Prognosis of Lamb Wave Reflections in Complex Geometry Structures

Abstract

Lamb waves are highly used in Structural Health Monitoring systems and are commonly studied to investigate the mechanical properties and integrity of different structures. The reflections from the boundaries of any studied structure complicate any data post-processing techniques and it is almost impossible to eliminate such reflections when working with small-sized structures. In addition, the more complex the geometry is the more reflections from edges and bents appear. This study focuses on understanding Lamb wave's boundary reflections and the effect of a bent on the signal. Tests are done on aluminum plates of different sizes and geometries, a step towards moving to composite materials and more complicated structures. In this work, the boundary reflections of Lamb waves propagating a thin metallic plate are investigated and a new theoretical Lamb wave boundary reflection (LBR) theorem is proposed that accurately predicts all boundary reflections and generates a full field signal based on a virtual actuating and sensing network. The LBR theorem locates virtual actuators that are used to transform the distance covered by the wave during the boundary reflection into a direct distance from the virtual actuator to the sensor. Using commercially available software to generate the incident wave between an actuator and a sensor, the proposed boundary reflection theorem is then used to predict the overall signal including reflections from all boundaries. The predicted signals are first validated using finite element analysis which also assists to visualize and analyze the actuation, propagation, and reflection of the waves in the plate. Additionally, an ultrasonic piezoceramic contact transducer and non-contact laser Doppler vibrometer are utilized experimentally to verify the validity of the proposed technique. The predicted signals at different excitation frequencies have shown a very good agreement between the theoretically predicted, computational, and experimental results.