Quantitative Evaluation of Multiple Cracking Damage by Means of Direct Current Potential Difference Method with Multiple-Type Probe

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The objective of the present research is to establish a method of quantitative evaluation of crack distribution from two-dimensional distribution of potential difference which can be measured by means of multiple-type probe. The multiple-type probe is composed of a lot of probes which are aligned in two rectangular directions. Although a conventional method, where only a pair of probes are used for measuring potential difference, has been applied to multiple cracking damage, it was impossible to determine whether relatively small number of large cracks exist or relatively large number of small cracks exist. In this study, electric field analysis and experiment were carried out and the following results were obtained.1. The distribution of potential difference between two adjacent probes in multiple-type probe was calculated for known distribution of cracks and the relationship between the distribution of potential difference and that of cracks was formulated.2. Using the formula, a method of quantitative evaluation of areal density (i. e. , the number of cracks in unit area) and mean length of cracks from mean and standard deviation of potential difference was proposed. The validity of the method was assured by numerical experiment.3. A new evaluation method of the distribution of microscopic strain on the surface of specimen was proposed. In this method, micro-grid at intervals of about 10μm was described in advance on the surface of specimen using roughness tester, and a change in the shape of each grid was measured to evaluate the strain. This method was applied to small fatigued plate specimens of pure copper. Strain showed a relatively large scatter in microscopic scale and this seemed to be caused by microstructural inhomogeneity such as the difference in crystallographic orientations among neighboring grains. It was thought that the non-uniform strain of grains brought about crack initiation on the surface of specimen.