Rayleigh Waves for Continuous Monitoring of a Propagating Crack Front

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Abstract
A simple ultrasonic system which uses Rayleigh (surface) waves is briefly described. Applied to a part-through crack (PTC) specimen the wave is attenuated according to the crack depth. Functional relations of the crack depth and hence the stress intensity factor versus the number of load cycles N or the crack growth rate da/dN versus the cyclic stress intensity range ΔK are therfore easily obtained by continuously monitoring such a surface wave. The system is sufficiently sensitive to actually show an “apparent” crack depth change during a single load cycle: at a low load level, the crack depth seems to be smaller than at high load level. This indicates that at low load levels internal compressive stresses at the crack tip resist crack opening. At high load levels, however, these internal stresses are overcome and the crack opens up to its actual size. This observation should be useful for interpretations of crack closure behaviors, crack retardation effects during spike loading, and other plasticity effects in the vicinity of the crack tip.

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