Application of a reference-free damage detection technique to complex structures

Abstract

A new Lamb wave based nondestructive testing (NDT) technique, which does not rely on previously stored baseline data, has been developed for crack monitoring in plate-like structures. The reference-free NDT technique takes advantage of two collocated lead zirconate titanate transducers (PZTs) pairs placed on the top and bottom surfaces of a plate. In a thin uniform elastic medium such as an aluminum plate, the formation of a crack or the change in thickness causes propagating Lamb modes to be transformed to other modes. This phenomenon is called mode conversion. In the previous study, it has been shown that converted Lamb wave modes due to crack formation can be extracted form instantaneously measured signals. In this study, the proposed approach is further advanced so that it can be also applied to plate-like structures with complex structural features such as holes, stiffeners and varying thickness. The applicability of the proposed technique to a plate with through-the-thickness holes is tested first. An array of holes creates multiple reflections and refractions of Lamb waves, and these multiple reflections and refractions may cause difficulties in analyzing measured Lamb waves. However, because the through-the-thickness holes do not produce mode conversion, it is shown that these holes do not affect the performance of the proposed technique. Next, the applicability of the proposed technique to a plate with a stiffener is validated. Due to the stiffer in the wave path, converted Lamb waves modes and additional reflections are produced even in the absence of damage. A signal processing techniques is developed so that mode conversion produced by a crack can be differentiated from that caused by the stiffener. Finally, the effect of thickness variation on the reference-free damage detection technique is investigated. Especially, the effect of the symmetrically tapered thickness on Lamb wave propagation is discussed. When Lamb waves propagate through the tape...