Ultrasonic detection and sizing of plastic zones surrounding fatigue cracks

The problem of detection and sizing of fatigue cracks that are closed due to the reverse plastic deformation is addressed in this paper. In particular, it is suggested that the zone of plastic deformation can be identified by measuring the changes in the wave speeds in the plastically deformed material. This is supported through experimental results, first by determining the changes in wave speed with monotonic plastic deformation and then by investigating the plastic zones near fatigue cracks. The characteristic changes in the wave speeds are interpreted in terms of the theory of wave propagation in nonlinearly deformed media and elastic plastic fracture mechanics.