Lamb Wave Propagation Across a Lap Joint

Abstract

Lap joints are common elements of aircraft and other engineered structures. They are often subject to hidden defects which are caused by corrosion and fatigue, and are very difficult to detect. Development of accurate and efficient methods for the early detection of corrosion and fatigue cracks in lap joints is of considerable current interest. Ultrasonic techniques using guided waves offer the possibility of improving the technology of detecting and characterizing flaws within lap joints. It is well known that the characteristics of guided waves can be used to detect defects in plates [1]. However, the geometry of the lap joint makes it difficult to extend these techniques to lap joints. In this paper we consider the theoretical problem of the propagation of guided waves across a simple model of the lap joint in an effort to understand the interaction of the guided waves with the geometrical features of the lap joint. The geometry of the lap joint, including the vertical stress free boundaries, the rectangular corners, and the change in thickness, makes it impossible to derive a closed form solution to the problem of wave propagation across it. The problem can only be attacked by numerical methods. Conventional finite element methods fail when dealing with problems with infinite domains. In order to handle problems with local inhomogeneities or irregular shapes in an infinite domain, hybrid methods must be used.