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

The proportion of carbon fibre-reinforced polymer (CFRP) composites being used in aerospace, automotive and marine structures is increasing year on year [1]. Fatigue tolerance of composite structures is found attractive in addition to lightweight and corrosion resistance. In the composites carbon fibres of high strength and Young’s modulus are embedded in a polymer resin as a matrix. Carbon fibres are usually produced from natural cellulose, synthetic polyacrylonitrile (PAN) and pitch, by carbonisation and/or graphitisation at high temperatures to eliminate other chemical elements and generate polycrystalline graphitic structures [2]. In general, fibres with a diameter of 5 to 10 μm are the primary load-carrying members, while the surrounding matrix acts as a load transfer medium and keeps the fibres in the desired location and orientation [3].

Defects like air voids and surface unevenness may be generated during the manufacturing process. And in the field carbon fibre composites remain vulnerable to various hazards, like impact caused by hailstones, runway debris, collision with ground equipment, tool drops and bird strikes. The types of the composite damage induced by impact include surface dents, delamination, matrix cracking and fibre breakage. And in many occasions, these happen internally and are hardly visible. Hence, it is of importance to routinely and accurately assess the conditions of composite structures with an effective inspection approach. In the aircraft industry visual inspection is commonly conducted 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 the impact 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 and digital image correlation (DIC). At present, no single method exists that can detect all types of manufacturing defects and in-service damage [4]. Each method has its own specific advantages and limitations. For example, couplants (e.g., water or gel) are needed in ultrasonic testing, and the acoustic emission sensors should be placed near the damage source for accurate measurement [5]. For thermography, the possibility of unwanted thermal damage to the structures should be taken into account caused during the test. In the setups of shearography and vibration testing, it is required to apply mechanical loads to the structure. In optical fibre sensing systems, the weight penalty, possibility of failures in the wiring network, manufacturing and installation costs are significant. For Lamb wave-based techniques, piezoelectric transducers are permanently mounted on the surface of the test piece [6]. And before the DIC measurement, the surface 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. Microwaves are electromagnetic (EM) radiation with frequencies between 300 MHz (wavelength of 1 m) and 300 GHz (wavelength of 1 mm). Microwaves can propagate in air and dielectric materials with low attenuation. The amplitude and phase of the microwave signals 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 a number of attributes when applying microwave NDT, such as non-contact, no need for transducers 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 be accessed, so the microwave NDT methods are more advantageous. Safety precautions are usually not needed since the power of the signal used is relatively low (few milliwatts). It is noted that the microwave techniques should be differentiated from the lower frequency 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 the field that exists only around the coil cannot propagate. In addition, the ECT and CSI do not work well for dielectric materials.

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

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

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

For the mixture the effective permittivity is a parameter of interest for evaluation of the overall electromagnetic performance. As CFRP is non-magnetic, its permeability μ is equal to that of free space μ₀. The permittivity ε can be written as [19].where ε₀ is the permittivity of free space (i.e., 8.8542 × 10⁻¹² F·m⁻¹), and ε_r is the relative permittivity. The real part \( {\varepsilon}_r^{\prime } \), or dielectric constant, is related to the ability of a material to store the electric field energy, while \( {\varepsilon}_r^{{\prime\prime} } \) accounts for the dissipation of energy within the material in the form of heat. \( {\varepsilon}_r^{{\prime\prime} } \) is positive due to energy conservation. The transmission line method, open-ended coaxial/waveguide probe method, resonant cavity method and free space measurement can be used for permittivity characterisation [20, 21].

The permittivity of the composite is anisotropic, as the fibres and fibre architecture affect the polarisation of the induced currents. In [22], it was reported that at 10 GHz, the relative permittivity of a T300 composite laminate was approximately 101-j73 when the fibres were parallel to the incident electric field, while it was 107-j31 when the fibres and the electric field were orthogonal. The relative permittivity of a neat resin sample cured under the same conditions as the composite was 2.90-j0.10. The differences in the permittivity are associated with the multiscale nature of the composite laminate. As illustrated in Fig. 1, there are four geometric 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 by the resin. However, the fibres, especially in 2D and 3D woven architectures, may contact at several 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 and dielectric epoxy make up a series of capacitors. The capacitance always exists when the incident field is applied at an arbitrary angle. Hence a similar energy storage capability is reflected in the real permittivity. And that permittivity is two orders of magnitude larger than that of the neat resin as would be expected. \( {\varepsilon}_{\mathrm{r}}^{{\prime\prime} } \) in the orthogonal case is approximately half of that in the parallel case. As the epoxy resin is a polar dielectric material [23], only the dipolar relaxation exists. The lower \( {\varepsilon}_{\mathrm{r}}^{{\prime\prime} } \) of the epoxy indicates that the epoxy itself does not contribute much to the high loss of the composite, so the primary loss is due to the carbon fibres. At the molecular level, when the electric field is parallel to the carbon fibres, the electrons move freely along the whole length of the fibres. In the orthogonal case, some electrons travel via the contact points between fibres, while the others 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 lossy medium, the power of the EM signal decays exponentially through the thickness. A parameter used for the evaluation is the penetration depth d_p, which is defined as the depth where the magnitude is reduced to 1/e (about 37%) in the medium. For a plane wave incident on a half space of a medium, d_p can be given by:where f is the operating frequency, and c is the speed of light in free space. For the same laminate discussed above, at 10 GHz the depths d_p in the parallel and orthogonal cases were 1.39 mm (approximately 11 layers assuming the thickness of a single layer is 0.125 mm) and 3.25 mm (approximately 26 layers), respectively.

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

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