A Study on the Detection of Compressed Micro-crack by Nonlinear Wave Modulation Technique

Abstract

We investigate nonlinear wave propagation through micro-cracks that are compressed by external forces by means of a nonlinear ultrasonic modulation technique. Nonlinear modulated waves are generated by the truncation of waves passing through cracks due to the opening and closing of the cracks with the nonlinear ultrasonic modulation technique, which is known to be effective when used to detect very fine cracks in comparison with other linear ultrasonic methods since the technique utilizes the breathing of the cracks rather than wave reflections or refractions. However, if the cracks are strongly compressed, the crack opening is hindered due to the excessive initial stress, and the nonlinearity does not become apparent.

In this study, an improved nonlinear modulation wave technique for the detection of micro-cracks under compression is devised. By analyzing photomicrographs of cracks with an algorithm designed to measure the crack width, a realistic crack model is generated and a chirp signal is applied to find the resonant frequencies which are used as the excitation frequencies. Experimental tests are conducted to verify the numerical results. An aluminum plate is compressed in the direction normal to the cracks' lateral surfaces and is excited using piezoelectric patches attached onto its surface. The experimental and numerical results show good agreement for various excitation frequencies and different compressions.