Crack tip and notch tip plastic zone size measurement by the laser speckle technique

Abstract

A laser speckle technique, together with a computer-based image processing system, was used to measure the extent of the elastic-plastic boundary ahead of a static mode 1 crack under small-scale yielding. The technique used a simple optical set-up and did not require surface contact with the specimen. A recently proposed correlation parameter called the Integrated Peak Spectral Intensity H i, was used to correlate the speckle images obtained with the state of strain around a crack tip and the subsequent mapping of the plastic zone. The experimentally obtained plastic zone around a narrow slit obtained by this technique showed reasonable agreement with predictions using Inglis' solution. For a fatigue crack, the extent of the experimental plastic zone ahead of a sharp crack was as expected larger than that predicted by linear elastic fracture mechanics, but agreed well with results corrected for load shedding at the crack tip. The present results indicate that the extent of the elastic-plastic boundary can be measured accurately with the laser speckle technique.