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

本文的研究目的是建立一种利用多探头测量的二维电位差分布定量评价裂纹分布的方法。多探头是由许多探头沿两个矩形方向排列而成的。虽然传统的方法（仅使用一对探针测量电位差）已经应用于多重裂纹损伤，但无法确定是存在相对较少数量的大裂纹还是存在相对较多数量的小裂纹。在本研究中，进行了电场分析和实验，得到了以下结果：在已知裂纹分布的情况下，计算了多型探头中相邻两个探头之间的电位差分布，并给出了电位差分布与裂纹分布之间的关系。利用该公式，提出了一种由电位差均值和标准差定量评价面密度（即单位面积裂缝数）和裂缝平均长度的方法。通过数值实验验证了该方法的有效性。提出了一种评价试样表面微观应变分布的新方法。该方法利用粗糙度仪在试样表面预先描绘出间隔约10μm的微网格，通过测量每个网格的形状变化来评估应变。将该方法应用于纯铜的小型疲劳板试样。应变在微观尺度上表现出较大的散点，这可能是由于邻近晶粒之间的晶体取向差异等微观结构不均匀性造成的。认为是晶粒应变不均匀导致试样表面裂纹萌生。

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.