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 &lt; p-value &lt; 0.58), but significantly higher than those of Kapton specimens (0.0001&lt; p-value &lt; 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.</p

Proof of concept for a smart composite orbital debris detector

Space debris particles with dimensions smaller than tens of millimetres are not trackable with existing monitoring systems and have sufficient energy to harm orbiting Earth satellites during impact events. This paper presents a proof-of-concept for an in-situ smart carbon fibre reinforced plastic (CFRP) composite orbital debris detector that is capable of localising space debris impacts on Earth satellites and measuring the direction and velocity of debris particles. This spacecraft detection system can be used to warn satellites about the impact occurrence and to enhance current Space Surveillance Networks by providing a catalogue of debris objects. The proposed orbital debris detector consists of two thin parallel CFRP composite plates, each instrumented with three piezoelectric transducers embedded into the laminate. The localisation method is based on the measurement of acoustic emissions generated by debris impacts on the CFRP plates, which are processed with the time reversal algorithm. The calculation of the direction of debris particles and their speed are accomplished by determining the arrival time of acquired signals and the speed of waves propagating within each CFRP plate. Experimental results showed accurate estimation of the impact location, direction and velocity, thus demonstrating the potential use of the proposed orbital debris detector in future Earth satellite systems

Nonlinear ultrasonic inspection of smart carbon fibre reinforced plastic composites with embedded piezoelectric lead zirconate titanate transducers for space applications

Carbon fibre reinforced plastic composites used in spacecraft structures are susceptible to delamination, debonds and fibre cracking that may arise during manufacturing, assembly or in-service operations (e.g. caused by debris impacts in near-Earth orbital spaceflights). Therefore, in situ and real-time health monitoring is necessary to avoid time-consuming and unsafe visual inspections performed either on-ground or during extra vehicular activities. In this article, a recently created ‘smart’ carbon fibre reinforced plastic composite structure with embedded piezoelectric lead zirconate titanate transducers was used to detect multiple areas of artificial delamination and real impact damage of different size using nonlinear ultrasound. The electrical insulation of embedded piezoelectric lead zirconate titanate transducers was achieved by interlacing a dry layer of woven glass fibre fabric between the sensor and the carbon fibre reinforced plastic plies before curing. Damage detection was successfully demonstrated using both second harmonic generation and nonlinear modulation (sidebands) of the measured ultrasonic spectrum. The material nonlinear response at the second harmonic and sidebands frequencies was also measured with a laser Doppler vibrometer to validate the nonlinear ultrasonic tests and provide damage localisation. Experimental results revealed that the proposed configuration of embedded piezoelectric lead zirconate titanate transducers can be utilised for on-board ultrasonic inspection of spacecraft composite parts

Tensile and fatigue testing of impacted smart CFRP composites with embedded PZT transducers for nonlinear ultrasonic monitoring of damage evolution

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

Smart composite detector of orbital debris and micrometeoroids particles

The presence of space debris and micrometeoroids particles in the space environment are a serious threat for Earth orbiting spacecraft. Hypervelocity impacts (HVIs) at the typical velocities of ∼7 to 10 km/s can severely damage or destroy satellites, so that debris detection systems are necessary. In the present research work, a “smart” composite detector of orbital debris and micrometeoroids particles is proposed and developed as proof-of-concept for future space missions. The presented detector consists of two thin parallel carbon fibre reinforced plastic (CFRP) composite plates, each instrumented with three piezoelectric transducers embedded into the laminate. The developed algorithm can estimate both directions and velocities of orbital debris and micrometeoroids particles by the knowledge of: (i) impact locations on the two plates, (ii) the time differences of arrival of acoustic emissions generated by impacts and (iii) the wave velocity profile in the composite plates. The localisation of the impact events is achieved by time reversal method, while the time of arrivals are calculated using Akaike Information Criterion method. A set of experimental tests were performed to validate the proof-of-concept using a small drop tower. Impact results showed the high accuracy of the proposed algorithm in the estimation of impact locations, directions and velocities of impact objects

Machine learning for impact detection on composite structures

In order to overcome the current limitations of the impact localisation process in composite materials, such as the a-priori knowledge of the mechanical properties and the direction dependency of the wave speed, a novel method is here proposed based on the machine learning approach. The algorithm is formed by two steps: the first is the training process, in which a baseline consisting of the structural responses due to impact tests is acquired; the second one evaluates the impact location exploiting the highest cross-correlation coefficient, obtained after the interpolation of the impact response baseline using the Radial Basis Function (RBF) method. Numerous experimental tests are performed on a simple carbon fibre reinforced polymer (CFRP) plate fitted with three piezo-sensors at three different drop heights to validate the training process. The results showed high accuracy in both the reconstruction and the impact localisation, with an error less than 10 mm

Impact Localization in Composites Using Time Reversal, Embedded PZT Transducers, and Topological Algorithms

Time reversal is a powerful imaging processing technique that focuses waves at their original source using a single receiver transducer when diffusive wave field conditions are met. This has been successfully proved on various engineering components and materials using elastic waves with surface bonded transducers. This paper investigates the performance of time reversal for the localization of impact sources on fiber reinforced plastic composite structures with embedded piezoelectric sensors. A topologic approach, here named as minimum average method, is proposed to enhance the accuracy of time reversal in retrieving the impact location. Experimental tests were carried out to validate the robustness and reliability of time reversal against traditional topological approaches by altering impulsive responses contained in the baseline signals. Impact localization results revealed that time reversal and the new topological approach provided high accuracy in identifying the impact location, particularly in the presence of double impacts and material damage, which were not accounted during the initial training process. Results indicate that time reversal with embedded transducers has potential to be effective in real operating conditions, where alterations of acoustic emission responses in the baseline signals are less predictable