Durability of Piezoelectric Wafer Active Sensors (PWAS) for health monitoring of composite structures

The aim of this Masters of Engineering (MEng) - Aerospace thesis was to investigate the durability of Piezoelectric Wafer Active Sensors (PWAS) for the health monitoring of composite structures.<br><br>One significant challenge faced by the aviation industry is the cost and time associated with maintaining aircrafts, in particular, aging airliners. Structural Health Monitoring (SHM) is a possible approach to reduce the high cost of inspection and maintenance. However, before SHM systems can be fully utilised in aerospace structures several questions need to be answered, in particular, concerns about their long-term durability and survivabilityAfter a comprehensive literature review, three specific areas were identified as warranting further investigation, namely, static and fatigue loading, impact events and numerical modelling.<br><br>For the experimental investigation, a carbon fibre/epoxy composite host was chosen with PWAS bonded to the surface and/or embedded in the mid-plane of specimens. A method for embedding the PWAS into the composite laminates was successfully developed. The performance of the sensors under different external loading was then monitored using the capacitance, output voltage and electro-mechanical impedance of the sensors..<br><br>Static and fatigue four-point bending tests were performed to study the durability of PWAS under tensile and compression strain. Both the surface-bonded and embedded sensors were used in these experiments. Those sensors subjected to compression strain suffered from cohesive failure under both static and fatigue testing. Under static testing, those PWAS bonded to the tensile surface of the host composites exhibited an initial increase in the capacitance followed by almost no change at higher strain levels. Under fatigue loading, the PWAS subjected to tensile strain performed much better than those under compression loading.<br><br>The effects of impact events on surface-bonded and embedded PWAS was also investigated. Due to their brittle nature, both composites and PWAS are extremely susceptible to impact loading, which for the case of aircrafts could be caused by events such as bird strikes, hail or tyre debris. Tests were conducted with the impact load directly over the PWAS and at a designated distance away, with different impact energy levels. Results showed that sensors bonded on the back face or embedded into the composite could survive impact energies up to 7 joules, after which significant changes in their performance, as well as physical cracking, was found to occur.<br><br>The final part of this investigation was focused on numerical modelling, in particular, developing a methodology to distinguish between sensor degradation and structural damage. The numerical model consists of a 3D representation of a free PWAS and a host composite laminate with a bonded sensor while the impedance for both cases was analysed. Findings were compared with the experimental results and good agreements attained for both instances. Using the bonded model as a base, three modes of damage were then investigated which included, debonding of the sensor, structural damage with delamination in the composite and a crack in the PWAS.<br><br>The Master's thesis concludes with a summary of the major finding from this investigation and provides several recommendations for future research into PWAS durability.