Localization and Imaging of Micro-Cracks Using Nonlinear Lamb Waves with Imperfect Group-Velocity Matching

Nonlinear Lamb waves have attracted increasing attention for detecting and identifying microstructural changes in structural health monitoring. However, most identification methods that determine the damage locations based on the intersections of the elliptical loci will inevitably cause positioning errors due to the change of the group velocity before and after interaction with the damage. In this work, a method focusing on elliptical rings was proposed for localization and imaging of micro-cracks in a three-dimensional structure using nonlinear Lamb waves with imperfect group-velocity matching. The width of the elliptical rings can be determined by the degree of the group-velocity mismatching of nonlinear S0 modes. The mode pair S0-s0, satisfying approximate group-velocity matching, is mainly introduced by interacting with the micro-crack. The effectiveness of the proposed methodology for damage localization is verified by the experimental testing and numerical simulation. Although the length of the being-tested small crack (about 1 mm) is smaller than the wavelength of the incident fundamental Lamb wave (around 20 mm), it can be well identified and localized using nonlinear Lamb waves. The experimental results show that the proposed method enables more reliable localization of the small crack with the crossover areas, as compared with the intersections based on the ellipse method. Furthermore, a breathing crack not situated in the propagation path can also be well localized by the proposed method in comparison with those by the probability-based diagnostic imaging in the simulation cases.