Progress in understanding of intrinsic resistance to shear-mode fatigue crack growth in metallic materials

Recently made experiments allowed measurement of effective data (free of crack tip shielding) and enabled a better understanding of intrinsic behaviour of shear-mode cracks. Influence of crystal structure and microstructure was distinguished. Experimental results for niobium showed that growth micromechanisms and effective thresholds (Delta K-IIeff.th approximate to 1.0 MPa1/2, Delta K-IIeff.th approximate to 1.4 MPa m(1/2)) of modes II and III cracks were different, which was also observed in other metals. Validity of the simple formula for mode II effective threshold for single-phase metals based on the dislocation emission model was confirmed. The cracks propagated in a coplanar manner along the maximum shear plane, which is a characteristic behaviour for bcc metals. Model of propagation of remote mode III loaded cracks by local mode II advances was confirmed. (C) 2016 Elsevier Ltd. All rights reserved.