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

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 co-planar 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 non-proportional 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.