Development of Automatic and High-Precision Displacement Gage of Laser Interferometry for Measuring Opening Displacement of Small Fatigue Cracks

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1. An interferometric strain/displacement gage(ISDG)with linear image sensors was developed and linked with a servo hydraulic fatigue testing machine. This system is called System A. A hysteresis loop of crack opening displacement and load was obtained by step-wise loading. The resolution of System A can reach lnm for the gage length of 100mum.2. A laser diode was used as a light source in order to chop the laser light synchronously with the load wave and to measure the displacement during cyclic loading. This system called System B is applicable up to a frequency of 3OHz.3. A mechanical shutter was also equipped with He-Ne laser system to chop the laser beam mechanically and synchronously with the load wave. This system called System C is useful for measuring the crack opening displacement at load frequencies below 3Hz.4. System A was used measure the crack opening displacement of surface cracks in smooth specimens of a low carbon steel. The effective stress intensity range calculated from the crack closure measurement was constant at the non-propagation of small cracks.5. The effect of the second phase such as martensite and pearlite on the crack propagation behavior of small fatigue cracks was discussed on the basis of the crack closure measurement by System A. The crack closure was found to be responsible for irregular behavior of crack growth rate. The roughness-induced crack closure plays a significant role.6. The ISDG system was also applicable to metal matrix composites such as SiC particulate reinforced aluminum alloy. The propagation behavior of small fatigue cracks was discussed on the basis of crack closure measurement.