Non-destructive damage localization in built-up composite aerospace structures by ultrasonic guided-wave multiple-output scanning

This paper focuses on defect localization in a composite skin-to-stringer stiffened panel representative of modern aircraft construction by an ultrasonic scanning system using multi-channel transfer function extraction. An especially made mini-impactor is utilized as the excitation to generate a broad wave frequency range. At each scanning position, multiple excitations are concatenated to extract the Guided-Wave Structural Transfer Function (GWSTF) between two points of the test structure. This extraction is performed using an averaged, normalized cross-power spectrum operation that minimizes the influence of the excitation and that of uncorrelated noise. The GWSTF extracted from different sensor pairs is then related to the presence of damage at different locations. The system is thus able to locate damage in the panel in 2D (along and across the stringer direction), in a substantial improvement over a 1D scanning system developed by the authors previously. The system is tested on damage in the form of added masses and real damage in the form of through-thickness stringer notches. The results show good detection and location of damage, suggesting the potential for a practical inspection system targeting internal damage in aircraft structures when probed from the outer skin side.