Investigation of Damage in Composites Using Nondestructive Nonlinear Acoustic Spectroscopy

The presented experimental work describes the nondestructive damage examination of polymer-matrix composites using acoustic methods under the consideration of nonlinear effects. The aim is to analyze these nonlinear effects in order to provide a quantification of the nonlinear acoustic transmission which is related to the damage state and its severity in the composite material. The first objective was to study the effectiveness of the distortion evaluation method and its related parameter: the “Total Difference Frequency Distortion” (TDFD) parameter. The TDFD was utilized as a new damage indicator to quantify the progressive damage state in composite materials. The TDFD method had initially been proposed to characterize the distortion of audio amplifiers. A custom-made setup was developed that imposes acoustic signals to the structure. The samples’ vibrations were afterwards analyzed by a laser vibrometer and further spectrum evaluations. The developed method was applied to two composite materials, both reinforced with taffeta woven glass-fibers, but having different thermoset polymer matrix, i.e. vinylester and epoxy. The damage was introduced in the specimen by tensile tests with a stepwise increase of the tension loading. It was observed that damage influences the intensity of nonlinear intermodulation after having introduced two harmonic and constant signals of different and randomly chosen frequencies in the specimen. The nonlinear intermodulation was then quantified by computing the TDFD parameter. In the specific case of epoxy based composites, high frequency peaks were noted for the high tensile loading levels only. The TDFD parameter was then modified in order to take into account this effect. For both studied composites, the modified TDFD parameter increases with the damage accumulation caused by the applied stepwise tensile loading.