Assessment of Thermal Damage in Polymethyl Methacrylate Using Quasi-static Components of Ultrasonic Waves

A nonlinear acoustic response can be an effective indicator of early-stage damage in a material. However, it is challenging to use this approach to assess thermal damage in highly attenuating materials such as polymethyl methacrylate by measurement of the generated second harmonics, which are present at double the fundamental frequency of the primary waves. Using the quasi-static components (QSCs) generated by ultrasonic-wave propagation in damaged structures—an approach that combines the high sensitivity of acoustic nonlinearities with low attenuation—has great potential for evaluation of early-stage damage in highly attenuating materials. In this paper, an experimental approach for directly detecting the QSC pulses from ultrasonic-wave tone bursts is proposed. In this approach, a high-frequency longitudinal-wave transducer is used for exciting the primary ultrasonic-wave tone burst, and the co-propagating QSC pulse is detected using a low-frequency ultrasonic transducer. The phase-reversal technique is employed to enhance the signal-to-noise ratio of the QSC pulse and to counteract the signals of the primary ultrasonic waves. In the experimental results, variations in the amplitudes of the QSC pulse in response to different degrees of thermal damage are clearly illustrated in a repeatable manner, even though the changes in the linear ultrasonic features in these specimens are negligible. These results indicate that the measurement of QSC pulse generation from ultrasonic-wave propagation is a promising alternative for evaluating thermal degradation in highly attenuating materials.