Durability Assessment of Lamb Wave-Based Structural Health Monitoring Nodes

Structural health monitoring (SHM) is an emerging technology leading to the development of systems capable of continuously monitoring structures for damage to improve safety and reduce life-cycle costs. SHM involves integration of one or more nondestructive test methods into a vehicle in order to facilitate quick and accurate damage detection with minimal human intervention. Aerospace structures have one of the highest payoffs for SHM systems since damage can lead to catastrophic and expensive failures, and the vehicles involved undergo regular costly inspections. Current work in SHM has focused on damage detection methods and sensor optimization, however, the topics of durability, reliability, and longevity of these systems has not been sufficiently addressed. Experimental results from durability testing of piezoelectric Lamb-wave nodes (transceivers) are presented and a framework for developing SHM test standards is offered. Existing standards for the durability, reliability, and longevity of commercial and military aircraft components are identified, and the relation of their standards to SHM systems is discussed. These standards include susceptibility to environmental testing, mechanical durability, and electro-magnetic interference (EMI), as well as a host of other extreme aircraft conditions (shock, vibration, fluids, etc.). Using these existing standards, a test matrix to assess the durability of the SHM sensors is developed, as well as criteria to establish whether a sensor/structural system has been affected by the various environments. Lamb-wave sensors have been tested in a variety of environments— including high temperature and large strain—so that their operational envelop can be characterized. Future environmental testing will include low temperature, high humidity, fluid susceptibility, low-velocity impact, and high altitude (low pressure). While the aircraft component industry is in general well regulated, it is evident that there is a need for a supplemental standard geared specifically towards smart structure technologies. This would incorporate SHM and other embedded or surface mounted smart structure components and systems, including interactions between the smart/active component and the structure. The field of SHM has progressed significantly in recent years, and it will become critical to address these topics explicitly before SHM systems can be successfully utilized in prognostic applications.