Analysis of fatigue crack propagation under mixed mode II + III in ARMCO iron

• Mixed-mode II + III fatigue cracks propagation experiments in ARMCO iron were done.
• Small-scale yielding was verified by both analytical and numerical approaches.
• The most appropriate expression of mixed-mode crack growth data was found.
• Mode III component of mixed-mode loading was less efficient than that of mode II.

Near-threshold propagation of shear cracks under mixed-mode II + III loading was investigated in ARMCO iron. Special specimens for simultaneous growth of cracks in II, III and II + III loading modes were employed in these experiments. All experiments were performed under small-scale yielding as verified by both analytical and numerical approaches. The most appropriate description of mixed-mode crack growth was found in terms of Δa/ΔN vs. ΔKeq   dependences, where ΔKeq=ΔKII2+β·ΔKIII2 and β = 1.1. Also another mixed-mode criterion based on averaged using weight coefficients revealed to be useful. These analyses revealed that the mode III component of the mixed-mode loading was slightly less efficient for the mixed-mode crack-growth mechanism than that of the mode II which is in agreement with results reported for some other metallic materials.