Nondestructive Evaluation of Composite Substrate Below Thick Top Coat Materials

The feasibility of two complementary time-domain terahertz (THz) reflection non-destructive evaluation (NDE) methods to detect sub-surface bulk damage in conductive composite aircraft outer mold line (OML) panels covered with thick specialty coatings will be demonstrated. The first method, traditional THz reflection tomography, is an electromagnetic analog of pulsed ultrasound. The coatings are transparent to the THz pulses, and a THz C-Scan image can be made of the subsurface coating/composite interface. The THz reflection C-Scan image shows any disruption to the conductive coating/composite. The novel second method of THz pulse displacement ultrasound C-Scan imaging detects damage to the bulk composite that does not manifest itself on the conductive coating/composite interface. Traditional ultrasound C-Scan imaging would require removal of the coating over the scan area because the coating is highly attenuative to high frequency ultrasound. The proposed method uses a single stationary lower frequency transducer to generate an ultrasound wave in the bulk composite, minimizing or eliminating coating removal. The microscopic displacement of the THz beam generates an ultrasound A-Scan beneath the coating at the interface with the composite. The THz pulse beam acts as a non-contact sub-surface microphone that is raster scanned over the panel to generate a C-Scan.