Artificial closed cracks were made by the diffusion bonding of one stainless steel block with another block, to make the crack gap of sub-micro meter. The crack surfaces were visualized by an immersion higher harmonic ultrasonic method. As excitation voltage was increased, the crack area visualized by the higher harmonic increased. This could be explained by the closed crack surfaces being opened by a large amplitude incident wave. Furthermore, we confirmed that the closed cracks where the gap was decreased to 0 from the maximum gap of about 6μm were made, by cutting the specimens after higher harmonic measurement.
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