DEVELOPMENT OF REAL TIME OBSERVATION AND IMAGE PROCESSING TECHNIQUE WITH THE AID OF MICRO-COMPUTER CONTROL OF FATIGUE TESTING APPARATUS INSTALLED TO SCANNING ELECTRON MICROSCOPE

利用配备专门设计的伺服液压疲劳加载系统的场发射型扫描电子显微镜（SEM）的扫描和扫描信号的微机控制，可以发展对小于0.2 Hz循环载荷下疲劳现象的直接实时观测技术。对晶粒取向硅铁进行了疲劳裂纹扩展试验，并对裂纹扩展机理进行了研究。通过直接、实时观察，发现裂纹微观和宏观扩展方向随裂纹扩展速率的变化而变化，表明疲劳裂纹扩展速率与扩展方式之间存在着密切的相互关系。疲劳裂纹扩展速率与显微照片测量的裂纹尖端张开位移（CTOD）密切相关。讨论了Hi-Lo两阶载荷下疲劳裂纹扩展速率、扩展方向与CTOD的关系。为了能够以更高的频率观察疲劳裂纹在循环载荷下的扩展行为，利用高速a /D转换器（瞬态存储器）将扫描电镜的二次电子信号直接输入计算机，开发了扫描电镜图像的边缘检测和裂纹边缘模式识别等处理技术。实验结果表明，该方法可在160 msec内对一帧扫描电镜图像进行采样，从而在1 Hz的正弦循环载荷下实现实时观测。基于所开发的图像处理技术，研究了裂纹张开位移（COD）沿裂纹长度的分布、加载循环下裂纹周边的位移以及裂纹扩展增量。

Techniques of direct, real time observation of fatigue phenomena under cyclic loadings of less than 0.2 Hz could be developed with the aid of micro-computer control of sweep and scan signals of a field emission type scanning electron microscope(SEM) equipped with specially designed servo-hydraulic fatigue loading system.Fatigue crack growth tests were conducted on grain-oriented silicon iron, and crack growth mechanism was investigated. Through direct, real time observations, both microscopic and macroscopic crack growth directions were found to change with growth rates which indicated that there exists close interrelationship between fatigue crack growth rate and growth mode. It was also found that fatigue crack growth rates were well related to the crack tip opening displacement(CTOD) measured on the microphotographs. Relationships of fatigue crack growth rate, growth direction and CTOD under Hi-Lo two step loadings were also discussed.In order to make it possible to observe fatigue crack growth behavior under cyclic loading with higher frequency, secondary electron signals of the SEM were taken directly into the computer by using a high speed A/D converter(transient memory), and processing techniques of SEM image such as edge detection and pattern recognition of crack periphery were developed. It was found that an SEM image of one frame could be sampled in 160 msec by above mentioned method so that the real time observation was made possible under a sinusoidal cyclic loading of 1 Hz. Distributions of crack opening displacement(COD) along the crack length, displacements of crack periphery under loading cycle and crack growth increments were examined based on the developed image processing techniques.