ACOUSTIC-EMISSION FOR IDENTIFICATION OF STAGE IIA TO IIB TRANSITION DURING STAGE-II FATIGUE-CRACK GROWTH IN PARENT AND WELD METAL IN AISI-316 AUSTENITIC STAINLESS-STEEL

The acoustic emission during stage II Paris fatigue crack growth in pa rent and weld specimens of AISI type 316 stainless steel has been repo rted. Substages IIa and IIb occurring during state II fatigue crack gr owth, could be distinguished by a change in the acoustic emission acti vity, corresponding to a sharp change in the crack growth rate. The tr ansition point in the plot of acoustic emission cumulative ringdown co unt v. number of cycles agrees well with that of the crack growth rate v. number of cyles. The transition from stage IIa to IIb is found to occur at a crack growth rate da/dn = 3 x 10(-7) m cycle(-1), which is in reasonable agreement with the values reported in the literature. Th e high acoustic emission activity during stage IIa is attributed to th e extensive cyclic plastic deformation and the increasing size of the cyclic plastic zone with Delta K (the cyclic stress intensity factor) under the plane strain conditions prevailing during stage IIa. The low acoustic emission activity during stage IIb is attributed to the redu ction in the mean free path for dislocation movement and the decrease in the size of the cyclic plastic zone under the plane stress conditio n prevailing during stage IIb. The higher acoustic emission activity i n weld specimens compared with the parent metal is attributed to the c ombined influence of cyclic plasticity, residual stress induced microc racking, and roughness induced crack closure phenomena.