Mixed-mode fracture mechanisms near the fatigue threshold of aisi-316 stainless-steel

<span style="font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif; line-height: 18px">Near threshold, mixed mode (I and II), fatigue crack growth occurs mainly by two mechanisms, coplanar (or shear) mode and branch (or tensile) mode. For a constant ratio of Δ</span><em style="margin: 0px; padding: 0px; border: 0px; outline: 0px; vertical-align: baseline; font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif; line-height: 18px">K</em><sub style="margin: 0px; padding: 0px; border: 0px; outline: 0px; font-size: 0.8em; white-space: nowrap; line-height: 0.7em; font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif">I</sub><span style="font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif; line-height: 18px">/Δ</span><em style="margin: 0px; padding: 0px; border: 0px; outline: 0px; vertical-align: baseline; font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif; line-height: 18px">K</em><sub style="margin: 0px; padding: 0px; border: 0px; outline: 0px; font-size: 0.8em; white-space: nowrap; line-height: 0.7em; font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif">II</sub><span style="font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif; line-height: 18px"> the shear mode growth shows a self-arrest character and it would only start again when Δ</span><em style="margin: 0px; padding: 0px; border: 0px; outline: 0px; vertical-align: baseline; font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif; line-height: 18px">K</em><sub style="margin: 0px; padding: 0px; border: 0px; outline: 0px; font-size: 0.8em; white-space: nowrap; line-height: 0.7em; font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif">I</sub><span style="font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif; line-height: 18px"> and Δ</span><em style="margin: 0px; padding: 0px; border: 0px; outline: 0px; vertical-align: baseline; font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif; line-height: 18px">K</em><sub style="margin: 0px; padding: 0px; border: 0px; outline: 0px; font-size: 0.8em; white-space: nowrap; line-height: 0.7em; font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif">II</sub><span style="font-family: Arial, 'Lucida Grande', Geneva, Verdana, Helvetica, 'Lucida Sans Unicode', sans-serif; line-height: 18px"> are increased. Both shear crack growth and the early stages of tensile crack growth, are of a crystallographic nature; the fatigue crack proceeds along slip planes or grain boundaries. The appearance of the fracture surfaces suggest that the mechanism of crack extension is by developing slip band microcracks which join up to form a macrocrack. This process is thought to be assisted by the nature of the plastic deformation within the reversed plastic zone where high back stresses are set up by dislocation pile-ups against grain boundaries. The interaction of the crack tip stress field with that of the dislocation pile-ups leads to the formation of slip band microcracks and subsequent crack extension. The change from shear mode to tensile mode growth probably occurs when the maximum tensile stress and the microcrack density in the maximum tensile plane direction attain critical values.</span>