Characterization of delamination and exfoliative corrosion using the ultrasonic pulse-echo method and the Compton backscattering technique

A Compton backscattering imaging system has been developed to obtain a cross-sectional profile of impact-damaged laminated composites and corroded lap-splices of aging aircraft, from the density variation of the cross section. A slit-type camera was designed to focus on a small scattering volume inside the material, from which the backscattered photons are collected by a collimated scintillator detector for interpretation of material characteristics. The cross section of the layered structure is scanned by moving the scattering volume through the thickness direction of the specimen.;A nonlinear reconstruction model is introduced to overcome distortion of the Compton backscatter data due to attenuation effects attributed to beam hardening, and noise from many sources including multiple scattering and irregular distributions of the fibers and the matrix in composites.;To quantitatively assess the impact damage in laminated composites, both Compton backscatter imaging and ultrasonic pulse-echo C-scan imaging have been performed on quasi-isotropic laminated composites which were impacted by a drop-weight tester. Delaminations masked or distorted by the first few delaminations in an ultrasonic C-scan image have been detected and characterized by the Compton backscattering technique, both in width and location, by application of the inversion analysis using the error minimization algorithm. The Compton backscatter technique has also been applied to a lap-joint in an aircraft structure in order to determine mass loss due to exfoliative corrosion of the aluminum alloy sheet skin. The mass loss of each layer has been estimated from a Compton backscatter A-scan to obtain the thickness of each layer, including the aluminum sheet, the corrosion layer, and the sealant.;The theoretical framework for the Compton backscattering technique presented herein is based on Boltzmann transport theory. Theoretical results are used for the characterization of delaminations in laminated composites and exfoliative corrosion in aging aircraft, through a deconvolution procedure using a nonlinear least-square error minimization method (BFGS method). It produces quantitative information such as location and width of planar defects (delaminations and corrosion) in layered structures, such as laminated composites and lap-splices in fuselages of aircraft. These defects generally cannot be detected by conventional NDE techniques, including the ultrasonic method.