A Study of Microstructure-Sensitive Crack Nucleation and Wear in Fretting
McCarthy, Oliver J.
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This thesis presents the development of a computational and experimental methodology for predicting crack initiation and wear in fretting of engineering materials. A bridge-type fretting fatigue test rig is designed and developed to characterise the fretting fatigue behaviour of 316L stainless steel (SS). The plain and fretting fatigue testing of the material demonstrated a significant reduction in fatigue life due to fretting, as well as a significant effect of normal load. A cylinder-on-flat crystal plasticity (CP) finite element (FE) fretting model is developed to simulate the experimental test. A J2 cyclic plasticity model of the fretting test rig is also developed for comparative purposes. The CP, based on a UMAT user subroutine, and J2 material constants are calibrated against published cyclic plasticity data for 316L SS. A microstructure-sensitive crack initiation prediction methodology is implemented based on accumulated crystallographic plastic slip as a fatigue indicator parameter (FIP). This accumulated plastic slip parameter is calibrated against the experimental plain fatigue crack initiation data for subsequent application to fretting fatigue crack initiation. Significant non-uniform (inhomogeneous) micro-plasticity effects are predicted by the microstructure-sensitive model due to the representation of inhomogeneous material behaviour at the micro-scale. A mixed mode short crack growth propagation methodology is implemented for the CP model. The CP predicted total fretting fatigue lives are shown to be in agreement with the experimental data and, more specifically, give improved predictions over a critical-plane Smith Watson Topper (SWT) approach for the J2 model. An experimental fretting life reduction factor due to fretting is reasonably well captured by the CP model. Furthermore, a novel fretting wear prediction methodology was developed using the microstructure-sensitive crack initiation FIP and the CP model. This method was shown to give reasonable correlation to the measured wear scars and published wear coefficients. A CPFE model of the cylinder-on-flat fretting wear test rig at the University of Nottingham was developed. CP dual phase and single phase material models were developed and implemented for Ti-6Al-4V within a UMAT user subroutine. Published experimental data based on a laboratory fretting wear test rig is used to validate the microstructure-sensitive prediction methodology. The constitutive behaviour of individual phases of the Ti-6Al-4V material are calibrated against experimental cyclic stress-strain curves (CSSC) from the literature to obtain phase specific CP material constants. The accumulated crystallographic slip parameter is shown to capture the low cycle fatigue (LCF) response of the material vis-à-vis the published Coffin-Manson relationship. A crystallographic random orientation study demonstrated the ability of the methodology to generate scatter in fatigue life for plain fatigue and fretting situations, akin to measured experimental fatigue scatter. The microstructure-sensitive prediction methodology successfully captured crack initiation lives, crack locations and orientations, when compared to the published experimental data from the University of Nottingham. The microstructure-sensitive fretting damage methodology based on accumulated plastic slip is implemented to distinguish between fretting wear and cracking. This technique was used to predict wear scar profiles which gave reasonable correlation with experimental measurements, and also allowed for the estimation of a wear coefficient for Ti-6Al- 4V, that is consistent with the previously published values for the same test rig and Ti-6Al-4V material.
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