Load and damage measurements on fibre-reinforced composites with embedded piezoelectric sensors

Abstract

Fibre-reinforced composites are increasingly being used in maritime structures. Piezoelectric sensors can be used to monitor both loads acting on a fibre-reinforced composite structure and elastic waves emitted from damage initiation within the structure. In certain marine applications, due to a harsh environment or for hydrodynamic reasons, piezoelectric sensors cannot be placed on the surfaces of the structure. Fortunately fibre-reinforced composite materials allow piezoelectric sensors to be embedded inside the structure. The focus in this research is on embedded piezoelectric sensor design, performance and behaviour. Carbon fibre beam specimens with embedded piezoelectric sensors have been manufactured and subject to low-frequency bending, high-frequency elastic wave emission, electrical excitation and monotonic bending to failure. The low-frequency bending test has given insight in the effects of sensor discharge, that is, decreasing and distorting the sensor response. Also it has been used to check consistency between different specimens. The effect of electrical discharge is practically absent around 1.8Hz but becomes apparent at lower frequencies. The test was simulated numerically. The simulation captures the effects of electrical discharge but underestimated sensor response amplitude by 40% to 65% for the small and large sensor respectively. High-frequency elastic wave emissions, originating from fibre-reinforced composite failure during monotonic bending, have been measured by an embedded piezoelectric sensor. The signals measured by the embedded piezoelectric sensor have a median signal-to-noise-ratio of 17dB. Using the spectral element method, a parameter study has been performed for a similar, two-dimensional setup. Embedded piezoelectric sensor length and embedment location in the plate thickness has been varied. Both a symmetric and antisymmetric elastic wave have been excited. It has been found that increasing sensor size has a negative effect on the sensitivity of the sensor. The antisymmetric elastic wave is better captured by the sensor response than the symmetric elastic wave. Effects of electromechanical resonance were not noticeable. Monotonic four-point bending failure tests were performed to compare structural integrity of a carbon fibre-reinforced beam specimen with an embedded piezoelectric sensor to a specimen without sensor. With embedded specimens, an average stiffness drop from 3 to 8% is noted. Specimens with small sensors on the tension side of the composite had a decrease lower than 4% in ultimate strain and at maximum 8% reduction in ultimate load and failure toughness. For specimens with embedded sensors on the compression side, the decline was larger, with the larger sensor outperforming the smaller sensor.