Evaluation of nanoscale cracks by low-pass filter effect in nonlinear ultrasound

Cracks in solids can be detected by ultrasound if they are opened. However, it is not easy when they are closed, or with only nanoscale opening. It is the fundamental problem in ultrasonic NDE. Harmonics (2f, 3f,) is a good candidate, but it has a problem since it is generated also by transducers and liquid media (water), and it disturbs the measurement. So, we propose that subharmonics (f/2) and DC effect, which are regarded as 'low pass effect', is the best approach. Through theoretical and experimental investigations, we found that subharmonics is useful to detect closed fatigue cracks in aluminum, not easy to detect by conventional methods. Since it was successfully analyzed by approximate and detailed theories, it will be possible to estimate the size and property of closed crack planes by fitting measured waveforms to theories.