Redirection of Lamb Waves for Structural Health Monitoring

Currently, structures are designed without structural health monitoring (SHM) in mind. It is proposed that SHM should be addressed at the design stage of new structures. This paper explores the benefit which can be gained from such considerations. The scope encompasses Lamb-wave-based SHM and a given fatigue critical location (FCL). Optimization is performed using specialised ray tracing. A case study is carried out using a specimen that simulates a hard-to-inspect region in a fuel vent hole in wings structures of aircraft. This work will report on the potential use of the focussing of stress wave to improve detectability of defect in this hard-to-inspect location. Following optimization, results are produced numerically and experimentally. The results revealed sensitivity to damage is nearly doubled while minimum detectable damage size is significantly decreased. As a result, this study brings together an assortment of specialised tools to form a workflow ready for implementation. 1. Introduction Propagating Lamb-wave-based structural health monitoring methodologies have been widely reported to be convenient and efficient in detecting fatigue cracks in metallic structures [1, 2] and disbonds and delaminations in composite structures [3]. The propagating characteristics of Lamb waves in metals [4] and multilayered materials [5] are well documented. An attractive feature of this stress-wave-based method of structural health monitoring is that it can be used in “hard-to-reach” areas because the actuators and the sensors required can be bonded onto the structure. This is commonly known as “insitu structural health monitoring” (ISHM). The investigators of this project are amongst the many others who have contributed to the development of the Lamb-wave-based method of ISHM. Many authors have published on optimizing and improving probability of detection (POD) and damage sensitivity. However, the works published have been focused on topics such as sensor placement, sensor selection, frequency selection, data capture, and data analysis. Such topics indicate that the structure itself is not included as a part of the optimization. SHM is not a part of the design process, rather an afterthought. A successful SHM friendly structure can be designed with less tolerance and therefore less material and weight. Consequently, the weight gained from implementing SHM systems can be offset. A variety of tools acquired over former years have been applied to this problem. Ray tracing specialised in propagating Lamb waves has been used to redirect waves. Laser vibrometry and