Composite structures used in modern military aircraft are subject to a range of loading conditions such as foreign object impacts, which can produce sub-surface damage involving delamination, ply cracking, and honeycomb core crushing. Current damage detection methods such as ultrasound and tap testing must be applied locally and the inspection results are highly dependent on the experience of the operator. The U.S. Marine Corps is interested in developing methods of damage detection for composite materials that are field expedient and less dependent on the operator’s experience.

A vibration-based method was investigated for detecting damage in composite materials based on a measurement of the nonlinear forced response that damaged materials are assumed to exhibit. A damage feature was extracted for a structural component by quantifying the degree to which the reciprocity between two input-output structural paths fail due to the nonlinearities associated with damage.

Experimental results were obtained from carbon fiber composite samples as well as fiber reinforced plastic samples subjected to various levels of damage. It was determined that reciprocity measurements could be used to identify the presence of laminate-to-core disbonding beneath sensor locations in fiber reinforced plastic composite material. It was also found that reciprocity measurements were capable of identifying damage due to impact energies of 8 ft·lbf beneath sensor locations in carbon fiber composite material.

A static nonlinear theoretical model was used to develop a better understanding of why reciprocity fails for networks of nonlinear components. In addition, simulations of a tuned dynamic nonlinear theoretical model were shown to exhibit the same qualitative results as in the experiments that were performed on damaged composite material.

The benefits of this method were that it was less dependent on reference measurements from the undamaged component, that it was likely to be more robust to environmental and boundary conditions than methods based on the use of linear modal parameters for damage detection, and that it was less susceptible to operator error when compared to traditional methods such as tap testing and ultrasonic.