A fractographic study of crack propagation under creep-fatigue conditions: Koterazawa, R., Nosho, T. and Mukai, Y. J. Soc. Mater. Sci., Jpn. July 1989 38, (430), 802–808 (in Japanese)

Abstract

Crack growth and roughness induced closure mechanisms of shear cracks under 180° out-of-phase strain histories having constant amplitude straining and periodic compressive overstrains (PCOs) were investigated. The results revealed that shear cracks initially nucleated on a slip band at 45° to the axis of the specimen which coincides with the plane of maximum shear strain. Growth on the shear planes (microcracks) into the specimen surface occupied up to 90–95% of fatigue life during which time the surface length of the microcracks remained nearly constant. Failure then occurred by a rapid linking of microcracks at the end of a test.The opening stress of microcracks was taken to be the applied static stress level at which the crack depth stopped increasing with increasing stress. Observations indicated that as the number of cycles increased the crack depth on the maximum shear plane increased but the crack opening stress did not change appreciably.The crack growth rate in the depth direction on shear planes increased significantly and fatigue strength was reduced by a factor of 1.4 and 1.7 at short and long lives, respectively, when PCOs of near yield point magnitude were applied. Confocal scanning laser microscopy and scanning electron microscopy examinations of fracture surfaces revealed that these PCOs flattened mismatch asperities near the crack tip. The reduction in the height of these irregularities on the fracture surface was accompanied by a reduced crack closure stress and a higher crack growth rate.