PSI - Issue 13

Shigeru Hamada et al. / Procedia Structural Integrity 13 (2018) 1026–1031 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction

Rolling contact fatigue (RCF) occurs in mechanical parts subject to cyclic rolling contact. The essence of RCF is the initiation and propagation of fatigue cracks, via “Mode II fatigue crack propagation.” Many Mode II fatigue crack propagation test methods have been proposed and studied. However, its understanding has not progressed as much as that of Mode I. Therefore, the current design methods to avoid RCF are insufficient. The authors think that this cause is the name: “Mode II fatigue crack propagation.” Originally, the labels “ Mode I ” and “Mode II ” were assigned to the loading form (mode) for a still crack in fracture mechanics by Irwin (1958), and do not represent the fatigue crack propagation mechanism. However, there are many cases where the knowledge of fatigue crack propagation under Mode I loading is applied to that under Mode II loading without consideration. For example, a study by one of the authors used d a /d N - Δ K II relations for organizing crack propagation under Mode II loading (Murakami et al. 1997). These relations were used without considering whether (1) Δ K II governs the fatigue crack propagation or not, (2) d a /d N : the fatigue crack propagates once with one loading or not. Before organizing the relation, we have to discuss the applicability of the d a /d N - Δ K II relations. On the other hand, in the RCF region where the fatigue crack propagates under Mode II loading, large plastic deformation is caused by the rolling contact load before fatigue crack propagation. Therefore, it is necessary for the test method to reproduce the effects of the actual machine to test a material possessing large plastic deformation. Therefore, in this study, we aim to classify the fatigue crack propagation phenomena, regardless of Mode I and II loadings, and re-examine the mechanisms. The conditions of the fatigue test method to examine these phenomena were first established, for which we developed a novel test method that enables pure Mode II loading and satisfies the conditions mentioned before. We used a micro-thin film disc as a specimen, making it possible to cut out and test a part subjected to a large plastic deformation from an actual machine. Thus, by the fatigue crack propagation test and results, we proposed a novel crack propagation mechanism, namely, damage accumulation type fatigue crack propagation mechanism under Mode II loading, which is different from the opening type fatigue crack propagation. 2. Classification of fatigue crack propagation mechanism Fatigue crack propagation is complicated; therefore, to express it, approaches using mechanical approximation have been developed. Figure 1 shows methods for fatigue crack propagation under Mode I loading. One method employs dislocation mechanics, which is a material engineering method. This method is close to reality, because the main fatigue crack propagation mechanism under Mode I loading is cyclic plastic deformation due to dislocations emission from the crack tip. The other method employs continuum mechanics, which is a mechanical engineering method. This method is advantageous over the method employing dislocation mechanics in that quantification is possible. In this study, we use the advantages of both these methods to understand fatigue crack propagation under Mode II loading and clarify its mechanism, which has not been elucidated thus far. Fatigue crack propagation encompasses multiple phenomena. Here, we will focus on the phenomena at the fatigue crack tip and near the fatigue crack tip. 2.1. Phenomena at the crack tip The phenomenon that occurs at the fatigue crack tip is caused by dislocation, owing to Mode I loading. The fatigue crack propagates due to coarse slip (Neumann 1969). Figure 2 shows the schematics; clearly, fatigue crack propagation may occur under both Mode I and II loadings. If there is no particular ingenuity, a phenomenon that occurs when a Mode II loading is applied to a crack is this phenomenon. Hereafter we term this phenomenon “coarse slip type fatigue crack growth” (CSFCG). 2.2. Phenomena near the crack tip Damage accumulation typically occurs near fatigue cracks. Specifically, dislocations are emitted from a Frank – Read source, leading to two possibilities. In one possible phenomenon, dislocations accumulate, substructures form, and voids initiate and coalesce to form a crack as shown in Fig. 3; hence, in this case, fatigue cracks propagate with repeated occurrence of this phenomenon. The first half of this phenomenon exactly constitutes the initiation of a fatigue crack. Therefore, here, the cracks are