Abstract
Ultrasonic systems based on ‘smart’ composite structures with embedded sensor networks can reduce both inspection time and costs of aircraft components during maintenance or in-service. This paper assessed the tensile strength and fatigue endurance of carbon fibre reinforced plastic (CFRP) laminates with embedded piezoelectric (PZT) transducers, which were covered with glass fibre patches for electrical insulation. This sensor layout was proposed and tested by the authors in recent studies, proving its suitability for nonlinear ultrasonic detection of material damage without compromising the compressive, flexural or interlaminar shear strength of the ‘smart’ CFRP composite. In this work, CFRP samples including PZTs (G-specimens) were tested against plain samples (P-specimens), and their mean values of tensile strength and fatigue cycles to failure were found to be statistically the same (910 MPa and 713 000 cycles) using the one-way analysis of variance method. The same tests on P- and G-specimens with barely visible impact damage (BVID) showed that the corresponding group means were also the same (865 MPa and 675 000 cycles). Nonlinear ultrasonic experiments on impacted G-samples demonstrated that embedded PZTs could monitor the growth of BVID during fatigue testing, for a minimum of 480 000 cycles. This was achieved by calculating an increase of nearly two orders of magnitude in the ratio of second-to-fundamental harmonic amplitude. Finally, PZT transducers were confirmed functional under cyclic loading up to ∼70% of sample’s life, since their capacitance remained constant during ultrasonic testing.
Highlights
This paper assessed the tensile strength and fatigue endurance of carbon fibre reinforced plastic (CFRP) laminates with embedded piezoelectric (PZT) transducers, which were covered with glass fibre patches for electrical insulation
With reference to the ASTM D3479/D 3479M standard, this failure mode was valid. As it is shown in figures 8 and 9, there was not any noticeable difference between the two specimen groups (P- and G-specimens). This proved that the evolution of material damage in the G-samples was not affect by the presence of the internal transducers, and it was true for both cases of initial sample condition: pristine and impacted
The ultimate tensile strength of the Pand G-specimens was similar in both tests; ∼910 MPa for undamaged samples and ∼865 MPa for impact damaged samples
Summary
A large percentage (∼50%) of metal components that were traditionally used in aerospace structures has been replaced. This study focused on the assessment of (i) the mechanical performance of the ‘smart’ CFRP composite under cyclic loading, and (ii) the capability of the embedded PZTs to monitor the increase in damage size with repeated loading based on nonlinear ultrasonic testing. To achieve these two objectives, impact damaged CFRP samples containing pairs of glass fibre insulated PZTs (G-specimens) were subject to tension–tension fatigue testing, and their endurance was compared with that of plain samples (P-specimens). In relation to the nonlinear ultrasonic experiments, the PZTs inside the G-specimens were used for the propagation of elastic waves through the material, and the calculation of the nonlinear parameter b at different stages of fatigue testing
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