Automatic detection of crack initiation and propagation using optical techniques

Two new experimental methods to detect crack initiation and monitor crack propagation during fatigue testing have been tried. Both methods are optical and use different kinds of image analysis to locate cracks on images acquired at regular intervals during the fatigue test. In the first method the specimen is illuminated by a laser to create a speckle pattern. By computing the correlation coefficient of the laser speckles, a growing crack tip will appear as a local decrease in the correlation coefficient field. In the other method the specimen is illuminated by white light. The deformation of the natural structure in the images is evaluated by computing the displacement field. The crack tip positions are obtained by analysing the displacement fields. Both methods have shown to provide useful results from fatigue tests on notched aluminium specimen. In the present state some manual work is needed during the analysis of the images but the experience from the tests can be used for future development of an automatized crack detection system.