Influence of micro structure on acoustic emission behavior during stage 2 fatigue crack growth in solution annealed, thermally aged and weld specimens of AISI type 316 stainless steel

Abstract

Acoustic emission (AE) behavior during fatigue crack growth (FCG) in solution annealed, thermally aged and weld specimens of AISI type 316 austenitic stainless steel is reported in this article. The presence of two substages 2(a) and 2(b) during stage 2 FCG could be distinguished by a change in the AE behavior corresponding to a sharp change in the crack growth rate (da/dn). The transition point in the AE parameter vs. number of cycles (n) plot matches well with that of the (da/dn) vs. 'n' plot and is found to occur at a da/dn ≈ 3 × 10^-7 m cycle^-1 which is in agreement with the reported value. The high AE activity observed during the substage 2(a) is attributed to the extensive cyclic plasticity within the cyclic plastic zone (CPZ) and the increasing size of the CPZ with ΔK under plane strain conditions prevailing during stage 2(a). The low AE activity observed during stage 2(b) is attributed to a reduction in the mean free path for dislocation movement and a decrease in the size of the CPZ under plane stress condition prevailing during stage 2(b). The AE during stage 2(a) is found to have a strong influence on the microstructure. The presence of carbide precipitates in thermally aged specimens reduces the AE activity. The high AE activity in weld specimen is attributed to the combined influence of cyclic plasticity, residual stress induced micro cracking and roughness induced crack closure phenomena.