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

Abstract

The acoustic emission behaviour during stage II Paris fatigue crack growth in parent and weld specimens of AISI type 316 stainless steel has been reported. Substages IIa and IIb, occurring during stage II fatigue crack growth, could be distinguished by a change in the acoustic emission activity, corresponding to a sharp change in the crack growth rate. The transition point in the plot of acoustic emission cumulative ring down count v. number of cycles agrees well with that of the crack growth rate v number of cycles. 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. The high acoustic emission activity during stage IIa is attributed to the extensive cyclic plastic deformation and the increasing size of the cyclic plastic zone with ?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 reduction in the mean free path for dislocation movement and the decrease in the size of the cyclic plastic zone under the plane stress condition prevailing during stage IIb. The higher acoustic emission activity in weld specimens compared with the parent metal is attributed to the combined influence of cyclic plasticity, residual stress induced microcracking, and roughness induced crack closure phenomena.