Development of Genetic Algorithms based System for High-Precision Reconstructions of Three-Dimensional Topographies using Stereo Fractographs

开发了一种软件，可以在没有人工帮助的情况下重建骨折表面的三维(3-D)形状。它以计算机图像处理和模式识别技术为基础，使用一对立体扫描电子显微图像。该过程包括两个子过程：在两幅图像之间搜索匹配点和利用匹配点的相对移位计算高度。通过使用之前开发的系统，在搜索过程中不可避免地会出现一些失配，特别是对于低对比度的扫描电子显微镜图像，如条纹、晶间小面等。为了提高搜索的精度，在开发的系统中引入了遗传算法。用遗传算法成功地重建了解理断裂、沿晶断裂、韧窝和疲劳条纹等多种断口的三维形貌，具有足够的精度。比较了遗传算法和以前提出的粗搜索和近搜索两步算法的搜索过程。证明了遗传算法在搜索过程中具有精度高、运行时间短的优点。因此，以足够的精度和相对较短的运行时间获得了详细的3-D形状，或超过120×120重建的点大小的形状。将该系统应用于不锈钢(SUS316 NG)和低合金钢(SGV420)的疲劳断口，讨论了定量形貌测量与应力强度因子和/或裂纹扩展速率之间的关系。

A software is developed which enables reconstruction of the three-dimensional (3-D) shape of fracture surfaces without human assistance. It is based upon computer image processing and pattern recognition techniques by using a stereo-pair of scanning electron micrographs. The processing consists of two subprocesses : searching the matching points between two images and computation of heights using the relative shift of the matching points. By using the previously developed system, some mismatches were inevitable in the search process, in particular, for low-contrast SEM images such as striations, intergranular facets, and so on. In order to improve the accuracy of the search, a Genetic Algorithm (GA) was implemented into the developed system. By using the GA method, the 3-D shapes of a wide variety of fracture surfaces including cleavage failures, intergranular cracking, dimples and fatigue striations, were successfully reconstructed with sufficient accuracy. The searching processes by the GA method and the previously developed two-step algorithm of coarse and close searching were compared. These proved that the GA method has the advantage of accuracy in the searching process and of short-run-time. A detailed 3-D shape, or more than a 120×120 reconstructed point-sized shape, was thus obtained with sufficient accuracy and with a relatively short-run-time. The developed system was also applied to the fatigued fracture surfaces of a stainless steel (SUS316NG) and a low-alloy steel (SGV420), and the relationship between quantitative topographic measures and stress intensity factors and/or crack growth rates was discussed.