Large plastic zones and extensive influence of notch under near-threshold mode II and mode III loading of fatigue cracks

Plastic zone shapes and sizes were determined for a cylindrical specimen for mode II and mode III crack testing made of ARMCO iron. Three methods of calculation were applied: Irwins approximate solution, HRR field and finite element method. The plastic zones were found to be very large under mode II and mode III loading even at the loading level corresponding to fatigue crack growth threshold. The difference between sizes of plastic zones under tensile-mode and shear-mode loading was explained by different stress gradients and corresponding stress concentration factors of notches under shear and tensile type loading. This is also the reason for a much further influence of notch on stress intensity factors for mode II and mode III cracks emanating from the notch than the influence under mode I loading. Preliminary results for reversed plastic zone size indicated that it is smaller than the theoretical size of 1/4 of the monotonic zone which explained the presence of large monotonic zone at threshold loading. (C) 2018 The Authors. Published by Elsevier B.V.