Ultrasonic guided wave tomography for damage detection in harsh environment

Guided wave tomography is an attractive tool for the detection and monitoring of the critical area in a structure. Using signal difference coefficient (SDC) as the tomographic feature, RAPID (Reconstruction Algorithm for the Probabilistic Inspection of Damage) is an effective and flexible tomography algorithm. In this algorithm, signal changes are exclusively attributed to the structural variation. However, external environmental factors (e.g. operating temperature, sensor bonding agent aging, rain) also change signals significantly. Particularly, the presence of anti-symmetric mode with a predominant out of plane displacement makes it very sensitive to the interferences like water loading or oil loading and leads to false alarms. In this paper, Lamb wave is excited in the low-frequency range. As a result, only the fundamental modes A0 and S0 exist. More importantly, the significant difference in group velocity between the two modes makes it possible to separate them in time domain. Benefit from that, a new method is proposed to extract pure S0 mode signal from the raw measurement data to improve the algorithm in interference (i.e. water loading) resisting. The results of the experiment show that the improved algorithm has the capability of providing accurate identification of damage in the presence of water loading.