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It is difficult to detect a closed crack by conventional ultrasonic testing. However, nonlinear ultrasonics may be useful for detection of a closed crack in structures. This study experimentally examined propagation of ultrasonic waves through the interface between contacting solids. We simulated a closed crack by compressing two aluminum blocks. The input frequency was changed in the range of 1.0-2.0MHz and two different types of surface roughness of specimens were used. As a result, generation of low-frequency components at about 600 kHz was found regardless of the input frequency. Such a feature of low-frequency components is different from that of subharmonics. Additionally, this phenomenon was not confirmed in specimens with smoother surface. Therefore, surface roughness can be one of important factors of generating low-frequency components. The results suggest that these low-frequency components can be useful for detecting a closed crack.

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