CRACK-GROWTH AND CLOSURE MECHANISMS OF SHEAR CRACKS UNDER CONSTANT AMPLITUDE BIAXIAL STRAINING AND PERIODIC COMPRESSIVE OVERSTRAINING IN 1045-STEEL

Crack growth and roughness induced closure mechanisms of shear cracks under 180 degrees out-of-phase strain histories having constant amplit ude straining and periodic compressive overstrains (PCOs) were investi gated. The results revealed that shear cracks initially nucleated on a slip band at 45 degrees to the axis of the specimen which coincides w ith the plane of maximum shear strain. Growth on the shear planes (mic rocracks) into the specimen surface occupied up to 90-95% of fatigue l ife during which time the surface length of the microcracks remained n early constant. Failure then occurred by a rapid linking of microcrack s 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 i ncreasing with increasing stress. Observations indicated that as the n umber of cycles increased the crack depth on the maximum shear plane i ncreased but the crack opening stress did not change appreciably. The crack growth rate in the depth direction on shear planes increased sig nificantly 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 scannin g electron microscopy examinations of fracture surfaces revealed that these PCOs flattened mismatch asperities near the crack tip. The reduc tion in the height of these irregularities on the fracture surface was accompanied by a reduced crack closure stress and a higher crack grow th rate. (C) 1997 Elsevier Science Ltd.