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

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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.