Long co-planar mode III fatigue crack growth under non-proportional mixed mode loading in rail steel

Abstract

Fatigue tests have been performed to obtain the co-planar crack growth rate in rail steel under non-proportional mixed mode I and mode III cycles. In addition, a finite element analysis (FEA) has been performed to investigate the crack growth behaviour under this loading. The experiments showed that a long co-planar crack could be produced under this loading. Based on fracture surface observations obtained using a scanning electron microscope and FEA, the long coplanar crack growth was thought to be driven mainly by mode III loading and the role of mode I was as an assistant; keeping the crack face open. It was also observed that the crack tended to branch when the degree of overlap between the mode I and mode III cycles increased. Comparing the crack growth rate data under loading with that under nonproportional mixed mode I and II cycles, it was found that the co-planar growth rates were similar regardless of whether the main driving force was in-plane shear or out-of-plane shear.

Fatigue crack growth under non-proportional mixed-mode I + II. Role of compression while shearing

“…Several experimental studies, motivated by similar rolling contact fatigue crack growth problems, were devoted to fatigue crack growth under sequential mode I + II [2,[7][8][9][10][11] or sequential mode I + III [12,13].…”

Fatigue Crack Growth under Non-Proportional Mixed Mode Loading in Rail and Wheel Steel Part 2: Sequential Mode I and Mode III Loading

“…WT exhibited the fastest coplanar FCG rates and, among the rail steels, the rates for RP were higher than those for RF when plotted against their appropriate equivalent SIF ranges, for both loading cases. Furthermore, Akama and Kiuchi [11] reported that FCG rates in RP under these two loading conditions (mixed modes I/II and I/III) were almost equal when plotted against the SIF ranges considered to be the main driving forces, ∆K II and ∆K III for the mixed mode I/II and I/III loading conditions, respectively. Therefore, the mechanism of shear-mode FCG under non-proportional mixed mode loadings that were subject to the RCF cracks can be considered to be the same even if the main crack driving force is in-plane shear or out-of-plane shear.…”

“…In the calculation results, both ∆σ max and ∆τ max planes were slightly oriented toward the ϕ direction within 7 • for all cases. First, the maximum tangential stress range was investigated to elucidate the effect of δ on crack branching direction [11]. The analysis was suggested to be based on an elasto-plastic stress field [21].…”

“…Akama and Kiuchi [11,12] conducted fatigue tests to determine the coplanar FCG rate on both rail and wheel steel under non-proportional mixed mode I/III loading cycles. They could induce long coplanar cracks under this condition.…”

“…As mentioned above, the previous studies about FCG under non-proportional mixed mode loading cycles were apparently focused on the mode I/II loading and coplanar growth in rail steel. Only a detailed study on coplanar and branch FCG under non-proportional mixed mode I/III loading cycles has been performed, by Akama and Kiuchi [11,12], for rail and wheel steel. However, the elucidation of the phenomena inherent to FCG under non-proportional mixed mode I/III loading is not sufficient in their studies.…”