Acoustic emission from surface fatigue cracks in SS41 steel

Crack penetration behaviour by fatigue crack propagation and the measurements of acoustic emission (AE) before and after crack penetration were examined using SS41 steel plate. The experimental crack shape of the SU type which has an out-of-plane degree of freedom was in good agreement with the calculated shape. But this was not so for the S type which has a restriction to out-of-plane freedom. The crack propagation behaviour on the front surface appears not to change markedly after penetration. However, crack growth on the back surface appears to accelerate. As a crack propagates, substantial acoustic emission occurred just prior to penetration. Then it decreased and the crack penetrated. Intense AE emanation has been attributed to a transition from a plane strain condition to a plane stress condition.