Measurement of displacement field around a crack tip using fractal correlation method

摘要

The key problem to enhance the precision of displacements measurement with image correlation is to make the most of correlation behavior of speckle images. In fact, the artificial random speckles or natural texture, especially the microcosmic texture, on the object's surface have fractal characteristics, and the digital images of these patterns are also fractals. It is found that there is correlation between the fractal characteristics in un-deformed and deformed states of the images. Based on this fact a method, called fractal correlation method, can be used to measure displacements. In this paper, the fractal correlation method was used to measure the displacement field on a crack tip of a metal specimen. The microcosmic natural texture on specimen's surface was taken as images, and the area (vision field) was 1.397x1.397mm. By correlation operation with the fractal correlation method, curves of displacement around the tip can be obtained. The position of crack tip can be clearly distinguished from the displacement curves. The relationship of COD and distance behind the tip is linear. This agrees with the theory of fracture mechanics. The results also show that the plastic field does not distribute in uniform.