Lamb wave–based detection of a controlled disbond in a lap joint

Abstract

Lap joints are widely used across many critical structures such as aircraft and bridges. Lamb waves have long been proposed to monitor lap joints against defects such as disbonds. However, there are many challenges which must be answered to make use of Lamb wave technology. Frequency selection is often overlooked, and many authors will select a single frequency without knowing if other frequencies will result in better sensitivity. Another challenge is the features (mode conversion, attenuation, reflection) associated with damage are also inherent in a lap joint. This sharing of features can lead to confusion (false positive/negative) depending on the chosen damage detection strategy. Furthermore, almost all proposed methods require a baseline reading of the structure in its flawless state. Relying on a baseline reading can result in false positives due to shifts in sensor outputs caused by ageing and inconsistent environmental conditions. Instead of a baseline, this article proposes a technique which uses strategically positioned sensors to detect Lamb wave modes generated only in the presence of a disbond. The technique is first developed using a numerical study and then verified with an experimental study. Several frequencies are trialled and detailed in this article which shed light on the ideal frequency selection when using this method.