CFRP Composites with Embedded PZT Transducers for

Nonlinear Ultrasonic Inspection of Space Structures

Christos Andreades and Francesco Ciampa*

Department of Mechanical Engineering, University of Bath, Bath, BA2 7AY, UK

*Corresponding Author: f.ciampa@bath.ac.uk

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 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.

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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 all-

aluminium 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

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 (A2) 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 A2 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 A2

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

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

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

2.3.1 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

6

time of 50 ms. Propagation of ultrasonic elastic waves through the AD- and the UD-

laminate was performed simultaneously and the two FFT spectrums were directly

compared (Figure 5). This allowed to distinguish the A2 harmonics generated due to

damage excitation from those generated due to noise (e.g. instrumentation noise). In

fact, the A2 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 184.7 kHz.

7

The transmitted signal was swept from 20 kHz to 500 kHz in steps of 30-40 kHz,

and higher-harmonic generation due to damage excitation was detected at two input

signal frequency ranges; 102-106 kHz and 180-185 kHz. The 104.5 kHz and 184.7 kHz

frequencies corresponded to the A2 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 (A1) 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 A1 and A2 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 184.7 kHz) the A1 and A2 amplitudes were rising with

increasing input signal voltage. The A2 amplitude was around two orders of magnitude

smaller than the A1 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 104.5 and 184.7 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

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 104.5 kHz and 184.7 kHz in

the AD-laminate and at 128 kHz and 310 kHz in the ID-laminate. In each case, the out-

of-plane vibrational velocity of the plate surface at the A1 and A2 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 A2 amplitude was

approximately an order of magnitude smaller relative to the A1 amplitude. The results

also revealed that the in the AD-laminate, the input signal at 104.5 kHz caused

excitation only to the big FEP patch whereas at 184.7 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.

9

(a) SmalldamageinAD-Laminate

A1freq.=184.7kHz

A1amp.=305μm/s

A2freq.=369.4kHz

A2amp.=43μm/s

(b) BigdamageinAD-Laminate

A1freq.=104.5kHz

A1amp.=257μm/s

A2freq.=209kHz

A2amp.=32μm/s

(c) SmalldamageinID-Laminate

A1freq.=310kHz

A1amp.=7038μm/s

A2freq.=620kHz

A2amp.=284μm/s

(d) BigdamageinID-Laminate

A1freq.=128kHz

A1amp.=2999μm/s

A2freq.=256kHz

A2amp.=108μm/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 A2

harmonic generation in the frequency spectrum of the received signal, and A2 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 A1 and A2 harmonic frequencies. The results

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.

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