Early Damage Detection in Composites by Distributed Strain and Acoustic Event Monitoring

Advances in the development of fibre reinforced polymer composites and their manufacturing techniques have led to their increased use as structural materials. They offer superior corrosion resistance and high specific strength and stiffness, compared to metals, while designing anisotropic structures can provide weight reductions. However, there is uncertainty associated with understanding the consequences of out-of-plane damage in composites. Non-destructive evaluation techniques are adopted in many cases, but represent significant down-time and labour costs. Attached and/or embedded structural health monitoring systems have shown promise in improving the reliability and safety of composites, while reducing lifecycle costs, and improving design and manufacture processes. In this work, a distributed optical fibre sensor is embedded in a six ply carbon fibre-epoxy composite laminate during fabrication, to give three strain sensing regions near the top, middle, and bottom surface of the laminate, which allow in-situ and real time monitoring of strain development in the panel during resin infusion and curing. Four point bending is then conducted on the plate. Acoustic emission events are collected using four bonded piezoelectric wafer active sensors, thus allowing the comparison of strain data with damage formation and growth during progressive loading cycles. Distributed strain data demonstrates the sensitivity of the optical fibre through-the-thickness of the panel. The top, middle, and bottom sensing regions clearly indicate the development of compressive, neutral, and tensile strain, respectively. The strain values obtained from the optical fibre are in good agreement with strain data collected by surface mounted strain gauges. Acoustic event detection suggests the formation of matrix (resin) cracks, with measured damage event amplitudes in agreement with values reported in published literature on the subject.