Baseline-Free Damage Diagnostic Imaging Approach Relying on the Extraction of Converted Modes of Ultrasonic Guided Waves

Abstract

Large-scale plate-like structural components have been widely used in various aerospace structures. Subject to complex loading conditions and environmental factors, the integrity of plate-like structures is threatened by the occurrence and accumulation of structural damages, calling for the development of structural health monitoring (SHM) methods and techniques. Guided wave-based SHM methods have been demonstrated with a high sensitivity to small-scale damage. In particular, the probability-based diagnostic imaging (PDI) algorithm is able to achieve enhanced accuracy and efficiency of damage localization. Typically, the PDI defines damage index (DI) in terms of the deviation between the baseline and real-time guided wave signals. However, as the baseline signals are highly sensitive to environmental factors, such as temperature, the DI may exhibit large instability subject to varied environmental factors and lead to inaccurate damage detection results. In this study, a novel damage diagnostic imaging approach free of a baseline signal is presented for nonpenetrating damages. The approach is established based on the modification of conventional baseline-dependent PDI, into which the converted mode extraction (CME) strategy is integrated to give rise to the DI in terms of the energy of the converted modes. As the essential part of the CME-PDI approach, the converted modes caused by nonpenetrating damages as structural thickness variation are extracted from the primitive multimode wave signals according to a recently proposed CME method featured by intrinsic basis mode reconstruction. The effectiveness of the approach is first examined numerically, and then an experiment is carried out by taking into account the temperature effect. Compared with conventional PDI, the CME-PDI approach shows apparently enhanced accuracy and stability in damage localization.