Detection of planar subsurface defects in fiberglass plastic composite panels by optical-acoustic method

An optical-acoustic method for detecting subsurface defects in laminated composite structures is considered. The method is based on a new approach to detecting such defects by forming and visualizing the mode structure of oscillations of a thin layer of material (region of interest (ROI)) located directly above the defect. The oscillations of the ROI occur at frequencies corresponding to the resonant frequencies of an edge-clamped thin membrane excited by flexural ultrasonic (US) waves. To technically implement the method, an experimental breadboard of a hybrid optical-digital system was created, in which laminated composite specimens are excited by harmonic US radiation in the frequency sweep mode in the range of 5-150 kHz. For research, laminated fiberglass panels with square and round planar subsurface defects were used. A series of experiments was carried out to detect square defects with dimensions of 20x20 mm2 located in composite panels at different depths. It is shown that spatial responses from defects correspond to the mode structure of the nodes of a rectangular membrane at its fundamental and multiple resonant frequencies. Dependences between the depth of the defects and their resonant frequencies are obtained. They indicate a monotonous increase in resonant frequencies with an increase in the depth of the defect. The main reasons for the deviations of the experimental results from the values of calculating the fundamental and multiple frequencies for the planar square subsurface defect using known formulas are analyzed.