A hidden corrosion detection method based on robust multimodal Lamb waves

A Lamb wave is a kind of elastic wave propagating in a plate-like structure with stress distributing through the whole thickness of the plate. A Lamb wave is multimodal, and each mode has unique through-plate-thickness stress profiles, called wave structure. The existence of a specific structure flaw at a certain position in the thickness direction will weaken the ability of the plate to support a specific component of stress, which means each Lamb wave mode would interact with structure flaws in a different way, or that each Lamb wave mode has a unique sensitivity to flaws. Conventionally, a sensitive single mode has been selected for defect detection. However, due to the inconsistency in actuators or sensors, the robustness of detection is limited. In this paper, the difference in the sensitivity of each mode to flaws, rather than a single mode, was used for defect detection. Based on this idea, numerical and experimental studies of the interaction between hidden corrosion and multiple modes were first conducted. Then, a new index, the relative amplitude coefficient, which evaluates the difference in sensitivity between lowest-order modes and higher-order modes, was proposed. Next, by using a double crosshole array and combining it with a probabilistic reconstruction algorithm, a multimodal Lamb wave corrosion detection method was developed. The results show greater robustness in corrosion location and better accuracy, compared with the location results of conventional single mode defect detection. Furthermore, by applying the double sensor strategy, the effective operating frequency range is extended and the location accuracy enhanced.