The barely visible impact damages reduce the strength of composite structures significantly; however, they are difficult to be detected during regular visual inspection. A guided wave based damage imaging condition method is developed and applied on a curved composite panel, which is a part of an aileron from a retired Boeing C-17 Globemaster III. Ultrasonic guided waves are excited by a piezoelectric transducer (PZT) and then captured by a laser Doppler vibrometer (LDV). The wavefield images are constructed by measuring the out-of-plane velocity point by point within interrogation region, and the anomalies at the damage area can be observed with naked eye. The discontinuities of material properties leads to the change of wavenumber while the wave propagating through the damaged area. These differences in wavenumber can be observed by deriving instantaneous wave vector via Riesz transform (RT), and then be shown and highlighted with the proposed imaging condition named wavenumber index (WI). RT can be introduced as a two-dimensional (2-D) generalization of Hilbert transform (HT) to derive instantaneous phases, amplitudes, orientations of a guided-wave field. WI employs the instantaneous wave vector and weighted instantaneous wave energy computed from the instantaneous amplitudes, yielding high sensitivity and sharp damage image with computational efficiency. The BVID of the composite structure becomes therefore “visible” with the developed technique.
A vision-based damage detection technique was proposed for the identification of damages in composite honeycomb structures. The motion above the damage area extracted from the wave field image with the developed image decomposition and image signal processing method reveals rich information to determine damage severity.
The standing wave prevailed at its resonant frequencies above the barely visible impact damage (BVID) on the surface of a CFRP/honeycomb composite sandwich plate, which was excited by a Q-Switch Nd:YAG pulse laser system for generating a broad-band guided wave, and the wavefield was captured by a Laser Doppler Vibrometer (LDV). With the developed image processing technique, the wavefield image that contains incident waves, reflected waves and standing waves could be separated from different wavenumber vectors and propagating directions. Phases, orientations and resonant frequencies derived from the separated standing wave were taken advantage of, for either emphasizing or magnifying the motion and illustrating the modal behavior on the damage surface. The barely visible impact damage (BVID) of the composite structure was therefore “visible” with the developed technique.