CFRP Composites with Embedded PZT Transducers for Nonlinear Ultrasonic Inspection of Space Structures

Transcription

1 CFRP Composites with Embedded PZT Transducers for Nonlinear Ultrasonic Inspection of Space Structures More info about this article: Christos Andreades and Francesco Ciampa* Department of Mechanical Engineering, University of Bath, Bath, BA2 7AY, UK *Corresponding Author: Abstract Spacecraft structures are made of carbon fibre reinforced plastic (CFRP) composites due to their high strength-to-weight ratio. However, material damage such as microcracks 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.

2 1 Introduction Carbon fibre reinforced plastic (CFRP) composites are extensively used in spacecraft structures because of the high strength-to-weight ratios, corrosion/fatigue resistance and design flexibility they offer, especially when compared to metals [1]. Moreover, when CFRP composites are used as the inner wall material in dual-wall shield systems on spacecrafts the ballistic performance is improved relative to allaluminium dual-wall systems of the same weight, and they can also be repaired easily after impacts using adhesively bonded patches [2, 3]. Spacecrafts in the lower earth orbit are susceptible to micro-cracks and delamination caused by hypervelocity impacts from micrometeoroids and pieces of orbital debris (MMOD) [4, 5]. Currently, satellite components are inspected only visually through long and dangerous extravehicular activities (EVAs) carried out by trained crew [6]. Therefore, monitoring of cracks and delamination in composite components of crewed spacecrafts is necessary. Over the past years, various non-destructive testing (NDT) techniques have been developed for the detection of material damages. Some examples include techniques based on linear ultrasonic wave propagation [7, 8], acoustic emission [9-11] and X-ray scanning [12, 13]. Although there is an ongoing research towards the automation of these techniques, currently they are labour-intensive, time-consuming and expensive [14]. Nonlinear elastic wave spectroscopy (NEWS) techniques such as those based on higher-harmonic generation [15-18], time reversal [19-21] and wave modulation [22-24] are also very popular due to their higher sensitivity over linear ultrasonic methods to detect damage at early stages of formation (e.g. micro-cracks, delamination and voids). NEWS techniques can also be implemented for the inspection of large structures using only few transducers for the propagation of guided ultrasonic waves [25]. Piezoelectric zirconate titanate (PZT) is the most common type of transducers because they are capable of converting changes in strain, pressure, force and acceleration into electrical signals, based on the piezoelectric effect [26]. Also, they offer fast sensing and actuation response, high stiffness and resistance to high temperatures [26, 27]. In previous studies, NEWS techniques were successfully applied for the detection of damage in composite materials, mainly using surface-bonded PZTs [14, 28, 29]. In the case of monitoring impacts on spacecraft components, external PZTs can be permanently damaged even if the impact energy is not enough to damage the monitored components [30]. For this reason, there is a growing interest in the development of composites with integrated PZTs capable of providing real-time ultrasonic monitoring of damage, and preventing the direct exposure of PZTs to the space debris clouds. It is believed that damage detection transducers integrated into the spacecraft shielding can help operations determine safe encounter distances from the threat [30]. For example, if shield integrity was confirmed good, more risky near approaches could be planned with higher science return [30]. In the past, researchers inserted PZTs between the plies of both CFRP and glass fibre reinforced plastic (GFRP) composites for structural health monitoring applications. [32-35]. However, the main challenge in the manufacture of smart CFRP composites is the need for insulation of embedded PZTs from the electrically conductive carbon fibres. In the majority of previous studies, insulation was achieved by interlaying polyimide (Kapton) films between the PZT and the CFRP plies [33-35]. It is known though that polymeric films such as Kapton and Teflon are commonly placed in composites to constrain the adhesion between layers during the 2

3 curing process, and hence simulate artificial delamination [36]. Therefore the presence of such films can affect the structural integrity of the composite. A previous study of the authors proposed an alternative insulation technique where a woven E-glass fibre fabric layer was interlaced between the conductive surface of the PZT and the CFRP plies, without affecting the interlaminar properties of the composite [37]. By conducting NEWS experiments on CFRP laminates with artificial delamination it was shown that the damage sensitivity of the proposed configuration of embedded PZTs, based on the second harmonic (A 2 ) generation was around two times higher than the sensitivity of the same PZTs that were bonded onto the composite surface. In this study, additional NEWS experiments were performed on a CFRP plate with two artificial damages and a plate with two impact damages. The aim was to assess the capability of the proposed type of embedded PZTs to detect multiple damages of different size on the same plate, based on the generation of A 2 harmonics due to damage excitation. In particular, by propagating elastic waves through the material at selected frequencies was expected to force the debonded layers at the damage location to either oscillate ( clapping motion) or move relative to each other ( rubbing motion) leading to the generation of nonlinear elastic effects that would be detected as higher harmonics (even and odd multiples) of the input signal frequency [38]. The main difference between the two types of damages is that the artificial damages cause debonding at a single interface (in-plane delamination) which is a common type of manufacturing defects whereas the impacts can cause delamination, fibre breakage and matrix cracking at multiple interfaces (through-thickness damage). Moreover, the out-of-plane vibrational velocity of the material surface was measured at the locations of damages using a laser Doppler vibrometer (LV) to examine whether the detection of A 2 harmonics at specific input signal frequencies occurred indeed due to excitation of the debonded layers. 2 Experimentation 2.1 Laminate Manufacturing Three 140 x 180 mm laminates were manufactured for this study using unidirectional carbon/epoxy prepregs (T800/M21) in [90 /0 /90 /0 /90 /0 ]s lay-up giving a thickness of around 3.5 mm. The plates were cured in an autoclave for 180 minutes at a pressure of 0.7 MPa and a temperature of 150 C with a ramp rate of 3 C/min. As illustrated in Figure 1, the laminates included two PZTs for the transmission and reception of elastic waves through the material. A woven glass-fibre fabric layer (10 x 10 mm) was also interlaced between the top surface of each PZT and the CFRP ply, for electrical insulation from the carbon fibres. The first laminate included two double-layered patches of different size that were made from Fluorinated Ethylene Propylene (FEP) release film. Specifically, each patch consisted of two square layers of FEP film (12µm thick) stacked one on top of the other. The double FEP patches were used to generate controlled artificial in-plane delamination. The PZTs and the FEP patches were embedded between layers 8 and 9 from the bottom. To minimise internal material distortion, the thin wires from to the anode and the cathode of the PZTs were directed outside the top surface of the plate through small slits on the CFRP plies, in the fibre direction of each ply (i.e. no fibre cutting was involved). The wires were connected to 50Ω straight Bayonet Neill-Concelman (BNC) plugs through low 3

4 noise cables (RG174/U). The other two laminates were identical to the first one but without the Figure 1: Dimensions of the CFRP plates used in the NEWS experiments. two double FEP patches. One was kept in pristine condition (control plate) whereas the other one was impacted at its centre with a hemispherical indentor of 20 mm diameter and used for the detection of actual damages. It must be noted that two damages of different size were created by applying two levels of impact energy (low and high level). Detection of the small impact damage was experimentally demonstrated before creating the big impact damage. In this paper, the first laminate is referred as artificially damaged (AD) laminate, the second one as undamaged (UD) laminate and the last one as impact damaged (ID) laminate. 2.2 Damage Evaluation Prior to performing the NEWS experiments, the AD- and ID-plates were subject to stepped linear C-scanning to evaluate the size of internal damages. That was achieved using a phased array system (National Instruments NI PXIe-1062Q) with a 128-element probe. The C-scan was performed in steps of 12 elements and the damages were assessed based on the signal amplitude. As depicted in Figure 2 and Figure 3, delamination was detected at the locations of the double FEP patches and the impact damages. In AD-plate, the size of the two artificial damages was very similar to the size 4

5 of the double FEP patches. In ID-plate, the small and big impact damages were approximately 18 mm and 32 mm in diameter. Figure 2: Assessment of the small (a) and the big (b) artificial damages in AD-laminate using phased array system - Images not to scale. Figure 3: Assessment of the small (a) and the big (b) impact damages in ID-laminate using phased array system - Images not to scale. 2.3 Experimental Procedure NEWS Experiments The experimental setup used in the NEWS experiments is shown in Figure 4. An arbitrary waveform generator (TTi TGA12104) was used to send a continuous periodic signal to the transmitter PZT, through a voltage amplifier (Falco Systems WMA-300) with a 50x amplification factor. The receiver PZT was connected to an oscilloscope (PicoScope 4424) which enabled monitoring of the time domain and the frequency domain of the received signal, at a sampling frequency of 2 MHz with an acquisition 5

6 time of 50 ms. Propagation of ultrasonic elastic waves through the AD- and the UDlaminate was performed simultaneously and the two FFT spectrums were directly compared (Figure 5). This allowed to distinguish the A 2 harmonics generated due to damage excitation from those generated due to noise (e.g. instrumentation noise). In fact, the A 2 harmonics detected in both FFT spectrums were considered as noise whereas those presented only in the FFT spectrum of the AD-laminate were related to the nonlinear response of the material due to excitation of the debonded layers. Figure 4: Illustration of the set-up used in the NEWS experiments. (a) (b) Figure 5: Frequency spectrum of the received signal in the UD-laminate (a) and in the AD-laminate (b) - Input signal of 60 V at khz. 6

7 The transmitted signal was swept from 20 khz to 500 khz in steps of khz, and higher-harmonic generation due to damage excitation was detected at two input signal frequency ranges; khz and khz. The khz and khz frequencies corresponded to the A 2 harmonics with the highest amplitude, and thus they were chosen as the excitation frequencies of the two FEP patches. However, these two frequencies did not necessarily correspond to the fundamental harmonics (A 1 ) with the highest amplitude (transmitter PZT/material excitation). The same experimental procedure was repeated for the detection of the two impact damages in the ID-laminate. The input signal frequency that caused excitation to the small damage was found to be 310 khz, and for the big damage 128 khz. In both the AD- and the ID-plates the amplitude of the received signal was measured at the A 1 and A 2 frequencies for five different input signal voltages (60 V, 70 V, 80 V, 90 V and 100 V). As it was expected, the results obtained from the AD-plate (Figure 6) showed that for both input signal frequencies (104.5 khz and khz) the A 1 and A 2 amplitudes were rising with increasing input signal voltage. The A 2 amplitude was around two orders of magnitude smaller than the A 1 amplitude. These observations were also valid for the results acquired from the ID-plate (Figure 7) at the driving frequencies of 128 khz and 310 khz. (a) (b) Figure 6: Amplitude of the received signal in AD-laminate at the fundamental (a) and second (b) harmonic frequencies - Input signals of 60, 70, 80, 90, and 100 V at and khz (a) (b) Figure 7: Amplitude of the received signal in ID-laminate at the fundamental (a) and second (b) harmonic frequencies - Input signals of 60, 70, 80, 90, and 100 V at 128 and 310 khz 7

8 2.3.2 LV Experiments The LV experiments were performed to verify that the chosen input signal frequencies were indeed associated with the excitation of the damages. The experimental setup of the LV is shown in Figure 8. The transmitter PZT was used for the propagation of continuous periodic signals of 300 V at khz and khz in the AD-laminate and at 128 khz and 310 khz in the ID-laminate. In each case, the outof-plane vibrational velocity of the plate surface at the A 1 and A 2 harmonic frequencies was measured around the location of the damage using the LV scanning head (Polytec PSV-400). Three-dimensional plots of the results (Figure 9) proved that in all cases the vibrational velocity was higher at the damage position and A 2 amplitude was approximately an order of magnitude smaller relative to the A 1 amplitude. The results also revealed that the in the AD-laminate, the input signal at khz caused excitation only to the big FEP patch whereas at khz only to the small FEP patch. Similarly, the small and big impact damages in ID-laminate were only excited at the 128 khz and 310 khz respectively. This confirmed that the input signal frequencies were chosen correctly. Figure 8: Illustration of the set-up used in the LV experiments. 8

9 (a) Small damage in AD-Laminate A1 freq. = khz A1 amp. = 305 μm/s A2 freq. = khz A2 amp. = 43 μm/s (b) Big damage in AD-Laminate A1 freq. = khz A1 amp. = 257 μm/s A2 freq. = 209 khz A2 amp. = 32 μm/s (c) Small damage in ID-Laminate A1 freq. = 310 khz A1 amp. = 7038 μm/s A2 freq. = 620 khz A2 amp. = 284 μm/s (d) Big damage in ID-Laminate A1 freq. = 128 khz A1 amp. = 2999 μm/s A2 freq. = 256 khz A2 amp. = 108 μm/s Figure 9: 3D representation of the out-of-plane vibrational velocity at the location of the small (a) and big (b) artificial damages, and the location of the small (c) and big (d) impact damages at the fundamental and second harmonic frequencies. 3 Conclusions This study demonstrated the capability of a novel configuration of embedded PZTs in CFRP composites to detect material damage. This embedding technique involves direct insertion of the PZTs between CFRP plies with the conductive surface of the PZTs being covered by a single layer of woven E-glass fibre fabric for electrical insulation. Pairs of embedded PZTs were used to perform NEWS experiments on two CFRP plates of the same dimensions and lay-up. One plate included two artificial damages and the other plate two impact damages. By propagating continuous periodic ultrasonic waves through the laminates, excitation of each damage was achieved only at a single input signal frequency. Damage excitation was detected based on the A 2 harmonic generation in the frequency spectrum of the received signal, and A 2 the amplitude of these harmonics was found to increase with increasing input signal voltage. In addition to the NEWS experiments, the material response at the chosen input signal frequencies was scanned with an LV. In fact, the out-of-plane vibrational velocity of the plate surface was measured at the A 1 and A 2 harmonic frequencies. The results 9

10 verified that in all cases the area at which the vibrational velocity was higher matched the position of the associated damage. The results of this study showed that the proposed type of embedded PZTs can be used to detect multiple damages of different size in composite plates which are excited at different frequencies. The experimental results also proved the ability of these internal PZTs to detect in-plane delaminations which are often caused due to manufacturing errors, as well as through-thickness damages (fibre breakage and matrix cracking at multiple layers) that usually occur due to impacts. Based on the above, this novel sensor embedding technique can be utilised to provide nonlinear ultrasonic monitoring of spacecraft composite components, without the risk of exposing the PZTs directly to the harsh outer space. References 1. A Francesconi, C Giacomuzzo, S Kibe, Y Nagao and M Higashide, Effects of high-speed impacts on CFRP plates for space applications, Advances in Space Research 50(5), pp , WP Schonberg and EJ Walker, Hypervelocity impact of dual-wall space structures with graphite/epoxy inner wall, Composites Engineering 4(10), pp , WP Schonberg, Studies of hypervelocity impact phenomena as applied to the protection of spacecraft operating in the MMOD environment, Procedia Engineering 204(1) pp 4-42, WP Schonberg, Protecting Earth-orbiting spacecraft against micrometeoroid/orbital debris impact damage using composite structural systems and materials: An overview, Advances in Space Research 45(6), pp , S Yashiro, K Ogi, T Nakamura and A Yoshimura, Characterization of highvelocity impact damage in CFRP laminates: Part I Experiment. Composites Part A: Applied Science and Manufacturing 48(1), pp , EL Christiansen, Meteoroid/debris shielding, NASA Johnson Space Center, pp 75-76, AS Birks, Nondestructive Testing Handbook 7: Ultrasonic Testing, ASNT Handbook, F Amerini and M Meo, Structural health monitoring of bolted joints using linear and nonlinear acoustic/ultrasound methods, Structural Health Monitoring 10(6), pp , F Cesari, V Dal Re, G Minak and A Zucchelli, Damage and residual strength of laminated carbon epoxy composite circular plates loaded at the centre, Composites Part A: Applied Science and Manufacturing 38(4), pp , M R Mili, M Moevus and N Godin, Statistical fracture of E-glass fibres using a bundle tensile test and acoustic emission monitoring, Composites Science and Technology 68(7), pp , AR Oskouei and M Ahmadi, Acoustic emission characteristics of mode I delamination in glass/polyester composites, Journal of composite materials 44(7), pp , MF Moura and AT Marques, Prediction of low velocity impact damage in carbonepoxy laminates, Composites Part A: Applied Science and Manufacturing 33(3), pp ,

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