Recent advances in the measurement of acoustic nonlinearity parameter for micro-damage detection of materials

Measurements of the acoustic nonlinearity parameter b are frequently made for early detection of damage in various materials. The practical implementation of the measurement technique has been limited to the through-transmission setup for determining the nonlinearity parameter of the second harmonic wave. For the purpose of practical applications, a pulse-echo measurement technique is more desirable which enables the single-side access of test components. The issue with using the second harmonic wave reflected from the stress-free interface is that such a boundary destructively alters the nonlinear generation process and consequently makes it difficult to obtain the reliable results of b. Our previous simulation study shows that the average displacement of the second harmonic wave reflected from the stress-free boundary is as large as one third or one half of the through-transmission mode of the same total propagation distance depending on the thickness. When the sample was rather thin, the average pulse-echo displacement of the second harmonic is very small and consequently the total correction to be made becomes very large. Therefore, the b determination in this case largely depends on the sensitivity of the second harmonic displacement measurement and on the accuracy of making total corrections. In this seminar, recent advances in the development of acoustic nonlinearity parameter measurements are introduced and some results are presented.