Assessment of thermal damage in polymethyl methacrylates using quasi-static components of ultrasonic waves

Abstract

The use of acoustic nonlinear response can be an effective method for the early-stage damage evaluation of materials. However, it is challenging to assess the thermal damage in highly attenuating materials, such as polymethyl methacrylates, by the measure of generated second harmonics with a double fundamental frequency of primary waves. Quasi-static component (QSC) generated by ultrasonic wave propagation in damaged structures, which combines the high sensitivity of acoustic nonlinearity and low attenuative effect, has a great potential for early-stage damage evaluation in highly attenuating materials. In this paper, an experimental approach for directly detecting the QSC pulse of an ultrasonic wave tone burst is proposed. While a high-frequency longitudinal wave transducer is used for exciting the primary ultrasonic wave tone burst, the co-propagating QSC pulse is captured using a low-frequency ultrasonic transducer. The phase-reversal technique is employed to enhance the signal-to-noise ratio of QSCs and to counteract the signals of primary longitudinal waves. Amplitude variations of QSCs under different thermal loadings are clearly illustrated in a repeatable manner, while those of linear ultrasonic features in the specimens are neglectable. Experimental results indicate that the measure of QSC generation of ultrasonic wave propagation can be a prospective alternative for evaluating thermal degradation of highly attenuating materials.