Vibration-Based Non-Destructive Evaluation of Internal Damage in Foam Cored Sandwich Structures Using Wavelet Analysis

Modern mechanical and civil structures are increasingly designed using polymeric composites that ensure great strength-to-mass ratio and are resistant to various environmental interactions, like corrosion. The application of sandwich structures in the design of mechanical and civil constructions is determined by their good stiffness properties and very low mass, which is a very attractive combination. Due to their wide applicability, these structures should be properly maintained and diagnosed, and thus, appropriate non-destructive testing (NDT) methods should be developed in order to detect and identify various types of damage. Special attention should be paid to internal damage (damage to the core of a sandwich structure), which cannot be detected during a visual inspection. In the paper, an NDT method based on modal analysis and further processing of modal shapes using a wavelet transform is proposed. Three sandwich structures with damage to the core of various types were experimentally tested using modal analysis, the damage positions were detected and identified using wavelet analysis, and verified by comparing the results of previously performed thermographic tests. The obtained results show a high effectiveness of the proposed approach, which could find an application in the industrial inspection of sandwich structures in a non-destructive and non-contact manner.