An in-situ real-time hidden damage inspection on C-17 Globemaster III composite aileron using LSP technique under thermal excitation

A non-contact, full-field vision-based non-destructive inspection (V-NDI) system was developed with multiple damages detection capabilities in composite structures under thermal excitation. In contrast to point-based nondestructive inspection (P-NDI) systems employing laser Doppler vibrometer (LDV) with discrete wavefield captured by pointwise scanning, the V-NDI system captures higher spatial resolution wavefield by a CMOS camera for every time instance without repeating the experiment tens of thousand times to reassemble the wavefield like P-NDI. An Advanced Damage Processing Network (ADPNet) was proposed with laser speckle photometry (LSP) employed in a V-NDI system for hidden damages inspection. The LSP/ADPNet system relies on observing the variation of speckle clouds in time sequence without a baseline and is very insensitive to ambient noise with statistics-based image processing where traditional holography/ESPI suffers greatly. Other advances of LSP/ADPNet system are its robust tolerance of laser coherence, larger illumination area, flexible choice of correlation functions, and more advanced post-processing techniques such as Bayesian updating/inference or unsupervised image segmentation that can be readily applied. Thermal excitation can have very large power throughput in hundreds of watts compared to traditional PZT actuator, merely in a few watt ranges. Laser speckle itself is the result of self-interference scatter field reflected from a rough surface, and each speckle can be treated as a sensing point from a randomly distributed speckle cloud. By observing the variation of speckle cloud on the structure surface, displacement related quantities in higher dimensions (e.g. hypercomplex envelope, phase between real-valued signal and its quadrature, phase congruency, etc.) can be deduced by the Riesz bp transform and correlated in time sequence to highlight the location of hidden damages in a very effective way. Another novelty of this paper is to make a super compact real-time LSP system on LabVIEW FPGA by applying ADPNet comprised of the Riesz bp transform, non-linear filter bank and unsupervised image segmentation to quantify/characterize barely visible impact damages (BVID) on a C-17 Globemaster III composite aileron. To conclude, the images processed by LSP/ADPNet of a V-NDI system show a very good agreement with ultrasonic C-scan and pulse laser/LDV wavefield reconstruction results. It is also demonstrated to be more accurate and robust than Digital Image Correlation (DIC) for minute deformation (sub-nano to nano meter) measurement and large area (meter by meter) inspection under industrial environment.