Visualization of Delaminations in Composite Structures Using a Baseline-Free, Sparse Array Imaging Technique Based on Nonlinear Lamb Wave Propagation

Environmental factors such as temperature and humidity influence the efficacy of defect imaging procedures based on the identification of changes between an intact state and the current state of a sample/component/structure in the presence of a defect. In this paper, we focus on the Reconstruction Algorithm for Probabilistic Inspection of Damage (RAPID) and propose a nonlinear Lamb wave version of RAPID to visualize a delamination in a composite structure without having to know anything about the intact state, i.e. a baseline free RAPID. Once the optimal frequency selection of Lamb waves in a pitch-catch configuration mode is performed, low and high excitation amplitude signal responses within a sparse array at that frequency are evaluated along each transducer-receiver path by analyzing a set of damage sensitive parameters: the correlation coefficient, the energy of the scaling subtracted signal, and the third harmonic ratio. Processing of this information leads to a corresponding probabilistic damage map of the area within the sparse array. The obtained results from a validation experiment demonstrate the capability of this nonlinear variant of RAPID for the identification of a delamination in a composite structure.