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Abstract:In this work quantitative results of applying nonlinear acoustic dynamics to study progressive damage in a polymer-based composite SMC (sheet molding compound) are presented. Via carefully controlled resonant plate experiment, nonlinear slow dynamics (SNLD) response of SMC in terms of relaxation time and frequency shift has been shown to be very sensitive to gradual damage induced using three-point bending tests. Besides, acoustic emission monitoring is used to characterize damage through the elastic energy released by SMC at every damage step. Interesting logarithmlike changes of the SNLD parameters as a function of the acoustic emission cumulated energy are found.

Sensitivity of nonlinear acoustic methods to the presence and the evolution of micro-damage has been proven in various studies on a wide range of materials. In this contribution, a guided wave approach is proposed to characterize polymer based composite plates taken at intact as well as damaged states. The changes in the nonlinear hysteretic parameters are observed by changing the order of the excited flexural resonances. The analysis is based on the evolution of the velocity frequency dispersion of the generated guided waves, where flexural modes are considered separately and together as a function of the dynamic strain.

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This work presents an acoustic emission (AE) monitoring of slow dynamics in micro-cracked polymer concrete (PC) samples. In order to obtain calibrated damage states, AE was first used to characterize the micro-damage mechanisms in real time when PC samples are submitted to three-point bending tests. Then, an unsupervised classification of AE data based on the Principle Component Analysis and the k-means clustering was applied to classify AE data. The AE monitoring of the nonlinear relaxation of PC samples revealed the existence of a silence period followed by AE hits belonging to two different damage classes. A similarity appeared between the properties of the detected AE hits obtained during the nonlinear relaxation and the quasi-static tests. Finally, this work shows that the dynamics of both mechanisms during the nonlinear relaxation are clearly different.

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