Principles and Applications of Microwave Testing for Wovenand Non-Woven Carbon Fibre-Reinforced PolymerComposites: a Topical Review

Zhen Li1,2 & Arthur Haigh3& Constantinos Soutis2 & Andrew Gibson4

Received: 12 July 2018 /Accepted: 26 July 2018 /Published online: 9 August 2018# The Author(s) 2018

AbstractCarbon fibre-reinforced polymer (CFRP) composites have been increasingly used by aerospace andother industries for their high specific stiffness and strength properties. When in service, non-destructive testing (NDT) methods are required to monitor and evaluate the structural integrity.Microwave-based detection techniques offer the advantages of non-contact, no need for a couplingmedium or sensors bonded to the object surface and relatively easy setup. This paper is intended toprovide a comprehensive overview of the currently available microwave techniques appropriate forcarbon fibre/polymer composites. The electromagnetic properties of carbon fibre composites associ-ated with microwave testing are discussed first. Then, the microwave methods are categorised intoself-sensing methods, near-field induction methods, near-field resonance methods, far-field sensingmethods and the methods with combination of other NDT (e.g., microwave-based thermography).Principles and applications of each kind are demonstrated in detail. Discussions of the advantages andlimitations in addition to research trends of microwave testing methods are presented.

Keywords Carbon fibre . Polymer composites . Non-destructive testing .Microwave testing

1 Introduction

The proportion of carbon fibre-reinforced polymer (CFRP) composites being used inaerospace, automotive and marine structures is increasing year on year [1]. Fatigue

Applied Composite Materials (2018) 25:965–982https://doi.org/10.1007/s10443-018-9733-x

* Constantinos Soutisconstantinos.soutis@manchester.ac.uk

1 College of Automation Engineering, Nanjing University of Aeronautics and Astronautics,Nanjing 211106, China

2 Aerospace Research Institute, The University of Manchester, Manchester M13 9PL, UK3 School of Electrical and Electronic Engineering, The University of Manchester, Manchester M13

9PL, UK4 Faculty of Science and Engineering, Manchester Metropolitan University, Manchester M1 5GD, UK

tolerance of composite structures is found attractive in addition to lightweight andcorrosion resistance. In the composites carbon fibres of high strength and Young’smodulus are embedded in a polymer resin as a matrix. Carbon fibres are usually producedfrom natural cellulose, synthetic polyacrylonitrile (PAN) and pitch, by carbonisation and/or graphitisation at high temperatures to eliminate other chemical elements and generatepolycrystalline graphitic structures [2]. In general, fibres with a diameter of 5 to 10 μm arethe primary load-carrying members, while the surrounding matrix acts as a load transfermedium and keeps the fibres in the desired location and orientation [3].

Defects like air voids and surface unevenness may be generated during themanufacturing process. And in the field carbon fibre composites remain vulnerable tovarious hazards, like impact caused by hailstones, runway debris, collision with groundequipment, tool drops and bird strikes. The types of the composite damage induced byimpact include surface dents, delamination, matrix cracking and fibre breakage. And inmany occasions, these happen internally and are hardly visible. Hence, it is of impor-tance to routinely and accurately assess the conditions of composite structures with aneffective inspection approach. In the aircraft industry visual inspection is commonlyconducted before each flight, while the accuracy depends on the lighting conditions,access for detection, inspector’s experience and time relaxation (e.g., dent depth of theimpact damage). In addition to that conventional method, various non-destructive testing(NDT) techniques have been applied, such as ultrasonic testing, acoustic emission,thermography, shearography, vibration testing, optical fibre sensing, Lamb waves anddigital image correlation (DIC). At present, no single method exists that can detect alltypes of manufacturing defects and in-service damage [4]. Each method has its ownspecific advantages and limitations. For example, couplants (e.g., water or gel) areneeded in ultrasonic testing, and the acoustic emission sensors should be placed nearthe damage source for accurate measurement [5]. For thermography, the possibility ofunwanted thermal damage to the structures should be taken into account caused duringthe test. In the setups of shearography and vibration testing, it is required to applymechanical loads to the structure. In optical fibre sensing systems, the weight penalty,possibility of failures in the wiring network, manufacturing and installation costs aresignificant. For Lamb wave-based techniques, piezoelectric transducers are permanentlymounted on the surface of the test piece [6]. And before the DIC measurement, thesurface of the sample under test should be speckled with black and white paints [7],which is not practical for large structures.

An alternative method for damage detection is the microwave technique. Microwavesare electromagnetic (EM) radiation with frequencies between 300 MHz (wavelength of1 m) and 300 GHz (wavelength of 1 mm). Microwaves can propagate in air anddielectric materials with low attenuation. The amplitude and phase of the microwavesignals can be affected by the variations of the thickness or electromagnetic properties(i.e., permittivity and permeability) of the material due to damage/defects. There are anumber of attributes when applying microwave NDT, such as non-contact, no need fortransducers bonded on the surface or couplants, no need for complicated signal post-processing, operator friendly, relatively inexpensive and one-sided scanning [8–10].During in-service inspections, commonly only one side of the object under test can beaccessed, so the microwave NDT methods are more advantageous. Safety precautions areusually not needed since the power of the signal used is relatively low (few milliwatts). Itis noted that the microwave techniques should be differentiated from the lower frequency

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electromagnetic detection methods, like the eddy current testing (ECT) (100 Hz -10 MHz) [11–13] and coupled spiral inductors (CSI) (1–100 MHz) [14–16], where thefield that exists only around the coil cannot propagate. In addition, the ECT and CSI donot work well for dielectric materials.

In recognition of the growing interest in microwave testing, in 2011 the Expertcommittee for microwave and THz testing procedures of the German Society of Non-Destructive Testing (DGZfP) was founded. And in 2014 the Microwave Testing Com-mittee of the American Society for Non-destructive Testing (ASNT) was established.Microwave testing was furthermore recognised as its own NDT method in the 2016edition of the ASNT standards, and the certifications for Level I and Level II microwavetesting inspectors are currently under development [9].

In this paper, the electromagnetic properties of carbon fibre composites closelyrelated to the microwave-based detection are addressed first. Four geometric scalesare adopted in the permittivity analysis to thoroughly study the differences in electro-magnetic responses due to the fibre direction. Based on the detection principles, themicrowave testing methods can be classified into five categories: self-sensing methods,near-field induction methods, near-field resonance methods, far-field methods andthe methods with integration of another NDT. The mechanism and applications of eachkind are discussed in detail. Finally, some viewpoints on the development of micro-wave detection research are presented.

2 Electromagnetic Properties of Carbon Fibre Composites

It is perquisite to understand the electromagnetic properties of each component first.Carbon fibres are both thermally and electrically conductive. The electrical conductivityof carbon fibres ranges from 3.84 × 104 S/m to 1 × 106 S/m depending on graphitisationtreatment [17]. The high conductivity makes them behave like metals, which reflect mostenergy of the incident signal. However, epoxy resin is nonconductive, i.e., dielectric. Itsdielectric constant is usually less than 6 at room temperature [18]. And the loss factor isnegligible, which means that little energy is absorbed in the material and it is consideredtransparent to microwave radiation.

For the mixture the effective permittivity is a parameter of interest for evaluation of theoverall electromagnetic performance. As CFRP is non-magnetic, its permeability μ is equal tothat of free space μ0. The permittivity ε can be written as [19].

ε ¼ ε0εr ¼ ε0 ε0r− jε

″r

� �ð1Þ

where ε0 is the permittivity of free space (i.e., 8.8542 × 10−12 F·m−1), and εr is the relative

permittivity. The real part ε0r, or dielectric constant, is related to the ability of a material to store

the electric field energy, while ε″r accounts for the dissipation of energy within the material in

the form of heat. ε″r is positive due to energy conservation. The transmission line method,open-ended coaxial/waveguide probe method, resonant cavity method and free space mea-surement can be used for permittivity characterisation [20, 21].

The permittivity of the composite is anisotropic, as the fibres and fibre architecture affectthe polarisation of the induced currents. In [22], it was reported that at 10 GHz, the relative

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permittivity of a T300 composite laminate was approximately 101-j73 when the fibres wereparallel to the incident electric field, while it was 107-j31 when the fibres and the electric fieldwere orthogonal. The relative permittivity of a neat resin sample cured under the sameconditions as the composite was 2.90-j0.10. The differences in the permittivity are associatedwith the multiscale nature of the composite laminate. As illustrated in Fig. 1, there are fourgeometric scales: macro level, meso level, micro level and molecular level. On the macro/meso-scale, a number of thin layers are stacked with a desired sequence of fibre orientations.On the microscale, in a lamina, ideally the fibres do not touch and are completely isolated bythe resin. However, the fibres, especially in 2D and 3D woven architectures, may contact atseveral points. At the molecular level, the electrons in carbon fibres (polycrystalline graphite)are delocalised, i.e., free to move along the planes of carbon atoms.

On the microscale, due to the presence of the applied EM field, carbon fibres anddielectric epoxy make up a series of capacitors. The capacitance always exists when theincident field is applied at an arbitrary angle. Hence a similar energy storage capability isreflected in the real permittivity. And that permittivity is two orders of magnitude largerthan that of the neat resin as would be expected. ε″r in the orthogonal case is approxi-mately half of that in the parallel case. As the epoxy resin is a polar dielectric material[23], only the dipolar relaxation exists. The lower ε″r of the epoxy indicates that theepoxy itself does not contribute much to the high loss of the composite, so the primaryloss is due to the carbon fibres. At the molecular level, when the electric field is parallelto the carbon fibres, the electrons move freely along the whole length of the fibres. In theorthogonal case, some electrons travel via the contact points between fibres, while theothers are trapped at the interfaces between the fibres and resin (Maxwell-Wagner(interfacial) polarisation [24]), which leads to a relatively lower conductivity.

The signal penetration is a practical factor that should be paid attention to. Due to the lossymedium, the power of the EM signal decays exponentially through the thickness. A parameterused for the evaluation is the penetration depth dp, which is defined as the depth where themagnitude is reduced to 1/e (about 37%) in the medium. For a plane wave incident on a halfspace of a medium, dp can be given by:

dp ¼ c

ffiffiffi2

pπf ε0

r

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ ε″r

ε0r

� �2r

−1

" #( )1=2ð2Þ

where f is the operating frequency, and c is the speed of light in free space. For the samelaminate discussed above, at 10 GHz the depths dp in the parallel and orthogonal cases were1.39 mm (approximately 11 layers assuming the thickness of a single layer is 0.125 mm) and3.25 mm (approximately 26 layers), respectively.

Fig. 1 Four geometric scales involved in the electromagnetic study of a composite laminate

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It is seen that there is an inverse relationship between the signal frequency and thepenetration depth. Hence, it is feasible that the distribution of the damage in thethickness direction can be revealed using multi-frequency inspection. For deeperpenetration, a lower frequency and an electric field not parallel to the fibre directionare preferred.

3 Existing Microwave Testing Methods for CFRP Composites

3.1 Self-Sensing Methods

In this type of methods, CFRP composite acts as a conductive component of a microwavecircuit. Any damage in the composite will lead to perturbation in the system. By detecting thechanges in the signal response, self-sensing of the composite structure is enabled without anyexternal setup.

(a) Part of a transmission line

The composite can be ground plane of a transmission line. Todoroki et al. [25–27]constructed a microstrip line with the use of a copper tape on a glass fibre reinforcedpolymer (GFRP) plate as substrate, Fig. 2. When there is surface damage or near-surface damage, the characteristic impedance at that location is changed, and theincident signal is split into the reflection and transmission components. This discon-tinuity can be located from the reflected signal in the time domain. This measurementtechnique is known as Time Domain Reflectometry (TDR). It was reported thatdelamination, matrix cracks, fibre breakage and lightning strike damage can besuccessfully detected.

In their setup shown in Fig. 2, a GFRP plate was cut with the same dimensions asthose of the CFRP plate and glued together, while the copper tape and CFRP weresoldered to a coaxial cable at the end of the transmission line. It is not suitable forpractical applications, since the microstrip line is bonded to the sample under inspec-tion. Li et al. [28] used an independent microstrip line on a Printed Circuit Board(PCB) for detection with the same TDR concept. As shown in Fig. 3, the stripconductor side of the board is placed close to the surface of the sample. Some fractionof the electromagnetic field is distributed between the strip conductor and the CFRP,which forms a new transmission line (similar to a covered microstrip line [29]). HereCFRP is separated from the measuring system, and the size of the sensor can besmaller than that of the CFRP sample. The whole area can be examined by conductinga line scan. In the test, a microstrip line was made on a FR4 substrate, and the distance

Fig. 2 Diagram of the cross section of the self-sensing sensor proposed by Todoroki [21–23]

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between the board and an impacted sample (usually called standoff distance) was100 μm. The microwave signal was generated with a vector network analyser(VNA). The frequency-domain data obtained were converted into the time domain byInverse Fast Fourier Transform (IFFT). It was revealed that the location of the peak inthe curve agreed with the real damage location.

(b) Part of an antenna

Matsuzaki et al. [30] treated CFRP as an element of a half-wavelength dipole antenna(e.g., CFRP wings in the electrically insulated case shown in Fig. 4) or a monopoleantenna. The feasibility of this method was studied using unidirectional laminates androtor blades of woven CFRP. It was demonstrated that the resonance frequency of thereturn loss was increased due to the damage in the specimen. Damage like fibre breakageor delamination caused the effective length of the dipole antenna shortened. However,the number of the damaged component (one or two blades) and damage location cannotbe readily determined.

3.2 Near-Field Induction Methods

The near-field induction methods are primarily based on Faraday’s principle of electro-magnetic induction. In the present case with CFRP composites it works the same as theeddy current technique, though microwaves can also be used for dielectric materials withthe same setup [31]. The signal can be radiated from an antenna or an open-endedcircular/rectangular waveguide performing like an antenna. To be accurate, the termBnear field^ refers to the non-radiative near field region, or reactive near field, whichexists in the immediate vicinity of the antenna where mature propagating waves have not

Fig. 3 Diagram of the detection system with a microstrip line sensor a experimental setup b cross section

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yet formed. The current distributions on the antenna are affected when an object isplaced in the reactive region, the boundary of which is commonly given by [32]:

R < 0:62

ffiffiffiffiffiffiD3

λ

sð3Þ

where D is the maximum linear dimension of the antenna, and λ is the wavelength. It isindicated that Eq. (2) is not suitable for evaluation of penetration in the near-field case, as theassumption of uniform plane waves is not valid. Here the penetration is associated with thestandoff distance, operating frequency, dimensions of the object under test, electromagneticproperties of the materials and the boundary conditions. Rigorous theoretical analysis of arectangular waveguide radiating into a laminated composite can be found in [33].

The schematic diagrams of the experimental setup and the equivalent lumped circuit model areillustrated in Fig. 5. When the CFRP sample is illuminated by the microwave radiation, currentsare induced and predominated along the direction of high conductivity (i.e., carbon fibredirection), and the currents flow from one fibre to another at contact points. The secondary fieldproduced can cause some energy reflected to the source, which is evaluated by the reflectioncoefficient S11 in the form of magnitude and phase. In the lumped circuit, the capacitance Cg islinked to the standoff distance, so surface unevenness can affect the value of this parameter. Theresistance Rs and inductance Ls are associated with the material properties and sample dimensions(for the relatively high conductivity of the composite, the effect of its capacitance is not consideredin the qualitative analysis). Change of the electric field direction with respect to the fibres will alsolead to changes in the resistance and inductance and subsequently the signal received.

(a) Detection of fibre direction

The basic application of this technique is determination of the fibre direction. When the fibresare not parallel to the electric field, less energy is reflected to the source, thus lower S11. Galehdaret al. [34] studied the effect of the orientation of the surface ply on the reflectivity. The carbon fibres

Fig. 4 Insulated CFRP rotor blades modelled as a dipole antenna (adapted from [30])

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that were parallel to the electric field behaved as good conductors, while the off-axis ply behaved aslossy dielectric layers with a finite conductivity. Kharkovsky et al. [35, 36] employed a waveguideto determine the fibre orientation in a CFRP patch with a thickness of 0.1 mm (one ply) and detectfibre breakage. It was implied that the electric field in the waveguide parallel with the fibres wasoptimal for fibre breakage detection. A small breakage with dimensions of 0.2 mm by 1 mm wasfound. The fibre orientation was determined by varying the angle between the fibre direction andsignal polarisation. Wilson et al. [37] investigated the detailed relationship between the angularorientation of a carbon fibre tow and the S11 responses using a broadband horn antenna. Thedependence on the tow orientation was strongly sinusoidal, and an average sensitivity of 97 kHz/degree resolution was obtained. A small angular misalignment of 1° was identified [38]. Thistechnique could be used for automated non-contact inspection during the manufacturing processwhen the carbon fibre tow is being laid down by a fabrication robot.

(b) Strain sensing

The engineering strain is the ratio of the total deformation to the initial dimension of thematerial alongthe force being applied, and it is an important indicator for stress concentration and crack growth. It isknown that the resistance and inductance of awire are linked to the length, and the deformation of theplate leads to changes in the capacitance associated with the air gap. Hence, by using the microwavenear-field induction approach, as seen in Fig. 5b, any change of the fibre length due to the appliedforce could be observed in the received signal. Wilson et al. [39] first introduced the non-contactmicrowave method for strain measurement. In the test, the antenna was placed 12 cm from a quasi-isotropic CFRP laminate. From the retrieved signal, the reactance (i.e., imaginary part of the compleximpedance) was calculated and provedmost sensitive to the strain change. A close linear relationshipbetween the reactance and the strain was established. However, the aperture of the antenna used wasrelatively large (approximately 24 cm by 14 cm), which suggested that the spatial resolution of thesensing was limited. Optimisation of the test setup, signal post-processing and characterisation of thetemperature and humidity effects are aspects of further improvement.

Fig. 5 Illustration of the near-field microwave induction method a experimental setup b equivalent circuit model

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(c) Damage detection

The near-field induction technique has been widely used for damage detection of carbonfibre composites. The information of the delamination and surface damage can be provid-ed. It is noted that there is a compromise between the signal penetration and the spatialresolution, as the resolution is highly dependent on the aperture size. For a rectangularwaveguide, the higher the operating frequency range, the smaller the aperture size.

Akuthota et al. [40] applied an open-ended waveguide to detect the disbonds between atwo-layer CFRP composite laminate and a concrete substrate. They pointed out that thefrequency and the standoff distance should be chosen optimally (or near optimally) tomaximise the sensitivity. It was found that the standoff distance variation, in the range of afew millimetres, had no adverse effect at 10 GHz, and a minimal effect at 24 GHz. Thesmallest detectable disbonded regions at 10 GHz and 24 GHz were 20 mm by 5 mm.

From theoretical simulations, Bin Sediq et al. [41] revealed that both rectangular andcircular waveguides could be capable of detecting defects inside carbon-loaded compos-ites. High attenuation was inevitable for the rectangular waveguide, due to the relativelylinear polarisation of the fields radiated out of the waveguide and the linear nature ofcarbon fibres. It was suggested that the circular waveguide with the characteristic ofcircular polarisation was more attractive for inspection.

Yang et al. [42] detected the impact damage in CFRP specimens by using a horn antenna,which is shown in Fig. 6a. In the test, a frequency range of 65–67 GHz was used, and the

Fig. 6 Microwave detection ofimpact damage in a CFRPspecimen with a V band (50–75 GHz) horn antenna [42] a setupb imaging result of 8.89 J impactenergy (unit: cm)

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E-field was applied parallel to the carbon fibres. The damage caused by impact energies of13.21 J and 8.89 J was reported to be easily distinguished from the image produced. With theuse of an edge detection image processing technique, a more natural image was produced andthe shapes of the damage were clearly identified.

3.3 Near-field resonance methods

(a) Near-field scanning microwave microscopy (NSMM)

As shown in Fig. 7, a near-field microwave microscope is generally composed of aresonant cavity and a sharp tip. The sample is placed close to the probing tip, from whichevanescent microwaves are emitted. The probe scans across the surface of the object at afixed standoff distance. The unevenness or material discontinuity of the surface can

Fig. 7 A general near-field microwave microscope for detection of surface damage a schematic diagram blumped circuit model

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result in a different capacitance of the air gap, subsequently changes in the resonancefrequency and quality factor (or Q factor) of the whole resonant circuit. Thus, a surfacecontour plot of the resonance frequency/Q factor can be produced. The sharper the tip is,the higher the spatial resolution that would be obtained. The penetration offered by thismethod is poor for the low intensity of the evanescent field.

Li et al. [43] utilised a near-field microwave profiler to detect low velocity impact damagein a 4-mm thick composite laminate. The damage categorised as barely visible impact damage(BVID) was created by a drop-weight impact energy of 20 J. In the test, the resonancefrequency at each measurement position was recorded from a Marconi 6200A scalar networkanalyser. The centre frequency was set to 3.05 GHz with a frequency span of 100 MHz. A 2Dscan was performed over the sample with a standoff distance of 100 μm and a step size of280 μm. As shown in Fig. 8, the area of the dent is clearly defined in the image generated, andthe symmetric and circular damage shape shows better image quality than that by themicrowave imaging with an open-ended waveguide.

(b) Microwave planar resonator

Instead of a resonant cavity, a microwave planar resonator was developed in [44]. Asillustrated in Fig. 9, a complementary split-ring resonator (CSRR) was made on the lower sideof a FR4 substrate, and a microstrip line with a width of 2.8 mm was made on the upper sidefor signal feed. Compared with the NSMM, this sensor exhibits some distinct advantages, suchas low cost, easy operation and simple design. From the simulation analysis, it was shown thatthe most sensitive region of the sensor was immediately under the resonator. In the test, thesensor was connected to an HP 3720D VNA by two coaxial cables and placed above a CFRPsample with a standoff distance of 100 μm. First, the location without impact damageunderneath was measured as a reference; then, the region with BVID was tested for compar-ison. The resonance frequency was shifted upwards from 2.23 GHz to 2.295 GHz with afrequency shift of 65 MHz.

Fig. 8 Scanned image of impact damage in CFRP by near-field scanning microwave microscopy

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3.4 Far-Field Sensing Methods

The far field is the region of operation for most antennae, in which the radiation does notchange with distance. The far-field (Fraunhofer) distance must satisfy three conditions:

R >2D2

λð4aÞ

R≫D ð4bÞ

R≫λ ð4cÞIt is mentioned that there is a radiative near field between the reactive near field and the farfield. The energy in the radiative near field is all radiant energy, but its mixture of magnetic andelectric components is still different from the far field. Depending on the values of R and thewavelength, this field may or may not exist. Hence the radiative near-field region is not in thescope of this paper.

Absorption of the radiation in the far-field region does not feed back to the transmittingantenna. Either bistatic configuration (a pair of transmitting and receiving antennae) ormonostatic configuration (a single antenna with a circulator) can be used. Each kind hasinherent advantages and disadvantages. The bistatic configuration has a high isolation betweentransmitting and receiving channels, but the dimensions are larger compared with themonostatic counterpart and accurate alignment between the transmitter and receiver is re-quired. The monostatic configuration is compact, as the microwave circulator can separate thetransmitting signal from the receiving one in a single component. However, the lower isolationof the circulator can lead to a significant leakage [45]. Additional measures must be undertakento reduce or suppress undesirable leakage.

Gubinelli et al. [46] used two ultra-wideband (UWB) antennae to detect the presence of ahole in CFRP. The experimental setup is presented in Fig. 10. UWB pulses were generatedwith a frequency range of 6.0–8.5 GHz, and the radar-to-sample distance was set to 40 cm. ACFRP sheet with no defect, one with a 3 mm through-thickness hole in the centre and anotherone with a 3-mm hole patched in the back with the same CFRP were tested separately. Theradar was in the central position with respect to the sheet. By scanning a healthy sheet(reference) and Bdamaged^ sheets, the difference in the signal responses was recognisable.

3.5 Integration with Other NDT

The microwave techniques can be combined with other NDT methods for better detectionperformance. Contact-free microwave-based thermography is commonly found in the litera-ture, in which microwaves are used as a heat source (like microwave curing [47]). When a

Fig. 9 Geometry of a microwave planar resonator for impact damage detection [44]

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high-power microwave signal is applied, the intensity of the induced currents in compositeswill increase and the temperature will rise due to the Joule effect. Damage and defects could berevealed in the thermogram with an infrared (IR) camera. Owing to good performance bycommercial infrared cameras, high sensitivity and high spatial resolution (much better than themicrowave wavelength) can be offered. In addition, rapid inspection can be achieved, as alarge area can be inspected within a short time. Compared with the available thermal excitationmethods, such as thermal lamps, laser, ultrasonic and pulsed eddy current (PEC), microwavescan provide more uniform, volumetric and selective heating [48–50]. If the penetration depthof the microwave signal is much smaller than the sample thickness, only the material withinthe penetration depth is heated and the rest of the material is heated by thermal conduction.Therefore, the penetration ability of the microwave-based thermography is better than that ofconventional microwave NDT techniques.

When using this specific technique, special attention should be paid to the electromagneticshielding as high power is used. According to the guideline issued by the InternationalCommission on Non-Ionizing Radiation Protection (ICNIRP) [51], the reference level foroccupational exposure to electromagnetic fields is 50 W/m2 over 2–300 GHz, which isequivalent to an electric field strength of approximately 137 V/m in free space. This healthand safety requirement should be carefully considered in the implementation.

A schematic diagram of the microwave thermography approach is illustrated in Fig. 11. Ahorn antenna is used to direct the waves into the region of interest. A microwave source can bea magnetron, travelling wave tube or signal generator [52]. An off-the-shelf IR camera islocated on the same side of the excitation source (reflection configuration). A personal

Fig. 11 Schematic diagram of theexperimental setup for microwave-based thermography

Fig. 10 Setup of an UWB radarsystem for far-field detection of ahole in a CFRP specimen [46]

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computer (PC) is used for control of the measuring instruments, data acquisition and signalpost-processing.

Keo et al. [53] used a commercial 2.45 GHz magnetron and an IR camera to detect a defect(absence of adhesive) in a CFRP plate. A 360 W microwave signal was applied to heat thesample for 150 s. It was found that the defect area was hotter than the non-defect area. Foudaziet al. [54] utilised a horn antenna to heat a cured mortar sample containing a 2-mm thicksurface-bonded CFRP, as shown in Fig. 12. The surface was under microwave illumination of50 Wat 2.4 GHz for 5 s. It was shown that the simulated delamination between the mortar andthe CFRP was visible in the thermal image.

In the work of Palumbo [55], sandwich specimens were put into a microwave chamber(power 750 W, frequency 2.45 GHz) for 2 s. The heating phase and subsequent cooling phasewere both used for impact damage evaluation. Good agreement was achieved in comparisonwith X-ray imaging and lock-in thermography using two halogen lamps with 500 W power.

Fig. 12 Microwave-basedthermography for delaminationdetection [54] a setup with astandoff distance of 6 cm bthermogram of the cured mortarsample

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4 Concluding Remarks and Future Work

An overview of the progress made in the development of the microwave testing methods forcarbon fibre-reinforced polymer composites have been presented here. The available micro-wave NDT methods have been summarised into five categories. Various applications havebeen demonstrated: strain sensing and detection of absence of adhesive, fibre direction, fibrebreakage, delamination, impact damage and lightning strike damage. The advantages andlimitations of the five categories of methods have been discussed for the readers’ benefit. It isnoted that the potential of microwaves for wider detection applications is yet to be fullyexploited. Near future work could focus on the following aspects:

(1) Theoretical analysis: the existing work is mainly based on experiments. It is also ofimportance to understand the wave propagation in the materials and interaction betweenthe wave field and the material system, which would help to optimise the setup fordetection and quantify the damage size. A multi-physics approach is needed wheninvestigating the microwave-based thermography, where electromagnetism and thermo-dynamics are involved.

(2) Damage identification: various types of defects/damage can be observed during fabrica-tion or in service, while there is a lack of research on differentiating the damage. Artificialintelligence (AI) can be introduced for automated identification of damage types. Forexample, an artificial neural network (ANN) can be used to build a classification system.The ANN can be modelled with input nodes that are matched to the input data format,output nodes in the form of a damage type probability and intermediate hidden layernodes. The connection parameters between the nodes are initially defined randomly, butcan be optimised by training, using a set of tests with and without damage.

(3) Far-field imaging: in the far-field region, microwave imaging methods can be adopted togenerate two-dimensional images of objects, such as synthetic aperture radar (SAR)imaging, phased array (digital beam forming) and microwave holography. The measure-ments can be performed with a single antenna raster scanning over the area, a 1D antennaarray sweeping over the area or a 2D antenna array covering the area.

(4) Use of the RFID technique: the active radio frequency identification (RFID) technologywith the capability of damage inspection has started to attract some attention [56]. With aRFID tag attached on the surface of the object under test, the local damage informationcan be wirelessly interrogated with a RFID reader. Hence, the microwave RFID conceptcan be introduced, and further Internet of Things (IoT) system can be integrated forremote inspection and long-term structural health monitoring (SHM).

(5) Wider applications of microwave-based thermography: the current applications ofmicrowave-based thermography are mainly limited to detection of absence of adhesiveand delamination. The potential of the technique for rapid detection of other damagetypes could be explored.

(6) Automated inspection: there is a high demand for automated inspection by the industry,while many of the existing applications are still at the experimental stages. Futureresearch could be focused on facilitating the industrial adoption.

(7) Potential applications in additive manufacturing (AM): additive manufacturing is anemerging technique where successive layers of material are formed under computercontrol to create a three-dimensional object. It significantly revolutionises the designof products that feature complex geometries and concept of spare parts management.

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However, at present the complex parts are posing difficulties for inspection, and astandard verification procedure for quality control is not established [57]. For thisreason, the 3D printing has not been widely used in industry. It is required that anydefect should be identified and corrected before finishing parts. For example, whenfabricating continuous carbon fibre reinforced polymer composites, porosity of eachlayer should be checked. Resin micro cracks [58] that could lead to delaminations[59] and fibre instability [60], especially under compressive loading, need also to beidentified. Microwave testing can be applied for in-process monitoring of additivemanufacturing. In addition, a customised microwave sensor can be made possible byAM to fit complex (e.g., curved) surfaces.

Acknowledgements This work was funded by Dean’s Doctoral Scholar Award, Faculty of Science andEngineering, The University of Manchester. Special thanks to Professors Christian Boller, Robin Sloan, AnthonyPeyton, Adrian Porch and Dr. Matthieu Gresil for their guidance and many helpful discussions.

Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 InternationalLicense (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and repro-duction in any medium, provided you give appropriate credit to the original author(s) and the source, provide alink to the Creative Commons license, and indicate if changes were made.

References

1. Soutis, C.: Fibre reinforced composites in aircraft construction. Prog. Aerosp. Sci. 41, 143–151 (2005)2. Xu, Z., Gao, C.: Graphene fiber: a new trend in carbon fibers. Mater. Today. 18, 480–492 (2015)3. Mallick, P.K.: Fiber-reinforced composites : materials, manufacturing, and design. CRC Press, Boca Raton

(2007)4. Boller, C., Chang, F.-K., Fujino, Y.: Encyclopedia of Structural Health Monitoring. Wiley, Chichester (2009)5. Rumsey, M.A., Paquette, J.A.: Structural health monitoring of wind turbine blades. In: Ecke, W., Peters,

K.J., Meyendorf, N.G. (eds.) Smart Sensor Phenomena, Technology, Networks, and Systems. p. 69330E.International Society for Optics and Photonics, San Diego (2008)

6. Qiu, L., Liu, B., Yuan, S., Su, Z., Ren, Y.: A scanning spatial-wavenumber filter and PZT 2-D cruciform arraybased on-line damage imaging method of composite structure. Sensors Actuators A Phys. 248, 62–72 (2016)

7. Saleh, M.N., Lubineau, G., Potluri, P., Withers, P.J., Soutis, C.: Micro-mechanics based damage mechanics for3D orthogonal woven composites: experiment and numerical modelling. Compos. Struct. 156, 115–124 (2016)

8. Kharkovsky, S., Zoughi, R.: Microwave and millimeter wave nondestructive testing and evaluation -overview and recent advances. IEEE Instrum. Meas. Mag. 10, 26–38 (2007)

9. Case, J.T., Kenderian, S.: Microwave NDT: an inspection method. Mater. Eval. 75, 339–346 (2017)10. Li, Z., Haigh, A., Soutis, C., Gibson, A., Sloan, R., Karimian, N.: Delamination detection in composite T-

joints of wind turbine blades using microwaves. Adv. Compos. Lett. 25, 83–86 (2016)11. Mook, G., Lange, R., Koeser, O.: Non-destructive characterisation of carbon-fibre-reinforced plastics by

means of eddy-currents. Compos. Sci. Technol. 61, 865–873 (2001)12. Yin, W., Withers, P.J., Sharma, U., Peyton, A.J.: Noncontact characterization of carbon-fiber-reinforced

plastics using multifrequency eddy current sensors. IEEE Trans. Instrum. Meas. 58, 738–743 (2009)13. Heuer, H., Schulze, M., Pooch, M., Gäbler, S., Nocke, A., Bardl, G., Cherif, C., Klein, M., Kupke, R.,

Vetter, R., Lenz, F., Kliem, M., Bülow, C., Goyvaerts, J., Mayer, T., Petrenz, S.: Review on qualityassurance along the CFRP value chain – non-destructive testing of fabrics, preforms and CFRP by HFradio wave techniques. Compos. Part B-Eng. 77, 494–501 (2015)

14. Li, Z., Haigh, A., Soutis, C., Gibson, A., Sloan, R., Karimian, N.: Damage evaluation of carbon-fibre reinforcedpolymer composites using electromagnetic coupled spiral inductors. Adv. Compos. Lett. 24, 44–47 (2015)

15. Li, Z., Haigh, A., Soutis, C., Gibson, A., Sloan, R., Karimian, N.: Detection and evaluation of damage inaircraft composites using electromagnetically coupled inductors. Compos. Struct. 140, 252–261 (2016)

16. Salski, B., Gwarek, W., Korpas, P.: Electromagnetic inspection of carbon-fiber-reinforced polymer com-posites with coupled spiral inductors. IEEE T. Microw. Theory. 62, 1535–1544 (2014)

980 Applied Composite Materials (2018) 25:965–982

17. Lodge, K.J.: The electrical properties of joints in carbon fibre composites. Composites. 13, 305–310 (1982)18. Pitt, C., Barth, B., Godard, B.: Electrical properties of epoxy resins. IEEE T. Compon. Parts. 4, 110–113 (1957)19. Pozar, D.: Microwave Engineering. Wiley, New York (2011)20. Chen, L.F., Ong, C.K., Neo, C.P., Varadan, V.V., Varadan, V.V.K.: Microwave electronics: measurement and

materials characterization. Wiley, Chichester (2004)21. Li, Z., Haigh, A., Soutis, C., Gibson, A., Sloan, R.: Dielectric constant of a three-dimensional woven glass

fibre composite: analysis and measurement. Compos. Struct. 180, 853–861 (2017)22. Li, Z.: Radio Frequency Non-destructive Evaluation of Impact Damage in Carbon Fibre Composites, (2017)23. Maity, P., Kasisomayajula, S., Parameswaran, V., Basu, S., Gupta, N.: Improvement in surface degradation

properties of polymer composites due to pre-processed nanometric alumina fillers. IEEE Trans. Dielectr.Electr. Insul. 15, 63–72 (2008)

24. Kremer, F., Schönhals, A. (eds.): Broadband Dielectric Spectroscopy. Springer Berlin Heidelberg, Berlin(2003)

25. Kurokawa, H., Todoroki, A., Mizutani, Y.: Damage monitoring of CFRP plate using self-sensing TDRmethod. JSME Int. J. A-Solid M. 6, 1053–1061 (2012)

26. Todoroki, A., Ohara, K., Mizutani, Y., Suzuki, Y., Matsuzaki, R.: Lightning strike damage detection at afastener using self-sensing TDR of composite plate. Compos. Struct. 132, 1105–1112 (2015)

27. Todoroki, A., Yamada, K., Mizutani, Y., Suzuki, Y., Matsuzaki, R.: Impact damage detection of a carbon-fibre-reinforced-polymer plate employing self-sensing time-domain reflectometry. Compos. Struct. 130,174–179 (2015)

28. Li, Z., Haigh, A.D., Saleh, M.N., McCarthy, E.D., Soutis, C., Gibson, A.A.P., Sloan, R.: Detection of impactdamage in carbon Fiber composites using an electromagnetic sensor. Res. Nondestruct. Eval. 29, 123–142 (2018)

29. Wadell, B.C.: Transmission line design handbook. Artech House, Norwood (1991)30. Matsuzaki, R., Melnykowycz, M., Todoroki, A.: Antenna/sensor multifunctional composites for the

wireless detection of damage. Compos. Sci. Technol. 69, 2507–2513 (2009)31. Li, Z., Haigh, A., Soutis, C., Gibson, A., Sloan, R.: Microwaves sensor for wind turbine blade inspection.

Appl. Compos. Mater. 24, 495–512 (2017)32. Balanis, C.A.: Antenna theory: analysis and design, third edition. John Wiley & Sons, Hoboken (2005)33. Bakhtiari, S., Ganchev, S., Zoughi, R.: A general formulation for admittance of an open-ended rectangular

waveguide radiating into stratified dielectrics. Res. Nondestruct. Eval. 7, 75–87 (1995)34. Galehdar, A., Rowe,W.S.T., Ghorbani, K., Callus, P.J., John, S., Wang, C.H.: The effect of ply orientation on

the performance of antennas in or on carbon fiber composites. Prog. Electromagn. Res. 116, 123–136 (2011)35. Kharkovsky, S., Zoughi, R.: Application of near-field microwave and millimeter wave nondestructive

testing for evaluation of fiber breakage and orientation evaluation in CFRP composite patches. In: AIPConference Proceedings. pp. 554–561. AIP (2005). https://doi.org/10.1063/1.1916724

36. Kharkovsky, S., Ryley, A.C., Stephen, V., Zoughi, R.: Dual-polarized near-field microwave reflectometerfor noninvasive inspection of carbon fiber reinforced polymer-strengthened structures. IEEE Trans. Instrum.Meas. 57, 168–175 (2008)

37. Wilson, W.C., Moore, J.P., Juarez, P.D.: Carbon fiber TOW angle determination using microwave reflec-tometry. In: 2016 IEEE SENSORS. pp. 1–3. IEEE (2016). https://doi.org/10.1109/ICSENS.2016.7808501

38. Wilson, W.C., Moore, J.P., Juarez, P.D.: Far field measurements of small angle carbon fiber misalignments.In: 2017 IEEE 18th Wireless and Microwave Technology Conference (WAMICON). pp. 1–4. IEEE (2017).https://doi.org/10.1109/WAMICON.2017.7930240

39. Wilson, W.C., Moore, J.P., Juarez, P.D.: Remote strain sensing of CFRP using microwave frequency domainreflectometry. In: National Aerospace & Electronics Conference & Ohio Innovation Summit (NAECON-OIS). IEEE, Dayton (2016). https://doi.org/10.1109/NAECON.2016.7856838

40. Akuthota, B., Hughes, D., Zoughi, R., Myers, J., Nanni, A.: Near-field microwave detection of disbond incarbon fiber reinforced polymer composites used for strengthening cement-based structures and disbondrepair verification. J. Mater. Civ. Eng. 16, 540–546 (2004)

41. Bin Sediq, A.S., Qaddoumi, N.: Near-field microwave image formation of defective composites utilizingopen-ended waveguides with arbitrary cross sections. Compos. Struct. 71, 343–348 (2005)

42. Yang, S.-H., Kim, K.-B., Oh, H.G., Kang, J.-S.: Non-contact detection of impact damage in CFRPcomposites using millimeter-wave reflection and considering carbon fiber direction. NDT&E Int. 57, 45–51 (2013)

43. Li, Z., Soutis, C., Haigh, A., Sloan, R., Gibson, A.: Application of an electromagnetic sensor for detection ofimpact damage in aircraft composites. In: 2016 21st International Conference on Microwave, Radar andWireless Communications (MIKON). pp. 1–4. IEEE (2016). ht tps:/ /doi.org/10.1109/MIKON.2016.7492004

44. Li, Z., Haigh, A., Soutis, C., Gibson, A., Sloan, R.: Applications of microwave techniques for aerospacecomposites. In: 2017 IEEE International Conference on Microwaves, Antennas, Communications and

Applied Composite Materials (2018) 25:965–982 981

Electronic Systems (COMCAS). pp. 1–4. IEEE, Tel Aviv, Israel (2017). https://doi.org/10.1109/COMCAS.2017.8244755

45. Kapilevich, B.Y., Harmer, S.W., Bowring, N.J.: Non-Imaging Microwave and Millimetre-Wave Sensors forConcealed Object Detection. CRC Press, Boca Raton (2014)

46. Gubinelli, S., Paolini, E., Giorgetti, A., Mazzotti, M., Rizzo, A., Troiani, E., Chiani, M.: An ultra-widebandradar approach to nondestructive testing. In: 2014 IEEE International Conference on Ultra-WideBand(ICUWB). pp. 303–308. IEEE (2014). https://doi.org/10.1109/ICUWB.2014.6958997

47. Green, J.E., Nuhiji, B., Zivtins, K., Bower, M.P., Grainger, R.V., Day, R.J., Scaife, R.J.: Internal modelcontrol of a domestic microwave for carbon composite curing. IEEE Trans. Microw. Theory Tech. 65,4335–4346 (2017)

48. Zhang, H., Yang, R., He, Y., Foudazi, A., Cheng, L., Tian, G.: A review of microwave thermographynondestructive testing and evaluation. Sensors. 17, 1123 (2017)

49. Salski, B., Olszewska-Placha, M., Karpisz, T., Rudnicki, J., Gwarek, W., Maliszewski, M., Zofka, A.,Skulski, J.: Microwave applicator for thermal treatment of bituminous surfaces. IEEE Trans. Microw.Theory Tech. 65, 3419–3427 (2017)

50. Swiderski, W., Szabra, D., Wojcik, J.: Nondestructive evaluation of aircraft components by thermographyusing different heat sources. In: Proceedings of the 2002 international conference on quantitative infraredthermography. pp. 79–84. QIRT Council (2002). https://doi.org/10.21611/qirt.2002.008

51. Ahlbom, A., Bergqvist, U., Bernhardt, J.H., Cesarini, J.P., Court, L.A., Grandolfo, M., Hietanen, M.,McKinlay, A.F., Repacholi, M.H., Sliney, D.H., Stolwijk, J.A.J., Swicord, M.L., Szabo, L.D., Taki, M.,Tenforde, T.S., Jammet, H.P., Matthes, R.: Guidelines for limiting exposure to time-varying electric,magnetic, and electromagnetic fields (up to 300 GHz). International Commission on Non-IonizingRadiation Protection. Health Phys. 74, 494–522 (1998)

52. Thostenson, E.T., Chou, T.-W.: Microwave processing: fundamentals and applications. Compos. Part A:Appl. S. 30, 1055–1071 (1999)

53. Keo, S.-A., Defer, D., Breaban, F., Brachelet, F.: Comparison between microwave infrared thermographyand CO2 LaserInfrared thermography in defect detection in applications with CFRP. Mater. Sci. Appl. 04,600–605 (2013)

54. Foudazi, A., Ghasr, M.T., Donnell, K.M.: Application of active microwave thermography to inspection ofcarbon fiber reinforced composites. In: 2014 IEEE Autotest. pp. 318–322. IEEE (2014). https://doi.org/10.1109/AUTEST.2014.6935164

55. Palumbo, D., Ancona, F., Galietti, U.: Quantitative damage evaluation of composite materials withmicrowave thermographic technique: feasibility and new data analysis. Meccanica. 50, 443–459 (2015)

56. Zhong, C.H., Croxford, A.J., Wilcox, P.D.: Remote inspection system for impact damage in large compositestructure. P. Roy. Soc. A-Math. Phy. 471, 20140631–20140631 (2014)

57. Waller, J.M., Parker, B.H., Hodges, K.L., Burke, E.R., Walker, J.L.: Nondes evaluation of additivemanufacturing state-of-the-discipline report. NASA (2014). https://ntrs.nasa.gov/search.jsp?R=20140016447. Accessed 6 Jun 2018

58. Katerelos, D.T.G., Kashtalyan, M., Soutis, C., Galiotis, C.: Matrix cracking in polymeric compositelaminates: modelling and experiments. Compos. Sci. Technol. 68, 2310–2317 (2008)

59. Kashtalyan, M., Soutis, C.: Analysis of local delaminations in composite laminates with angle-ply matrixcracks. Int. J. Solids Struct. 39, 1515–1537 (2002)

60. Soutis, C., Smith, F.C., Matthews, F.L.: Predicting the compressive engineering performance of carbonfibre-reinforced plastics. Compos. Part A: Appl. S. 31, 531–536 (2000)

982 Applied Composite Materials (2018) 25:965–982