Application of Wavelet Parameters for Impact Damage Detection in Plates

In this study, ultrasonic guided waves were used in a laser vibrometer set-up to measure impact damage responses in carbon fibre epoxy composite plates. The impact energies used resulted in internal delaminations, as well as small hairline cracks observed on the surface. By identifying a change in the baseline signal compared to the damage response signal and quantitatively characterising it, the impact damages in the specimens could be detected. Damage response could be determined from reflected peaks in the signal arriving after the initial pulse; however, the major difference found was a change in signal amplitude. In order to improve the observation of these changes, wavelet analysis was applied to the signals. Wavelet analysis provided efficient means of removing noise in the signals while allowing for useful data to be extracted from both the time and frequency domains. The maximum wavelet coefficient changes were also easily identified after normalisation to allow for the development of a quantitative index that corresponds to the damage. By relating the absorbed impact energy and the physical damage size to this index parameter, a method of characterising the damage was produced. In general, it was also found that the larger the impact energy and therefore the larger the physical damage area, the greater the calculated value of the Damage Index. This thus provides potential means of quantifying the impact damage in composites with improved efficiency compared to raw signals in the time domain.