Baseline-free guided wave imaging via adaptive source removal

Structural health monitoring systems employing in situ guided wave transducers are being considered for civil, mechanical, and aerospace applications. In particular, guided wave-based imaging methods have been applied to graphically depict the area of interest and localize the damage. Most existing imaging techniques have relied on scattered waves extracted by baseline subtraction to quantify the effects of damage on the guided waves. In many cases, however, baseline data may not have been recorded or may be ineffective for baseline subtraction because of uncompensated variations between baseline and in-service signals. Proposed here is an alternative approach whereby source signals are first adaptively estimated from the in-service signals, direct arrivals are then calculated by forward propagation, and scattered signals are obtained by subtraction of the direct arrivals. Images using these scattered signals can then be created by, for example, delay-and-sum methods. This approach is demonstrated using synthetic signals generated by ray tracing and conventional delay-and-sum imaging. Its feasibility is further demonstrated with experimental data recorded on an aluminum plate using a single in situ transducer as a source and a laser vibrometer to record guided signals around the periphery of the region of interest.