MARAXIL Project Effect of cold-rolling onto axle life prediction

Transcription

1 ESIS TC24 Workshop 30 March 2012 Milano, Italy Progetto cofinanziato da Regione Lombardia tramite il "Fondo per la promozione di Accordi istituzionali", attraverso il quale il progetto Maraxil è stato cofinanziato nell'ambito del Bando Cooperazione Scientifica e Tecnologica. MARAXIL Project Effect of cold-rolling onto axle life prediction M. Carboni

2 Introduction 2 Cold-rolling of railway axles could be a good strategy to improve safety and diminish costs by relaxing inspection intervals The main aim of the Maraxil Project is to investigate different aspects of this manufacturing technology applied to axles in order to analyse its feasibility From this point of view, the topic of this presentations are: residual stress measurements by XRD and HD FE modelling of cold-rolling estimation of the effects of cold-rolling on propagation lifetime of railway axles

3 Investigated axle Depth probing of residual stress

4 Measurement strategy D C B A Cutting of the axle is needed for depth probing of residual stresses Effect of cutting is controlled by XRD measurements before and after cutting: 1) XRD measurements at position "C and B, 0, 120, 240 2) cut the axle at position "D" 3) repeat XRD measurements (0, 120 ) at position "C" to see influence of cutting 4) depth probing of RS using X-ray diffraction (XRD) and incremental center hole drilling (HD): Position C only, sequence: - perform XRD + CHD (drilling depth 0.5 mm) - remove approx. 0.4 mm by turning - electro-chemically remove additional 0.2 mm (eliminate turning-rs) repeat sequence

5 Equipment XRD: STRESSTECH G2 Cr-radiation penetration depth 5 µm sin 2 y-methode HD: MTU, modified high speed turbine (60,000 rpm) hole diameter 0.9 mm differential calculation method (MPA Stuttgart) Material removal in workshop

6 Spannung [MPa] Typical results XRD: sin 2 y-distribution of lattice spacing standard deviation of measurements typically better 10 MPa Transverse to turning direction: linear sin 2 y-distribution due to normal stress HD (+ XRD at surface): 0 Messstelle C Oberfläche axial hoop XRD HD results near surface skipped due to insufficient sensitivity Parallel to turning direction: y-splitting due to shear stress HD results are only reliable up to 60% yield strength! Bohrtiefe [mm]

7 Comparison for sections B and C Remarks: - significant different surface RS at C and B - no differences at positions 0, 120, no influence of cutting on RS

8 Comparison of XRD and HD ** * *** Remarks: * HD results are only reliable up to 60% yield strength ** Below 2 mm no significant compressive RS exist *** Estimated depth profile of RS

9 Further residual stresses measurements The same measurements were carried out at PoliMi (XRD) and Lucchini (HD) on two twin cold-rolled axles with the same geometry of the one measured at IWM 200 Residual Stress (MPa) Direction 90 o Direction 45 o Direction 0 o Second Principal Third Principal Distance along axis (mm) Residual Stress (MPa) Depth (mm) Direction 0 o Direction 45 o Direction 90 o Second Principal Stress Third Principal Stress Results seem to be consistent between the laboratories

10 Finite element simulation of cold-rolling 10 A 3D explicit finite element model was build using ABAQUS In order to avoid a big unnecessary model, a 15 sector having the length of 50 mm was considered and meshed The longitudinal movement of roller was considered by applying 16 roller which were initially displaced by the amount of feed. At the beginning the first roller pushed the axle and rolled its surface. Then the same was repeated by other rollers

11 Finite element simulation of cold-rolling 11 Material Behavior (A4T): Roller: analytical rigid body was considered for the rollers Axle: monotonic and cyclic behaviors of the material were measured and applied to the simulation Residual Stress (MPa) Monotonic Cyclic Equivalent Plastic Strain (mm/mm)

12 Finite element results 12 von Mises stress distribution around the produced groove during rolling

13 Finite element results 13 Longitudinal stress distribution after rolling A uniform state of residual stress was generated at the treated surface

14 Finite element results Residual Stress (MPa) Longitudinal (XRD) Circumferential (XRD) Longitudinal (FE) Circumferential (FE) Depth (mm)

15 A1N Kiruna axle Load spectrum on the T-transition of the wheel seat (section C), representing 500 km of the typical service

16 A1N crack growth behaviour Crack growth predictions are based on the A1N NASGRO equations derived from the experimental tests, considering both CPLR and DKdecreasing techniques for deriving threshold SIFs DK th dependency on the stress ratio R NASGRO fitting of crack growth rate data

17 A4T Maraxil axle Predictions considered the same geometry of the axle used for residual stress measurements The fictitious crack is placed in the T- transition of the wheel The adopted load spectrum is the same already applied to the Kiruna axle

18 A4T crack growth behaviour Crack growth predictions are based on the A4T NASGRO equations derived from the experimental tests, considering both CPLR and DK-decreasing techniques for deriving threshold SIFs DK th dependency on the stress ratio R NASGRO fitting of crack growth rate data

19 Stress [Mpa] Effect of cold-rolling on crack initiation Cold rolled axles - Longitudinal stress Adopted approximation of experimental results Depth [mm] The effect of cold-rolling vanishes at about 2 mm depth As a function of crack depth, the right figure shows the amplitude of the stresses needed to reach the propagation threshold The minimum required stress value depends on the crack size and on the experimental threshold SIF

20 A1N predictions for the Kiruna axle Cold-rolling shows a big influence onto life predictions for A1N axles A crack of initial depth a 0 =2 mm is not able to propagate in presence of cold-rolling residual stresses The axle, applying coldrolling, has an estimated life of more than one million km even with a crack of initial depth a 0 =4 mm

21 A4T predictions for the Maraxil axle The same conclusions drawn for A1N axles can also be drawn for A4T ones

22 POD curves and inspection intervals The effect of cold-rolling on the performance of NDT for axles (i.e. on the POD curve of UT) is being analyzed The important consideration to be remarked is that the crack growth predictions shown in the previous slides seem to suggest the possibility to adopt a one million miles axle concept for cold-rolled axles In order to check the validity of this assumption, full-scale fatigue tests are being successfully carried out on cold-rolled axles It is also worth noting that the definition of the inspection interval is dependent on a number of factors together with the crack growth behavior of the material: the Maraxil project allowed to successfully study most of these relevant factors for cold-rolled axles