Embedded piezoelectric transducers in carbon fibre composites for nonlinear ultrasonic applications

There has been a growing interest over the last few years in developing "smart" carbon fibre reinforced plastic (CFRP) composites with integrated piezoelectric lead zirconate titanate (PZT) transducers, which are able to provide real-time ultrasonic monitoring of aircraft components. However, piezo-sensors embedded in CFRP laminates require electrical insulation layers due to the electrical conductivity of carbon fibres, which may compromise the mechanical properties of the composite. Typically, non-conductive materials such as polyimide (Kapton) films or polymer coatings are used as interlayers between the sensor and the composite ply for electrical insulation. Nevertheless, these materials increase the level of material distortion around the PZT and can dramatically reduce the adhesion between laminate plies. Thus, the structural integrity of the component may significantly be affected. This paper proposes a novel design of embedded PZTs in CFRP composites, which consists of insulating the conductive surface of the PZT with a thin layer of plain glass fibres. In this research work, the effect of PZT embedment on the flexural and interlaminar shear strength of CFRP laminates was experimentally investigated through long-beam and short-beam three-point bending tests. A failure analysis inspection of the laminates subject to mechanical loading was also conducted using an optical microscope. Test results revealed that the proposed configuration had no impact on the interlaminar properties of the CFRP laminate. In particular, the difference in maximum stress and deflection to failure between the laminates with embedded PZTs and plain laminates was less than 3%. Moreover, a pitch-catch nonlinear vibro-acoustic experiment was performed to study the sensitivity in damage detection of the proposed configuration of embedded PZTs compared to the case of conventional surface-bonded PZTs. The results obtained from CFRP laminates with in-plane delamination showed a higher level of second harmonic generation at the local damage resonance (LDR) frequency using this configuration of embedded PZTs. Besides applications on in-service monitoring of aircraft components, such "smart" composites have the potential to be used within other engineering sectors.