Characterisation of smart CFRP composites with embedded PZT transducers for nonlinear ultrasonic applications

Embedded piezoelectric lead zirconate titanate (PZT) transducers in carbon fibre reinforced plastic (CFRP) composites are typically electrically insulated by interlaying materials such as polyimide Kapton films between the PZT and the laminate ply. However, the presence of polymeric films may cause debonding at the layer interface, thus reducing the structural performance. This paper proposes an alternative insulation technique in which PZTs are covered by a thin patch of woven E-glass fibre fabric for enhanced adhesion with the surrounding epoxy matrix. An analysis of variance on experimental test results showed that the compressive, flexural and interlaminar shear strengths of plain CFRP specimens were equal to the means of the smart CFRP composite (0.41 < p-value < 0.58), but significantly higher than those of Kapton specimens (0.0001< p-value < 0.05). Moreover, a post-test fractographic analysis indicated that damage opening in Kapton specimens was significantly larger (p-value = 0.03) than that in plain specimens. Brooming failure compression was also the same for the smart CFRP composite and plain samples, whereas Kapton specimens failed by through-thickness shear. Finally, nonlinear ultrasonic experiments were conducted on CFRP laminates with artificial in-plane delamination using glass fibre insulated PZTs. Remarkably, the effect of second harmonic generation was found to be nearly two times higher than conventionally surface-bonded PZTs.