Fatigue Crack Growth Study in Polymer Matrix Composites

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1 Fatigue Crack Growth Study in Polymer Matrix Composites Arunakumara P C 1,P.Dinesh 2 1 Department of Mechanical Engneering, M.S.Ramaiah Institute of Technology, Bangalore Department of Mechanical Engneering, M.S.Ramaiah Institute of Technology, Bangalore Abstract The fatigue crack growth threshold defines the stress intensity level, ΔK, below which a crack will not propagate. Recent research has shown some of the threshold data generated based on ISO and ASTM testing standards has some limitations, influencing the data in unforeseen ways. One of these limitations refers to the steady-state condition of threshold value. In the present study in view of the limitation mentioned, an attempt has been made to determine the fatigue threshold value by regression analysis. In the present investigation polymer matrix composites(pmc s) with varying reinforcement mass fraction are fabricated using injection moulding technique,specimens were prepared and tested for fatigue crack growth (FCG) as per ASTM E-647 standards.servohydraulic controlled fatigue crack growth testing machine was employed for load ratio of 0.1 and frequency of 30Hz at constant amplitude cycle. The experimental results show an improvement in crack resistance for 40% fibre mass fraction compared to 20% and 30% mass fractions, this is mainly due to increase in fibre percentage hinders the crack growth under high cycle fatigue loading. Keywords ISO, ASTM, PMC, FCG. I. INTRODUCTION The fatigue crack growth threshold defines the stress intensity level, ΔK, where a crack will arrest or begin to propagate. The threshold is used to define a durability lifetime (or safe operating time) for a component, the same way as an endurance limit is used in stress-life based design methods. Therefore, accurate threshold data is critical in durability based designs. The development of fatigue crack growth threshold data is based on ISO and ASTM standards. The standards outline experimental procedure, specimen geometry and crack configurations along with tolerances on dimensions and operating parameters. Recent research into the fatigue crack growth threshold has exposed some limitations in the standards that could affect the data in unforeseen ways [1-7]. One of these limitations is the development, or lack thereof, of the steady-state condition.several researchers have proposed using constant ΔK testing to define steady-state conditions [8, 9]. Constant ΔK testing holds the driving force constant, which in turn develops the steady-state plasticity condition. If the test is conducted long enough, i.e. there is significant crack growth, the steady-state roughness condition will develop. If the environment where the test is being conducted is also controlled, then steady-state environmental conditions will develop, and subsequently steadystate cracking will exist [10]. Pippan, et al. [11] and James, et al. [12] have proposed using compression precracking to generate a sharp crack at a notch that can then be propagated at nearthreshold levels. The objective of this paper is to study the influence of heat treatment on fatigue threshold of various heat treated EN8 steel. II.THE FATIGUE RATE CURVE It is a crack growth rate (da/dn) versus stress intensity range ( K) curve as shown in Fig.1. The curve is often divided into three regions; Region I, Region II and Region III. In region I the early development of the fatigue crack represented, and the growth rate is in the order 10-6 mm/cycle or below. This region is very sensitive to micro structure features like grain size, the means stress of the applied load, the environment and the operating temperature. The most important feature in this region is the FCG threshold, Kth. This is the limit for the propagation of fatigue cracks start. All Rights Reserved 284

2 SIF ranges below the Kth crack growth will usually not occur.region II is the intermediate zone for growth rates in the order of 10-6 to 10-3 mm/cycle. In this region the crack growth is stable, the data is following a power equation and the plastic zone in front of the crack tip is large compared to the mean grain size. The pari s equation for region II of the curve is given by da/dn=cδk m Where C and m are material parameters, K is stress intensity range. Since the data is following a power equation, the fit will be linear on a log-log plot and the use of LEFM concepts is applicable. For region II the mean stress has the highest influence on the results, but the influence is small compared with region I. Fig. 1: Fatigue Rate Curve The last region, region III, starts where the curve again become steep, and is usually in the order 10-3 mm/cycle and above. This is high crack growth rates caused by the rapid unstable growth prior to final failure. The curve approaches in this region an asymptote corresponding to the fracture toughness, KIC, for the material. The influence of nonlinear properties cannot be ignored in this region due the present of large scale yielding. This result in that LEFM cannot be used for the data in this region and nonlinear fracture mechanics should be applied instead. FCG analysis for this region is therefore very complex and as the FCG rates are high and little fatigue life involved. This result often in that analysis for this region is ignored. III.EXPERIMENTAL DETAILS OF FATIGUE CRACK GROWTH TEST (FCG) 3.1 MATERIAL SELECTION In the present study polymer Matrix composites with varying mass fraction of glass fibers was selected for the study as this material finds wide applications in aircraft and automobile industries. The chemical composition of PMC s with varying mass fraction is shown in table I. Table I: Chemical Composition of PMC s Material Polypropylene(Matrix) Glass(Fibre) Aluminum Wax gel Oxide(Al203) Trial 1 75% 20% 2% 3% Trial 2 65% 30% 2% 35 Trial 3 55% 40% 2% All Rights Reserved 285

3 3.2 SPECIMEN PREPARATION Fatigue crack growth tests characterize the rate at which pre-existing cracks in metals, ceramics, composites and other materials propagate as a function of a cyclic driving force, most commonly the range of applied stress intensity. Fatigue crack growth tests generate data that can be used as a basis for setting inspection intervals in components designed for damage tolerance. Specimens as per ASTM E-647[13] were prepared with 2.5 mm thickness as shown in fig.2. Fig.2: Specifications of FCG Test Specimen as Per ASTM E 647 Standards III.FATIGUE CRACK GROWTH TEST Fatigue crack growth tests were carried out using servo hydraulic controlled fatigue crack growth testing machine of 25 KN capacity as per ASTM E-647 standards. FCG tests were carried out at a loading frequency of 30 Hz, Load ratio of 0.1 for maximum load of 2 KN, minimum load of 0.2 KN with Constant amplitude cycle. Fig. 4 shows the FCG test setup. Fig.3: Specimen Loaded on the Servo Hydraulic Controlled Fatigue Crack Growth Testing System IV.RESULTS AND DISCUSSIONS Tests with a constant load ratio, R = 0.1 were performed at room temperature in accordance with ASTM E-647 standard using compact tension specimens, to determine the threshold value. The test systems were calibrated to meet or exceed the requirements of ASTM E647.The displacement gages, strain gages and signal conditioners were calibrated to assure linearity in the operating regime. All testing was conducted under K-control with all crack length measurements verified using travelling microscope of the machine. The visual measurements were used to correct the compliance-based crack length values prior to data reporting per ASTM All Rights Reserved 286

4 Fig.4: Crack growth rate (da/dn) V/S Stress intensity range ( K) for PMC with 30% Glass Fibre Fig.5: Crack growth rate (da/dn) V/S Stress intensity range ( K) for PMC with 20% Glass Fibre Fig.6: Crack growth rate (da/dn) V/S Stress intensity range ( K) for PMC with 40% Glass Fibre Figures 4,5 and 6 show the curves of fatigue crack growth vs stress intensity range for PMC s specimens with 20%,30% and 40% glass fibres respectively.from the above graphs it is clear that stress intensity range varies linearly with crack growth rate in the region II of fatigue crack growth curve. The curve fitting process fits equations of approximating curves to the raw field All Rights Reserved 287

5 Nevertheless, for a given set of data, the fitting curves of a given type are generally not unique. Thus, a curve with a minimal deviation from all data points is desired. This best-fitting curve can be obtained by the method of least squares. R 2 is a goodness of fit of linear regression, which ranges between 0.0 and 1.0. The linear curve intersecting the region II of the fatigue crack growth curve is used to find the fatigue threshold and fracture toughness. The lower growth rate region is termed the threshold regime. The higher growth rate regime is where values of maximum stress intensity in the fatigue cycle are tending towards the fracture toughness and static modes of fracture (cleavage, intergranular) are adding to the fatigue induced growth rates. It was observed that specimen with 40% glass fibre pocesses good threshold value compared to specimens with 20% and 30% glass fibre. specimen with 20% glass fibre showd poor threshold and fracture toughness values.it is mainly due to decrease in resistance possesed by the PMC s.table 2 shows the values of fatigue threshold and fracture toughness for various PMC s. Table II: Fatigue Threshold and Fracture Toughness Values Material PMC with 20% fiber mass fraction PMC with 30% fiber mass fraction PMC with 40% fiber mass fraction Regression value R 2 Threshold (MPa m) KTh Fracture Toughness(MPa m) Kc Table II shows the fatigue threshold and fracture toughness values for PMC s with varying mass fractions of glass fibres.from the table it is clear that fatigue threshold fracture toughness value increases with the increase in fibre mass fractions.it is mainly due to the resistance offered by the glass fibres for the crack growth.pmc with 30% glass fibre offered moderate values of fracture toughness and threshold values. Fig.7: Number of cycles to failure for PMC s with varying mass fractions Fig.7 shows the number of fatigue cycles withstood by the PMC s under constant amplitude fatigue cycles.pmc with 30% glass fibre processed good resistance for crack growth, hence it withstood maximum number of fatigue cycles ie cycles.pms with 20% glass fibre withstood only cycles as the resistance to crack growth is All Rights Reserved 288

6 V.CONCLUSION PMC s specimens were fabricated with 20%,30% and 40% glass fiber mass fractions as per ASTM E-647 standards.pmc with 40% glass fibre specimen possesses higher value of fracture toughness due to the resistance offered by the glass fibre for the crack growth. PMC specimen with 20%glass fiber possess poor fracture toughness due to poor resistance offered by the matrix for the crack growth under fatigue loading. From the investigation it is clear that fatigue life of PMC s can be increased with the increase in mass fraction of glass fiber. REFERENCES 1. Tabernig, B., Powell, P., and Pippan, R., Fatigue Crack Growth Thresholds, Endurance Limits, and Design, ASTM Press, , Newman, J.C., Jr., Fatigue Crack Growth Thresholds, Endurance Limits, and Design, ASTM Press, , 2000, 3. Smith, S.W. and R.S. Piascik, Fatigue Crack Growth Thresholds, Endurance Limits, and Design, ASTM Press, , Liknes, H.O. and Stephens, R.R., Fatigue Crack Growth Thresholds, Endurance Limits, and Design, ASTM Press, , Garr, K.R. and Hresko, G.C., Fatigue Crack Growth Thresholds, Endurance Limits, and Design, ASTM Press, , Forth, S.C., J.C. Newman, Jr. and Forman, R.G., International Journal of Fatigue, vol. 25, 9-15, Leevers, P.S. and Radon, J.C., International Journal of Fracture, vol. 19, , Dawson, D.B. and Pelloux, R.M., Metall. Trans., vol. 5, , Hudak, Jr., S.J. and Wei, R.P., Int. Journal of Pressure Vessels and Piping, vol. 9, 63-74, Gangloff, R.P., Environmentally-Induced Cracking of Metals, NACE-10, , Pippan, R., Stuwe, H.P. and Golos, K., International Journal of Fatigue, vol. 16, , James, M. A., Forth, S. C., and Newman, J. A., Fatigue and Fracture Mechanics: 34 th Volume, ASTM Press, ASTM Standard E647, Standard Test Method for Measurement of Fatigue Crack Growth Rates, Annual Book of ASTM Standards, Vol , ASM Metal hand Book of Heat treatment, All Rights Reserved 289