Evaluating damage progression in fiber-reinforced polymeric composite structures is critical for effective condition-based monitoring techniques of these structures. In this study, three sets of woven fiberglass/epoxy specimens were investigated under axial loading and unloading conditions using embedded and surface-mounted piezoelectric wafers. The noisy ultrasonic waveforms were denoised and analyzed with Gabor wavelet transforms and their contour plots, in particular normalized contour perimeters. It was found that the trend of the selected contour parameters captures the specimens' strain reasonably well in some samples, replicating loading, unloading and residual strain. This technique shows promise for tracking damage progression, residual strains, and pending failure in fiber-reinforced polymeric composites.

This work describes a novel method of embedded damage detection within glass fiber-reinforced polymer composites. Damage detection is achieved by monitoring the spatially distributed electrical conductivity of a strain-sensitive multiwalled carbon nanotube thin film. First, thin films were spray-deposited directly upon glass fiber mats. Second, using electrical impedance tomography, the spatial conductivity distribution of the thin film was determined before and after damage-inducing events. The resolution of the sensor was determined by drilling progressively larger holes in the center of the composite specimens, and the corresponding electrical impedance tomography response was measured by recording the current-voltage data at the periphery of the monitored composite sample. In addition, the sensitivity to damage occurring at different locations in the composite was also investigated by comparing electrical impedance tomography spatial conductivity maps obtained for specimens with sets of holes drilled at different locations in the sensing area. Finally, the location and severity of damage from low-velocity impact events were detected using the electrical impedance tomography method. The work presented in this study indicates a paradigm shift in the available possibilities for structural health monitoring of fiber-reinforced polymer composites. © The Author(s) 2013.