Macro and micro delamination detection in adhesive structures: A synthesized linear and nonlinear air-coupled ultrasonic technique

Adhesive structures are widely used in industries. During manufacturing and operation, both macro and micro delamination between different materials or layers may occur within these structures. Despite the practical use of air-coupled ultrasonic testing (ACUT) techniques for detecting such delamination, the interaction mechanisms between elastic waves generated by air-coupled transducers and these two types of delamination have not yet been fully understood. Additionally, there is a lack of accurate identification methods for either macro or micro delamination. To address these issues, this study proposes a synthesized linear and nonlinear ACUT approach for detecting and identifying both types of delamination in adhesive structures. Using finite element simulations with a coupled ACUT model, the study thoroughly examines the interaction between ACUT-induced elastic waves and the delamination types. The results show that macro delamination causes wave reflection and transmission, while micro delamination leads to contact acoustic nonlinearity. The proposed method is validated through experiments and shown to be effective for detecting and identifying both types of delamination. The results of the study contribute to the advancement of ACUT theory and applications, elucidate complex wave features and pave the way for more sophisticated non-destructive evaluation techniques.