Fatigue analysis crack propagation

Brocklehurst [37] has written an exhaustive review of the early work (prior to 1977) on fracture in polycrystalline graphite. Much of this work focused on the fracture behavior of nuclear graphites. In most investigations considered, conventional fracture mechanics tests and analysis were performed for macroscopic cracks. LEFM provided an adequate criterion for failure. Additionally, results on work of fracture, strain energy release rate, and fatigue crack propagation were reported. 

The second part of the investigation was directed toward the understanding of the crack propagation stages. Two questions were especially addressed (i) the determination of the transition from fatigue to brittle fracture (ii) the determination of the crack arrest conditions. Within the frame of linear elastic fracture mechanics, such an analysis requires the... 

Fatigue failure in neat polymers or short fibre reinforced polymers involves a phase in which a defect zone initiates, with the development of crazes or microcracks, followed by crack propagation to final rupture [17], This statement means that an analysis of fatigue behaviour can use the tools already developed for metals. This will be demonstrated in the cases that follow. 

Borduas, H.F, Culver, L.E. and Bums, D.J. (1968) Fracture-mechanics analysis of fatigue-crack propagation in polymethylmethacrylate. /. Strain Anal., 3, 193. 

Work by Schapery, Saxena, Wilhams, and others details the analysis of cracks in creeping, strain rate dependent materials, and provides a predictive basis for the apparently brittle nature of FCP in UHMWPE [43-48]. Particularly usefid are the models developed by Schapery and Wilhams, which directly link the intrinsic, constitutive viscoelastic relaxation behavior of the material to the advance of a stable crack tip [46,48]. The power of these models is the predictive nature of the mechanics in relating ECP dynamics to the material s viscoelastic behavior that is eashy measured in a simple one-dimensional creep test The elementary consequences of the models result in the static mode fatigue crack propagation behavior that is observed in UHMWPE, and thus potentially provide a first-principles explanation of the fatigue and fracture behavior by the material. 

It is no longer neeessary to demonstrate the influence of oil contamination on Rolling Contact Fatigue (RCF). Previous studies have shown the consequences induced by the travel of solid particles inside the contacts. In fact, when particles enter the contact, they indent the surfaces. The resulting surface defects, called dents, become stress raisers and increase the risk of spalling and consequently of RCF. In order to compare numerical and experimental results, some tests were performed on a two disk machine with artificially dented surfaces. Cross-sections were performed to complete the surface observations, and to analyse the crack initiation sites and the crack propagation directions. This paper proposes an analysis of these cross-sections to identify the cracks that lead to micro and macro spalling. 

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