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

利用微机控制场发射型扫描电子显微镜（SEM）的扫频和扫描信号，并配以专门设计的伺服液压疲劳加载系统，可直接、真实的实时观察疲劳现象。通过直接的、真实的时间观测，发现微观和宏观裂纹扩展方向都随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关系。还发现，疲劳裂纹扩展速率与显微照片上测得的裂纹尖端张开位移（CTOD）有很好的关系。讨论了高低两级载荷下疲劳裂纹扩展速率、扩展方向与CTOD之间的关系，并利用高速A/D转换器将扫描电镜的二次电子信号直接输入计算机，以观察高频循环载荷下的疲劳裂纹扩展行为（瞬态记忆），以及裂纹边缘检测和裂纹周边模式识别等SEM图像处理技术。实验结果表明，在1Hz的正弦循环载荷下，用上述方法可以在160毫秒内完成一帧扫描电镜图像的采样，从而实现了真实的时间观测。基于图像处理技术，研究了裂纹张开位移（COD）沿着裂纹长度的分布、载荷循环下裂纹周边位移和裂纹扩展增量。

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.