Acoustic Emission from 4340 Steel During Stress Corrosion Cracking

Abstract

A commercially available system is employed to detect and count acoustic emissions emanating from the aqueous stress corrosion crack propagation in 4340 steel. Standard ASTM E 399 compact tension specimens with short notches, a/W=0.25 versus 0.50 standard, are prepared in four tempering temperatures: 204, 316, 427, and 538°C (400, 600, 800, and 1000°F). Crack length is monitored with a notch opening clip gage. Comparisons show the time rate of emission events to increase with stress intensity in rough correspondence to, but much more rapidly than, the crack velocity. Tempering back tends to suppress the count rate as a warning of fast fracture instability. Some similarity is observed between the areal rate of acoustic emission and factors which influence degree and intensity of plastic flow instability in the fracture process zone.