CFRP composites with embedded PZT transducers for nonlinear ultrasonic inspection of space structures

Spacecraft structures are made of carbon fibre reinforced plastic (CFRP) composites due to their high strength-to-weight ratio. However, material damage such as micro-cracks and delamination are likely to occur during spacecraft fabrication, assembly or on-orbit due to hypervelocity debris impacts. In the latter case, satellite components are visually inspected during time-consuming and risky astronauts’ extravehicular activities. Hence, there is a need for real-time monitoring of cracks in spacecraft composites, especially for future manned missions. The integration of piezoelectric lead zirconate titanate (PZT) transducers in CFRP composites is a possible solution for the development of “smart” structures capable of (i) providing in-situ ultrasonic monitoring of damage, and (ii) preventing the direct exposure of PZTs to the harsh outer space. In a previous study, the use of a woven E-glass fibre fabric layer between the PZT and the CFRP plies was proposed as a suitable technique for electrical insulation of embedded PZTs with no effect on the interlaminar properties of the composite. Nonlinear ultrasonic experiments on artificially delaminated CFRP plates revealed that the damage sensitivity based on the second harmonic generation was nearly two times higher than with conventionally surface-bonded PZTs. In this study, nonlinear ultrasonic experiments on CFRP test samples with both artificial (in-plane delamination) and real impact damage proved the capability of the proposed embedded PZTs to detect multiple defects of various dimensions. The ultrasonic response of damaged specimens was studied against that of a pristine one, and damage detection was achieved based on the generation of second harmonics at specific input signal frequencies. In addition, by scanning the material response with a laser Doppler vibrometer it was verified that for each of the chosen driving frequencies, the area on the sample’s surface at which the out-of-plane vibrational velocity was higher matched the position of the associated damage. Based on the results of this study, the novel sensor embedding technique has the potential to be used for in-service monitoring of composite spacecraft components and other critical engineering structures.