An acoustic emission investigation of microscopic ductile fracture

A high-strength 4340 steel fracture-toughness specimen was heat treated to give a ductile-rupture type of slow crack growth under rising load. For evaluation of the step-wise growth process, the specimen was instrumented with acoustic stress wave emission (SWE) detection equipment. The resulting crack area swept out by the advancing crack was correlated to the magnitude and number of the acoustic emission pulses. A crack growth model was developed which accounts for the direct relationship between crack area swept out and the sum of the individual SWE amplitudes, and for the experimentally observed bimodal distribution of the SWE amplitudes. The model postulates that slow crack growth takes place in a step-wise mechanism. This involves a repeated two-step process where the first step is the formation of a multitude of individual thumbnail cracks and the second step is the simultaneous interconnection of these thumbnail cracks to form a new continuous crack front.