Nondestructive testing of composites using a nonlinear acoustic spectroscopy method

Abstract

The presented experimental work describes the nondestructive examination of polymer based composites using an acoustic method under the consideration of nonlinear effects. The technique is based on the fact that material behaves more nonlinearly in the presence of damage than in the undamaged state during dynamic load. Damaged structures show an increased nonlinear transmission behavior. Consequently, the level of nonlinearity is an indication of the damage severity. The aim is to analyze the nonlinear transfer behavior and if a quantification can be used to determine the damage severity. The focus is on the type of nonlinear acoustic spectroscopy methods that analyses the modulation spectrum. When two harmonic waves are applied with different frequencies to a damaged material, the amplitude of the high-frequency wave will be modulated by the low-frequency wave and the created spectrum manifests an intermodulation by showing new frequencies such as sidebands and higher harmonics. Samples made of glass fiber reinforced vinyl ester respectively epoxy, both taffeta woven, and pre-damaged by application of tensile tests using different levels of tension have been considered. A new experimental setup has been developed and a clearly visible increase of the intermodulation with increasing damage has been noticed for both materials. Finally, the method is proven to nondestructively evaluate the damage state of composites.