PSI - Issue 39

Georg Schnalzger et al. / Procedia Structural Integrity 39 (2022) 313–326 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Rolling Contact Fatigue (RCF) is a degradation processes, which drives besides wear, rail maintenance and replacement (Grossoni et al. (2021), Magel et al. (2016)). In the field of railway research, various types of RCF defects are distinguished differing in the crack initiation location as well as the failure pattern and mechanism. Squats, initiating below the rail surface, and head checks propagating down into the rail from the surface, are two prominent RCF failure types relevant for the current work (Grossoni et al. (2021)). The main factor differentiating fatigue in rails from most engineering components is the repeated highly compressive rolling-sliding contact loading (Fletcher et al. (2009)). The surfaces are subjected to a combination of rolling and sliding motion relative to one another mitigating crack propagation in the opening Mode-I and promoting other mechanisms (Fletcher et al. (2009)). In the compressive stress regime of the wheel-rail contact, cracks are assumed at first to propagate by the sliding of the crack faces relative to each other due to a cyclic shear stress component (Mode-II), see Fig. 1 (Fletcher et al. (2009)). Second, fluids (e.g., rain water) can promote the growth of surface cracks, such as head checks, through a hydraulic mechanism (Fletcher et al. (2009)). The current work here is limited to the Mode-II propagation mechanism. The interested reader is referred to Fletcher et al. (2009) for details on fluid-assisted mechanisms.

Fig. 1. Shear crack growth of a surface crack (e.g., head check) shown in a cross-sectional view of a rail containing a crack, which is subjected to contact pressure p ( x ) and tangential traction q ( x ) by a passing wheel due to the rolling-sliding contact. Adapted from Fletcher et al. (2009).

The complexity of the RCF failure makes systematic investigations on rails in service difficult, where many different vehicle types and wheel profiles combinations exist (Fletcher et al. (2009)). Therefore, different laboratory tests have been developed to evaluate the RCF failure under well-defined conditions. In many cases, the wheel-rail contact is simplified by using a twin-disc system to produce rolling-sliding loading conditions on the laboratory scale, see for example Kammerhofer et al. (2014), Santa et al. (2019). In addition, full-scale test rigs have been realized such as presented in Stock and Pippan (2011). However, the primary focus of these studies lies on the initiation and early growth phase of RCF cracks as well as a semi-quantitative evaluation of crack growth. More accurate crack growth rates and laws are determined in laboratory Fatigue Crack Growth (FCG) experiments using well-defined fracture mechanics specimens and loading sequences. However, the shear crack mechanism is particularly difficult to observe in controlled experimental conditions because the cyclic shear loading rapidly leads to crack branch formation or bifurcations resulting in a mixed-mode propagation, see (Bold et al. (1992), Vojtek et al. (2013), Vojtek et al. (2017)). Bold et al. (1991) demonstrated that coplanar shear-driven growth is possible under the type of sequential mixed mode loading experienced in the railhead using a cruciform four-point bending specimen. Several further researchers investigated FCG in railway materials under Mode-II as well as mixed-mode I/II loading conditions. Wong et al. (2000) performed experiments to produce coplanar cracks and established empirical crack propagation laws. Akama et al. conducted FCG tests on rail and wheel steels to estimate the coplanar and branch crack growth rates and analyzed stress intensity factors for RCF cracks using the boundary element method (Akama (2019), Akama and Mori (2002)). Doquet and Pommier (2004) used tubular specimens as well as compact-tension shear specimens for Mode-II FCG tests as well as tests in sequential Mode-I and then Mode-II on ferritic-pearlitic rail steel. Bonniot et al. (2020) investigated FCG in R260 rail steel under non-proportional mixed-mode I/II loading and explored the role of compression while shearing on crack branching using tubular specimens and a Finite Element Analysis (FEA). Otsuka et al. (1996) developed a test apparatus to investigate FCG under cyclic Mode-II loading with a static compressive stress superimposed parallel to the crack plane to suppress out-of-plane crack growth in the tensile mode.