Digital Material Surface Image Analysis for Evaluating Geometric Fatigue Crack Parameters

A procedure for acquiring and analyzing digital fatigue crack propagation images is advanced, which provides automatic crack size detection and the deviation from the axis perpendicular to the loading direction. The procedure is based on comparing the original image with the subsequent ones and eliminating all elements present, only getting the changes in the fatigue crack extension. A slight shift of the images concerning each other was fixed with their cross-correlation. Choosing an optimum image segmentation level permits rejecting the elements that appear due to the digital matrix noise. The four-dimensional pixel assembly was used to get the geometric elements. As a result, a set of geometric figures characterizing the fatigue crack propagation was obtained. For analyzing the length and direction of crack components, the ellipses were applied whose second moments coincide with those of the components. The total change in the crack propagation direction and length is determined from the parameters of the vector connecting the centers of mass of the first and last components. The advanced procedure can be employed in crack resistance tests.