ends of the crack become significantly worse when the contact surface between the wheel and rail is hot, as the material above the crack expands, tending to open the ends of the crack. The heating of a rail is analysed by Ertz and Knothe [42, 11], and by Sawley [43]. Ertz and Knothe [42, 11] present a one-dimensional solution for the decay of temperature below the rail surface, which has a characteristic thermal penetration depth This is given by: 2a v Where a is the semi-axis of an elliptical contact patch, is thermal diffusivity of steel, and v is the train speed. As steel is a good conductor, the temperature dies away rapidly below the surface, making typically 0.1 mm or less. However, at low speeds, the depth of heating increases. For instance, at 1 m/s train speed, the train stays around to heat the rail and is about 0.5 mm, the value used by Fletcher [41]. Only near-surface crack tips corresponding to small squats can be affected, but early crack growth could be accelerated this way. To investigate this further, the plane strain XFEM code reported in [44] has been modified to include thermal strains produced by assumed rail surface temperatures decaying exponentially with depth below the contact zone. The contact zone has been given one temperature, although the analysis by Knothe predicts a temperature that varies over the contact zone. Two cases have been examined: a crack totally subsurface and a surface-breaking crack. The subsurface crack is shown in Figure 38. It could be thought of as a section through a squat that initiated out of the plane modelled. mm Rail surface 0.54 mm 0.48 mm 0.54 Crack Figure 38: Subsurface crack modelled 6 mm 6 mm The history of K I and K II at each end of the crack is reported in Figure 39 for no heating, and in Figure 40 with the contact patch passing over the crack at 200 C above the environmental temperature, and with a thermal penetration depth of 0.5 mm. Tractions are applied corresponding to using a coefficient of friction of 0.4. The peak wheel rail contact pressure is high at 1.5 GPa. Contact semi-length a is 9.2 mm. CRC for Rail Innovation December 2011 Pag

50 Chapter 3 Crack Growth This contact semi-length tends to open the trailing end of the subsurface crack after the wheel passes, as shown by a detail of the deflected mesh in Figure 41, and by the positive K I values after the wheel passes: the blue curves at the right of Figures 39 and 40. Comparing Figures 39 and 40 shows that both stress intensity values have significantly increased due to the thermal strains. Figure 39: Cycles of stress intensity at the ends of a subsurface crack due to a wheel passage Figure 40: Cycles of stress intensity with the contact zone at 200 C A detail in Figure 40 is that K I at the leading end of the crack (the red curve) is slightly positive when K II is large during the wheel passage, making mode II growth at the leading edge more possible, as the crack is not quite closed. CRC for Rail Innovation December 2011 Pag

51 Chapter 3 Crack Growth Figure 41: Portion of mesh deformed A surface-breaking crack was also considered: a 1 mm crack facing forwards at 20. It experiences a peak of KI just before a wheel arrives. With no heating, and the parameters above, the peak is estimated as 4.83 MPa m1/2. With a thermal penetration depth of mm, corresponding to a speed of 20m/s, and a +2000C contact zone, this peak value increases dramatically to 26.4 MPa m1/2. With a thermal penetration depth of mm, corresponding to 1m/s speed, the value increases to 49.1 MPa m1/2, which would cause rapid initial growth of a crack. Note the accuracy of these figures is limited by the fact that the imposed temperatures make no allowance for the crack, and the fact that the imposed contact pressures and tractions are treated as unaffected by the thermal expansion Stress intensity values at the tip of a squat-like crack 1 Bogdanski [45] has created a solid finite element model using LEFM to investigate water entrapment and crack pressurisation effects on stress intensity factors for a semi-elliptical surface crack with some resemblance to a squat. This model involves a block with a flat surface laid on fixed boundary conditions, except on the surface with the crack. Stress intensity values around the crack front are calculated and show that trapped water increases the stress intensity of the opening mode (K I ) 10 times. This model was replicated and improved to consider elastic foundation effects as well as railhead curvature. It still has very significant limitations, as no allowance was made for plasticity, for dynamics, or for any changes to contact pressure. The results are summarised here: for more detail see [8]. The analytical model of bending stress in a beam laid on an elastic foundation (i.e. the Winkler model [46]) is used, to obtain boundary displacements of the slice of 60 kg/m rail with a crack that is modelled. The stress intensity values around the crack front are calculated by estimating the crack opening displacements. Figure 42 demonstrates the local coordinates at a crack tip when a wheel approaches a crack. 1 The author of section 3.6 is M. Farjoo et al., CRC for Rail Innovation December 2011 Pag

52 Chapter 3 Crack Growth Figure 42: Local coordinate system showing displacements of nodes, when a wheel approaches a crack Stress Intensity Factor at a Crack Tip The stress intensity factors are related to displacements of nodes next to the crack tip as: ( ) ( ) ( ) where r is distance of the nodes from the crack front (Figure 42), and the subscripts 1, 2 refer to either side of the crack (Figure 35). The equivalent stress intensity (K eq ) can be found from [11]: (1) ( ) (2) Figure 43: Nodes at crack tip in general coordinate system. φ is the kinking angle Crack Propagation Angle In the absence of any shear mode (i.e., zero K II and K III ), a crack propagates along its crack face. Under a pure in-plane shear mode, the crack kinks. Different theories predict kinking angles such as maximum principal stress or minimum energy density theories [47, 48]. Crack propagation angle prediction equations for three dimensions were developed by Richard et al., [47] based on minimum energy density of an isotropic elastic solid: [ ( ) ] (3) CRC for Rail Innovation December 2011 Pag

53 Chapter 3 Crack Growth [ ( ) ] (4) where φ is a kinking angle (Figure 35) and ψ is a twisting angle at each point of the crack front due to the out of plane shear mode of fracture (mode III). φ < 0 (CW) for K II > 0, and φ > 0 (CCW) for K II < 0. Correspondingly, ψ < 0 (CW) for K III > 0, and ψ > 0 (CCW) for K III < 0. K I is always positive or zero. These equations can predict crack propagation angles at each node of the crack front. The stress intensity values to be selected and substituted in equations 3 and 4 are those that make K eq a maximum during a loading cycle (i.e., during a wheel passage) Finite Element Models (ANSYS) A block model matching that of Bogdanski [45] was created as shown in Figure 44. The second model adding railhead curvature and rail bending deformation from the Winkler foundation is shown in Figure 45. In both the block and rail models, the cracks have the same size and a 20 angle to the running surface. With ƴ = 90 0 in Figure 44, the major axis of the crack modelled is in the transverse direction. a. b. c. d. Figure 44: Crack model matching Bogdanski s [45] model a. Isometric view: the fine mesh area in the middle of the surface is shown dark. b. A zoomed view of the crack. c. A contact load passes a crack (side view). d. A crack face, midpoint, right and left edges are as shown. The major axis of the crack makes an angle γ with the traffic direction. CRC for Rail Innovation December 2011 Pag

54 Chapter 3 Crack Growth To model crack closure when a wheel passes the crack, contact elements are used between both faces of the crack. A friction coefficient of 0.1 is specified between both faces of the crack, matching that used by Bogdanski [45]. a. Slice model of a rail with a crack on the running surface. Associated displacements are applied at both ends to simulate elastic foundation. b. The crack on rail head, direction of travel is shown with an arrow. right Figure 45: A slice model of a 60 kg/m rail laid on an elastic foundation: a: isometric view; b: the crack on a rail head surface, direction of travel is left to A Hertzian contact pressure with peak pressure P 0 = 740 MPa passes the semi-elliptical 3D crack. Longitudinal traction ratios (TR) of 0 and 0.2 are considered, as well as lateral traction ratios (LR) of 0, 0.3 and 0.5. It is assumed the water pressure in a crack is equal to the amplitude of the contact pressure on the crack mouth. b. K a. K I, K II, and K III along the half-crack front (X/b f = - II and K III when the wheel passes the crack, compared with Bogdanski s results. 0.8) Figure 46: a. Dimensionless stress intensity factors around the crack front from midpoint up to the right node, for a particular wheel position. b. Dimensionless K I and K II at the crack tip midpoint obtained from benchmark and compared with Bogdanski s [45] results, varying as a wheel passes the crack. K III is zero due to symmetry The benchmark model of Figure 44 can be compared with Bogdanski s [45] in Figure (46), normalising K as in [11]. These results apply to the crack midpoint where K III is zero. The large cycle of K II is characteristic of rolling CRC for Rail Innovation December 2011 Pag

55 Chapter 3 Crack Growth contact. Note the crack is open (+ve K I ) before the wheel arrives. It was found that K I and K II of the benchmark model have differences of less than 12 per cent and 10 per cent respectively when a wheel passes the crack, considered to be satisfactory agreement with Bogdanski [45]. When modelling a curved railhead, as a wheel passes the crack, the amplitude of K I around the crack front changes. For instance, for X/b f > -0.6, K I = 0 at all points of the crack front due to crack closure, which differs from the benchmark problem, and results in a lower K eq. These nodes next to the left and right edges of the crack are affected by discontinuity of the stress field. Although they are plotted in Figure 46, the stress intensities obtained at those nodes are not reasonable. Lateral force ratios, L/P = 0.3 and 0.5, with the load to the left, were applied at the contact patch to investigate how the stress intensity values vary with a transverse rail curvature. The results obtained from a crack with γ=45 or 90 show that while the wheel has not passed the crack, the superposition of the lateral load along with bending stress due to the elastic foundation, opens the right side and closes the left side of the crack. But when the wheel has passed the crack, the lateral load opens the left side and closes the right side. Increasing lateral load ratio from 0.3 to 0.5 raises the maximum K eq 31 per cent at the sides of the crack front. The lateral load also affects the shear mode (K II and K III ) patterns significantly. Therefore, the K eq at the crack tip is strongly correlated with lateral load, especially near each end of the crack front. Figure 47 plots crack propagation angle in an oblique crack ( ), when a wheel passes the crack. The black curve shows propagation angles of a crack without any entrapped water. The second curve shows what happens if a very high traction ratio occurs, although a very high traction ratio is unlikely due to the use of traction control systems. The third curve plots crack propagation angles when the crack is pressurised due to trapped water. Negative φ implies the crack kinks clockwise and then turns down the rail, while positive φ represents the crack turning counter clockwise. As it has an initial 20 angle to the rail surface, implies that the crack propagates almost parallel the surface and can make a subsurface crack. Figure 47 indicates that the trapped water has as significant an effect on crack propagation direction as traction ratio. CRC for Rail Innovation December 2011 Pag

56 Chapter 3 Crack Growth a. K I along the crack tip when X/b f =-0.8 b. K II along the crack tip when X/b f =-0.8 c. K III along the crack tip when X/b f =-0.8 d. K eq along the crack tip when X/b f =-0.8 Figure 47: Stress intensity along the crack tip when the crack s minor axis is aligned with the traffic direction (γ=0) or has angle of γ=45. Different longitudinal traction ratio values and lateral traction ratio values are considered a. Crack kinking angle (φ) b. Crack twisting angle (ψ) Figure 48: Stress intensity along the crack tip when the crack s minor axis is aligned with the traffic direction (γ=0) or has angle of γ=45. Different longitudinal traction ratio values and lateral traction ratio values are considered. CRC for Rail Innovation December 2011 Pag

57 Chapter 3 Crack Growth Further Discussion of this Model These Finite Element (FE) models show the transverse railhead curvature along with the crack s direction both affect stress intensity plots. The slice model shows K I at the crack front is highly correlated with the elastic foundation stiffness, as increasing the stiffness increases crack closure, and reduces the peak K I. A lateral traction ratio can lead to high shear stress intensity values. These factors rise greatly when the contact load is over the crack. This leads to K eq being at least 400 per cent greater at both sides of the crack considered, than at the crack s midpoint. This is qualitatively consistent with the crack-face displacements reported for elliptical cracks in railway wheels, by Liu [16]. Therefore, the crack with the shape assumed can grow with a higher rate in the major-axis directions compared to the growth in the minor-axis direction. This variation of K eq was not in the first model where the running surface has no curvature. When the traction force is superimposed with lateral load and bending stress, the maximum K I and K II decrease. But K II does not change because the negative K II also decreases due to traction force while the wheel has not reached the crack. The traction force decreases K eq due to decreasing the contribution of K I. In an oblique crack with =45, K I is always zero in this model due to crack closure and K II and K III have a different pattern and amplitude compared with a crack with =90. K II in an oblique crack is reduced, but K III on the left side is increased dramatically. Varying elastic foundation stiffness can change stress intensity values and particularly K I, and it also varies crack growth rate. The elastic foundation effect here is quasi-static, causing a softer support to decrease crack closure. In reality, softer supports for track will increase the dynamic response of the wheel-rail system, increasing the peak contact loading. Changes in loading, due to dynamics or due to plastic deformation of the rail head, need to be modelled to achieve a more realistic picture. The model shows an oblique crack can develop parallel with the surface if some water trapped inside the crack pressurises the crack. This agrees with other studies that assumed trapped water is one of the main causes of squat formation process [50]. Given there is uncertainty about how well water can be sealed in the crack, this (current) study confirmed the crack can grow parallel the surface if the amplitude of K I is high enough to change the sign of. When the crack is pressurised, the crack front kink angle is around 30 and the twist angle is 5. This will lead to a saddle-shaped crack like those observed in practice (e.g., in Figure 33). Cracking of railway wheels involves similar cycles of stress intensity as a crack passes through the contact zone [51]. For cracked railway wheels, shelling occurs, and is thought to be due to the second peak in K eq associated with the second (-ve) peak in K II that occurs as the contact zone passes beyond the crack tip. Squats do not behave this way. CRC for Rail Innovation December 2011 Pag

58 Chapter 3 Crack Growth 3.6. Modelling Crack Growth by Extended Finite Element Method and by Element Removal While modelling a prescribed crack gives clues about how a squat forms, it cannot realistically simulate growth of a squat. It is better to let the model decide where the crack should form and grow. This can be achieved in two ways: by removing elements from a solid model, or by splitting elements in two using the extended finite element method (XFEM). Both approaches are much simpler to implement in 2D. The element removal approach is simpler. Crack tip elements are removed from a fine finite element mesh, to extend the crack in a direction normal to the current maximum principal stress direction at the crack tip. This approach does not predict if a crack will actually grow, but just the direction of growth if it did. A simple elastic model in 2D is shown in Figure 49. The wheel is pressed onto the rail repeatedly and each time elements are removed. This process does not simulate the history of stressing due to rolling contact, but just one moment in time, when the edge of the contact zone is adjacent to the mouth of the crack. This moment will give the worst state of stress, if growth of a closed crack is discounted, ignoring the possibility of fluid pressurising and lubricating the closed crack, as considered in the previous section 3.5, and assuming the wheel closes the crack when it passes over it. As the maximum principal stress on the surface is normal to the free surface, the crack initially grows straight down. This conflicts with extensive experimental evidence that shows the crack often initially grows at a small angle to the surface, but this type of initial growth can really only be explained by an anisotropic plastic model. However, the growth angle corrects itself and the direction of the left crack growing in the direction of travel in Figure 49 becomes parallel to the surface in a plausible fashion. Figure 49: Predicted squat growth by element removal in 2D. This approach can be extended to 3D at the expense of a huge increase in computation. An example of crack surfaces predicted due to Hertzian contact pressure and both lateral and longitudinal tractions, is shown in Figure 50. The 3D models show cracks that grow down at a realistic angle to the surface of the rail e.g., Figure 50. However, the photographs in Chapter 1 indicate that assuming a Hertzian CRC for Rail Innovation December 2011 Pag

59 Chapter 3 Crack Growth distribution of contact pressure is quite questionable, as is the assumption of crack closure. As can be seen in Figure 49 the contact stressing is quite local to the contact zone, and the loading at this point in time does not explain why the crack would continue to grow as the crack tip moves away from the stressed region. 5 mm 2 mm Figure 50: Half model of a squat crack showing elements removed. Z = direction of travel, TR = 0.1, XFEM offers the potential to perform estimates of crack growth in a more realistic way, and with less computation, but it has not been attempted in this project. Code has, however, been written to do this in 2D, as used in Section 3.5. XFEM in 3D is implemented in the ABAQUS package, but the package does not enable modelling of pressure due to fluid in a crack. ABAQUS uses continuum damage mechanics to describe crack growth in low-cycle fatigue. That is, elements accumulate damage with each cycle until the damage becomes critical and the element cracks. This approach is an alternative to LEFM that has some potential to better explain the early growth of a squat crack, but modeling the damage would need to be changed from ABAQUS to achieve better realism Measured crack growth The final results of crack growth studies undertaken by Railcorp at Erskineville and at Chatswood in Sydney are discussed in this section. Figure 51a gives a view of the rail at Erskineville, showing the crack outline and crack depth of one of several squat cracks that was monitored. Crack size versus time for six squats at Erskineville is plotted in Figure 51b Figure 51c shifts data in time to show the trend in size, measured as an equivalent radius versus time, including some data from Chatswood. Figure 52 shows a squat at Chatswood, early and late in its growth. Less data was collected here, as the rail was replaced. Growth versus time of squats at Chatswood was reported in this project s previous progress report [54]. Estimated tonnages of traffic using these lines are 15 MGT at Erskineville and 16 MGT at Chatswood. CRC for Rail Innovation December 2011 Pag

60 Chapter 3 Crack Growth Figure 51a: Crack growth at Erskineville. Shape of crack and crack depth mm Squat Growth Erskineville (area growth) 0 28/3/09 6/7/09 14/10/09 22/1/10 2/5/10 10/8/10 18/11/10 26/2/11 Figure 51b Crack growth at Erskineville. Growth v time Squat No CRC for Rail Innovation December 2011 Pag

61 Chapter 3 Crack Growth 100 Squat size (mm) Squat Growth in the Erskineville & Chatswood lines Months Figure 51c: Crack growth at Erskineville. Growth of equivalent radius time-shifting curves a. b. Figure 52: Crack growth at Chatswood. a: December 2007 b: May Growth rate of cracks in rail steel To assess damage tolerance of rail once a squat exists, it is helpful to have an accurate crack growth law; however, these laws rely on accurate stress intensity values, which are hard to obtain as the loading parameters need to be accurately determined as well, which can be difficult with variable rolling stock in use and the effects of variations in track stiffness. Paris law, which assumes a linear relation between the log of crack growth rate da/dn and the log of the range of the effective stress intensity K eff, can be corrected to better fit test data both the data obtained in this project, the data reported in [50] and from others fatigue tests on rail steels [52, 53, 54]. This was first done using a generalised Frost Dugdale law as reported in a previous project report [50], but it can be improved using a Hartman Schijve equation [55], which uses the change from threshold stress intensity range ( K eff K th ) to correct for short cracks and a factor 1/(1 K max /K c ) to accelerate the growth rate as the fracture toughness K c is approached, that is: CRC for Rail Innovation December 2011 Pag

62 Chapter 3 Crack Growth da dn =D(DK-DK th )a (1- K max K c ) As seen in Figure 53, the crack growth law fits rail steel test data well for both very short and very long cracks, with = 2. For head hardened rail steel, K c = 120 MPa m 1/2 and K th is 2.5 to 10 MPa m 1/2. This law could be rewritten as growth per tonnage, but it still requires knowledge of the evolution of the effective K with crack size. Figure 53: Growth of cracks in head hardened rails steels fitted to a Hartman Schijve law As the majority of the fatigue life is consumed in the short-crack regime that is in growing to a size of approximately 1 mm using the modified law is important. The dependence of the growth of longer cracks on K max implies that mean stresses, such as residual stresses, could influence the growth at this later stage, although these are stresses are mainly shear stresses that are not believed to affect the rate of growth. Residual stress does have an effect of influencing the direction of growth. In terms of LEFM, the sign of the peak K II controls whether a tension crack will kink up (-ve K II ) or down (+ve K II ). The residual compression horizontally that was in the rail above the crack disappears, as this material becomes free to expand, but it does increase the residual shear that would otherwise exist at the crack tip. This pre-existent damage may encourage the crack to continue ahead in mode II. A consequence of having a simple power law with an exponent of 2, is that measured growth rates should be approximately proportional to crack length, if the stress intensity shows the classic dependence on square root of crack size. Measured data in section 3.9 shows that the exponent of the rate of growth is actually reducing as CRC for Rail Innovation December 2011 Pag

63 Chapter 3 Crack Growth the crack grows, suggesting that the crack tip is less affected by contact force loading as it grows away from the running band or grows deeper into the rail. Scaling growth rates for different tonnages of traffic is possible by using the experimental data on growth of squats. The Sydney data can be fitted to two different slopes on a loglinear plot, as shown in Figure 54. Some data from Delft in Holland is included in this figure to show that international experience is similar. The log-linear form enables estimation of the effects of different tonnages. That is, as x in Figure 54 is really measuring the number of cycles, it can be regarded as tonnage/(reference tonnage) by time in months. Figure 54: Squat growth data replotted, where y = squat size (mm), x = months 3.9. Fractal Dimension of a Crack Classical long-fatigue cracks tend to a fractal dimension D of 1. Short-fatigue cracks without closure effects tend to have D ~ 1.2. Long cracks as they approach final failure have values that are greater than ~ 1.5. To evaluate this, the fractal dimension of a gauge corner squat (or stud?) in a specimen provided by Railcorp, was measured at Monash University at the locations shown in Figure 55. CRC for Rail Innovation December 2011 Pag

64 Chapter 3 Crack Growth Opening of subsurface crack behind the wheel. Figure 55: Squat crack surface tested for fractal dimensions Table 4 Fractal Dimension Fractal dimension: D X (transverse) Y (axial) Considering that the wheel is rolling over points 1 to 3, exerting a high-contact pressure, it is surprising that the values are high. The values obtained support Steenbergen s theory that plastic deformation causes the loading of a squat crack to shift to beyond the crack mouth, at 1 in Figure 55, as the wheel rolls by, as otherwise the crack closure should smooth the crack. Other squat crack surfaces, like that in Figure 3(a) of Chapter 1, are significantly smoother and would be expected to have a lower fractal dimension Conclusions About Crack Growth The growth of squat cracks is poorly understood. A number of investigations with LEFM do not account for plastic flow causing the loading on the rail to change. Preliminary tests at Monash University (not reported here) have been conducted that show water in a crack does promote crack growth, even without the water being sealed in the crack. This is a surprising result that remains unexplained. An accurate simulation of how a squat crack grows has not been conducted. The LEFM analysis reported here indicates that fluid pressure in the crack is needed to explain its angle CRC for Rail Innovation December 2011 Pag

65 Chapter 3 Crack Growth of growth, if it grows in accordance with published estimates of crack growth angle, with the angle estimated from the moment of maximum effective stress intensity. This moment occurs while the crack is nominally closed, and it will only grow if the effective coefficient of friction in the crack is low enough. Water is needed in the crack to make this plausible, as well as to pressurise the crack. Without lubricating the crack, if it does close, it will only grow before or after the contact with the wheel, as assumed in the element removal algorithm. As this assumption also gives a plausible angle of growth (Figure 50), and perhaps the crack grows under pressure until it becomes too large to be pressurised by a wheel passage, and then grows without lubrication. This may help to explain why crack growth does not accelerate as the crack gets larger (Figure 52). However, initially a crack grows more on the plane of maximum shear than normal to the maximum tension, as it typically does not start growing normal to the surface of the rail. This shear failure can only continue if the range of K II is above the threshold for mode II growth, which is higher than that for tensile mode I growth. Water is still needed to lubricate the crack, to make it grow in shear. The contact stressing is very local to the region of contact, and water pressure helps transmit this stressing to the crack tip. Crack closure may not occur as much as an elastic analysis or a 2D elasto-plastic analysis indicates. Observation of squats (Figures 1e and Figure 1f of Chapter 1) indicates a loss of contact of the wheel with material above the crack. The measured rate of crack growth shows this does not accelerate much as the squat gets larger and in fact the exponent of the growth rate reduces. The slowdown probably reflects that fact that the loading of the crack changes as it grows. CRC for Rail Innovation December 2011 Pag

66 Chapter 4 Squat and WEL Detection 4. Squat and WEL Detection A number of technologies for detecting squats or for detecting WEL have been explored. Train-based systems of flaw detection in track use ultrasonics, eddy current and accelerometers, either axle-mounted or associated with a roller trailing on the track. Train-based systems are generally not tuned to squat detection, but are potentially capable of it. Other technologies such as thermography and lasers to measure height variation of the track, or the Barkhausen noise method to detect WEL have been considered. However, a number of other approaches are possible. For example, a squat is expected to have a distinctive acoustic noise signature that could be recognised by processing a signal from a microphone. Types of squat and WEL detection methods are discussed in this chapter Eddy Current Detection of White Etching Layers Eddy current detection of WEL is explored in detail by in [56] and is reported in a previous project progress report [32]. The depth of penetration of an eddy current detector depends on the frequency used. To detect a WEL, a small penetration depth associated with a high-frequency probe is needed, for example 1 MHz giving penetration of about 100 m. A difficulty with using eddy current for inspection at speed is the need to precisely control the height of the probe above the rail (or lift-off), as readings are sensitive to this parameter. Hand-held examination of rail specimens with eddy current has been very successful, with the martensitic layer being consistently confirmed by microscopic examination of the specimens tested. An Olympus Notec 500D inspection device was used with the NEC 2236/500K-2M/7L probe at the testing frequency of 1 MHz with a gain of 58 db. To detect WEL, the change in the eddy current reading had to exceed a threshold that was determined by testing samples, as illustrated in Figure 56. Eddy current readings are also affected by temperature, and this effect had to be calibrated. A WEL Figure 56: Typical eddy current readings from a rail with WEL. Dashed line is the WEL threshold CRC for Rail Innovation December 2011 Page 57

67 Chapter 4 Squat and WEL Detection 4.2. Eddy Current Detection of Squats In Germany [57], eddy current equipment used for identifying head checks can also detect squats. The signal for a squat has a characteristic two-peaked shape associated with the forward and reverse growing cracks, as illustrated in Figure 57. No WEL Figure 57: Eddy current readings for squats Where squats are initiated from head checks, the head-check signal makes the squat signal hard to detect, as the effects from both are superimposed Thermographic Detection of Squats Figure 58: Lock-in thermographic image of two closely spaced squats CRC for Rail Innovation December 2011 Page 58

68 Chapter 4 Squat and WEL Detection Lock-in thermography was tested at Monash University in two imaging modes (E mode and S mode) on a Railcorp rail containing a number of closely spaced, gauge-corner initiated squats, as reported in a previous project progress report 357]. S mode, or second harmonic, images like the greyscale image in Figure 58, gave the best resolution of subsurface cracking. This technique is, however, better suited to laboratory use than train-mounted testing Magnetic Barkhausen Noise Detection of WEL (White Etching Layers) P. Bellette Magnetic Barkhausen Noise (MBN) has a possible role in detecting WEL. MBN has been applied in industry for the non-destructive evaluation of microstructural changes and longitudinal residual stresses in ferromagnetic metals. It may be an option for detecting WEL. MBN also involves inducing eddy currents in the test specimen, but the eddy current is not measured. The typical setup of an MBN evaluation of a ferromagnetic sample is shown in Figure 59. A time dependent current is applied to the electromagnet. This magnetic field causes an induced magnetisation of the sample, which is detected by the pickup coil. 3 squats Ferromagnetic Sample Electromagn et Signal Generator Spectral Analysis High Pass Filter Figure 59: Schematic Layout of a MBN Detector Barkhausen Noise is the name given to the observation that the magnetisation of the sample does not progress continuously, but instead consists of discrete jumps in magnetisation [4]. Residual stress typically results in an increase in the Barkhausen Noise Intensity for tensile stresses and a reduction for compressive stresses. Increased hardness broadly results in a lower Barkhausen Noise intensity and correspondingly lower hardness a lower intensity. Therefore, careful calibration is needed to isolate the effect of either the residual stress or the hardness of the sample. The depth of the Barkhausen signal is limited by induced eddy currents attenuating the signal exponentially with depth and is typically limited to examination of the first millimetre of the sample. The WEL seen on the surface of rails is in the range of microns thick. The suspected martensitic change in the steel microstructure results in an increased hardness (approximately three times higher than the bulk material) and may also result in compressive residual stresses due to an increase in volume associated with the Martensite. Both of these changes should reduce in a lower intensity MBN signal compared with an untransformed rail surface. MBN should not be influenced by the presence of cracks, unlike the eddy current method. However, correcting for elastic stresses may require careful calibration. MBN has also been applied in rail, most notably in finding the neutral stress temperature of inservice continuously welded rail. Commercial applications include RAIL SCAN and Thermit TRACKSAFE. CRC for Rail Innovation December 2011 Page 59

69 Chapter 4 Squat and WEL Detection The main difficulties of the MBN techniques is calibrating for the various factors that may make detection difficult, such as lift off, surface roughness, temperature and correcting for residual stresses. Due to the operation principle, very high speed detection is not possible; however, reasonably high speed detection is used in sheet metal applications (300m/min = 18kph) Acceleration measurements Accelerometer readings are routinely used to detect flaws in track. However, more can be done to interpret the signals to identify what type of flaw. Figure 60 illustrates the work of ProRail [5] who are using the way the frequency content of an accelerometer signal changes along the track to identify particular flaws by their characteristic signature in frequency versus their position (a wavelet spectrum). Figure 60 shows the wavelet spectrum of a squat, with distinctive features highlighted. Since acceleration signals, like that on the left of Figure 60, are already being obtained. This is a promising approach for development in Australia, as the software to generate the wavelet spectrum is available, so the research should be task to automatically identify features in the spectrum, and to find cracks before they become too severe to be removed by grinding. Figure 60: Wavelet spectrum of vertical axle box acceleration due to a squat 4.6. Conclusions Given the current signal processing technology, a lot more can be done to interpret signals from sensors such as accelerometers. Automatically recognising squats from train-based acceleration measurements is possible and is a technology that could be developed in Australia. To prevent squat problems developing, early detection is needed; detection from acceleration measurements may miss really small cracks. Systems that detect corrugation could also be developed to identify squats. Acoustic detection of squats is also possible because they produce a characteristic sound spectrum in rolling noise. Recognising WEL is probably most easily undertaken using eddy current measurements. A 1 MHz frequency probe has been reliably used to find WEL in manual inspections. Automating these inspections is possible by modifying similar equipment intended for crack inspection. CRC for Rail Innovation December 2011 Page 60

70 Final report on the rail squat project R3-105 Chapter 5 Open Questions and Findings 5. Open Questions and Findings At a squat workshop in Leoben, Austria in September 2011, the state of understanding of three key questions was evaluated [60]. These questions were: 1. Squat initiation. What is necessary for squats to develop in a rail network? 2. Squat detection. How can squats be detected at an early stage? 3. Maintenance. What maintenance measures have proven to be most effective to manage the squat problem? 5.1. Causes of Squat Initiation and Growth The answers given by delegates to Question 1 were: a. the presence of wheel slip (braking systems, traction system, changing friction) b. the presence of WEL and/or friction-induced martensite c. a low-wear regime, harder steel types vs geometric conditions vs. axle load d. surface- breaking defects (indentations, head checks, wheel burn) e. high tangential surface stresses. Open questions about initiation included: a. the influence of track form (slab track, ballast) b. the influence of suspension characteristics c. whether using bainitic rails would fix the problem d. whether to use hard vs soft pearlitic rail e. the potential of wheel rail conditioning and friction management f. does the anti headcheck profile used in Europe on tangent track cause squats instead of headchecks? There is evidence of squats growing initially along the lamellae of the distorted pearlite microstructure under the surface of the rail. The microstructure of bainite does not have such preferred failure planes, which could be advantageous. Many of these factors promote squat formation. Stock of Voestalpine represented this as Figure 61. Figure 61: Factors combining to enable squats to occur on track CRC for Rail Innovation December 2011 Page 61

71 Final report on the rail squat project R3-105 Chapter 5 Open Questions and Findings Stiff bogies have been suspected in Europe of making the squat problem worse, by putting higher dynamic loads on the track. Friction management probably can avoid the squat problem, but the practicality of implementing this is debatable. Softer rails may achieve a more appropriate wear regime. Policies on the use of head hardened rails in curves are being revised, with Denmark now using head hardened rail in curves below 500 m R rather than below 1000 m radius Squat Detection The answers given by delegates to Question 1 were a. measurement of longitudinal profile (e.g., axlebox acceleration, measurement during grinding) b. eddy current (no depth information) c. track walk inspection d. noise. Other approaches to squat detection are also possible, as discussed in Chapter 4. Improving processing of axlebox accelerometer readings to identify types of flaws is a practical option that could be developed by further research in Australia. Technologies could be combined, for example axlebox acceleration plus eddy current measurements to gain more confidence that a flaw is correctly identified. It is also possible to automate visual inspection by processing images of the railhead to recognise squats, or dips in the longitudinal profile of a rail could be detected by laser. Detection of WEL was not discussed a lot at the workshop, but is possible to use an eddy current probe at a higher frequency than that already used to find headchecks (e.g., 1 MHz) Maintenance The lack of squats reported in the US, except by Magel [2], suggests that preventative grinding in use in the US must help avoid squat formation. The workshop discussion raised the following open questions about maintenance: a. wheel/rail profile management (e.g., width of running band) b. calibration/limitation of traction system c. grinding at higher speeds (during normal traffic) d. friction management e. surface condition after grinding. The behaviour of traction control systems when they encounter a sudden change in traction is still being understood, and modification of traction control is likely in the long term, when the case is made that traction control systems are damaging the track. There is evidence that squats may develop from flaws left after grinding. The image in Figure 62 from Railcorp, shows the high rail of a moderate curve and suggests a previous grinding may have initiated the squats. Squats aligned with periodic grinding marks have also been observed in Denmark [60], as shown in Figure 63. CRC for Rail Innovation December 2011 Page 62

72 Final report on the rail squat project R3-105 Chapter 5 Open Questions and Findings Figure 62: Squats initiating near the gauge corner of a high rail (Railcorp), Sydney. The arrows mark the positions of small squat cracks in synchronisation, but not aligned to the pitch of the most recent grinding (the white marks). Figure 63: Periodic squat damage associated with grinding marks in two rails from Banedanmark, Denmark Removing squats requires milling up to 8 mm off the head of the rail. Squats reappear if they are not fully removed. Measuring how deep a squat is before grinding becomes an issue requiring more elaborate ultrasonic inspection than is currently undertaken Findings More testing and more modelling is being undertaken internationally to understand squats better. More research can be done in Australia as well. The exact conditions of temperature and pressure under which a WEL can form on the surface a rail need to be clarified. An improved test needs to be conducted as discussed in Section WEL detection by eddy current can be developed into a routine inspection tool. A convincing explanation of how squats grow is yet to be achieved. A model of squat growth incorporating plastic deformation in 3D and using continuum damage mechanics rather than linear CRC for Rail Innovation December 2011 Page 63