INFLUENCE OF MICROSTRUCTURE ON ACOUSTIC-EMISSION BEHAVIOR DURING STAGE-2 FATIGUE-CRACK GROWTH IN SOLUTION ANNEALED, THERMALLY AGED AND WELDSPECIMENS OF AISI TYPE-316 STAINLESS-STEEL

Acoustic emission (AE) behavior during fatigue crack growth (FCG) in s olution annealed, thermally aged and weld specimens of AISI type 316 a ustenitic 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 v s. 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 approximate to 3 x 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 exten sive cyclic plasticity within the cyclic plastic zone (CPZ) and the in creasing size of the CPZ with Delta K under plane strain conditions pr evailing during stage 2(a). The low AE activity observed during stage 2(b) is attributed to a reduction in the mean free path for dislocatio n movement and a decrease in the size of the CPZ under plane stress co ndition prevailing during stage 2(b). The AE during stage 2(a) is foun d to have a strong influence on the microstructure. The presence of ca rbide 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.