Wirelessly monitoring structural defects
Most research and development has focused on the diagnostic phase of SHM, including damage events and locations. The prognostic phase is equivalent to preventive medicine, identifying or predicting faults to save time, money and lives. EU-funded scientists working on the project 'Wireless/integrated strain monitoring and simulation system' (WISMOS) combined sensor-based diagnostic systems with computational structural mechanics-based predictive systems for an integrated diagnostic–prognostic SHM system (DPS). The built-in sensor–actuator networks detect in real time, the location and extent of structural discrepancies between actual structures and original designs. They also monitor the effects of structural usage and plan cause-based inspection (as opposed to periodically timed ones) to implement corrective measures before airworthiness is compromised. Conventional strain sensor systems require a large number of shielded wires as well as numerous modifications to existing aircraft structures. Scientists developed the wireless ZigBee sensor system to minimise such issues. This system was developed using an off-the-shelf strain gauge, sensor data transducer and a ZigBee wireless chip to communicate with a personal computer. Associated far-field strain mapping software was also created to match physical sensor locations to nodal locations in an advanced progressive failure analysis durability and damage tolerance numerical model. For compression testing, scientists chose a composite stiffened panel representative of fuselage and wing components, and utilised both a healthy panel and one with a diamond cut. Performance of the DPS with surface-mounted wireless ZigBee sensors was compared to that of conventional wired strain-measurement equipment. The acquired signals were in good agreement and, importantly, in the case of the diamond cut panel the DPS recorded higher strain reflecting the location of a sensor near a large delamination crack. WISMOS DPS technology has been successfully validated and future research should focus on both in-flight testing of the surface-mounted sensor system and strain-sensor embedding. In the meantime, applications abound in composites for commercial, military and space flight and in components for buildings, ground vehicles, ships and more.