APPLICATION OF THE LASER SPECKLE TECHNIQUE IN THE STUDIES OF CRACKS AND NOTCHES IN PLATES

Abstract

This project investigates the application of the laser speckle technique in three areas: Crack tip and notch root plastic zone size measurements, crack initiation detection and strain measurement in polypropylene. In this work, the changes in speckle patterns as a result of plasticity and fatigue damage are studied by using a newly defined parameter called the Integrated Peak Spectral Intensity Coefficient, Hi. This parameter is easier and simpler to compute than 1 other previously defined coefficients. It is found that Hi is closely related to strain and is suitable for plastic zone size measurement. Under mode 1 loading, the experimental notch root plastic zone agreed reasonably well with predictions using Inglis' solution. For a fatigue crack, the experimental plastic zone ahead of the sharp crack was larger than that predicted by Linear Elastic Fracture Mechanics, but agreed well with results corrected for load shedding at the crack tip. Under mixed mode loading, the measured notch root plastic zone deviated from Creager's solution when the slit was under predominantly mode 2 load. However, the agreement improved as the angle of the slit approached mode 1 load conditions. Results for the plastic zone sizes for angled slits with cracks are also presented. In a separate investigation, it was found that the proposed parameter Hi was sensitive to the initiation of fatigue crack at notches. Crack initiation studies using centre slit specimens with a plastically deformed notch root showed that when the laser beam fell on a 0.1 mm fatigue crack, Hi. showed a significant increase. It continued to increase as the crack grew. A definition for fatigue crack initiation as the intersection of two straight lines on the Hi vs cycle plot was proposed. Results showed that surface crack initiation at a crack length of less than 0.1mm, can be detected. Similar behaviour observed for angled slit specimens whose notch root was plastically deformed. Results showed that crack initiation occurred sooner as the axis of the angled slit approached the perpendicular of the load direction. Hi is found to change with strain in polypropylene when the specimens are subjected to uniaxial tension. The characteristic relationship between Hi and strain is similar to that in metal and is independent of strain rate. Some of the findings in this project have been accepted or submitted for publication [1-3].