Two new microwave sensors, an open cylindrical cavity resonator with a T‐shaped excitation element and a flanged coaxial line sensor, are used for the nondestructive thickness measurement of surface coatings on a unidirectional carbon fiber‐reinforced polymer (CFRP) laminate. Both sensors are found insensitive to the anisotropy of the composite substrate, offering easy implementation. Three commercially available plastic films, polyethylene (PE), polyethylene terephthalate (PET), and polyvinyl chloride (PVC), are used to simulate the coating. In the setup, the microwave sensors are directly mounted on the 4 mm‐thick composite plate. The coating affects the surface impedance of the composite, causing changes to the signal reflection. Calibration is performed by the measurement of samples with known coating thicknesses. For the cavity resonator, a linear relationship is obtained between the resonance frequency shift and the thickness; the difference between the measured and actual thickness values is around 17 μm. For the coaxial line sensor, the phase variation increases with increasing thickness. For both sensors used, the estimation error for a coating thickness in the region of 200 μm can be well within 10%.

The complex electric permittivity of Baijiu (one of the most consumed beverages) is measured over a frequency range of 2 to 20 GHz using a low-cost open-ended coaxial probe and a portable microwave analyzer. The Cole-Cole equation is used to fit the permittivity data, and the parameter values related are extracted using the particle swarm optimization. A linear relationship is obtained between these parameters and the alcohol by volume. A strong dependence of the permittivity on the alcohol content is also observed for other alcoholic drinks, including brandy and whisky. The study demonstrates that the microwave technique provides an efficient solution in the effort to determine the alcohol content and the principal component analysis is a powerful tool for the quick estimation of the dielectric properties. The sensor presented can contribute to efficient on-site quality control of alcoholic drinks. PRACTICAL APPLICATION: The microwave approach presented could offer an economical approach for the characterization of Baijiu and a general solution to the alcohol content determination for the alcoholic drinks industry.

ABSTRACTMicrowaves are a form of electromagnetic radiation commonly used for telecommunications, navigation and food processing. More recently microwave technologies have found applications in fibre-reinforced polymer composites, which are increasingly used in aircraft structures. Microwave energy can be applied with low power (up to milliwatts) for non-destructive testing and high power (up to kilowatts) for heating/curing purposes. The state-of-the-art applications at high power include curing, three-dimensional (3D) printing, joining and recycling, whereas low-power microwave techniques can provide quality checks, strain sensing and damage inspection. Low-power microwave testing has the advantage of being non-contact, there is no need for surface transducers or couplants, it is operator friendly and relatively inexpensive; high-power microwave energy can offer volumetric heating, reduced processing time and energy saving with no ionising hazards. In this paper the recent research progress is reviewed, identifying achievements and challenges. First, the critical electromagnetic properties of composites that are closely related to the heating and sensing performance are discussed. Then, representative case studies are presented. Finally, the trends are outlined, including intelligent/automated inspection and solid-state heating.

A low-cost microwave cavity resonator sensor is developed for the assessment of honey. When a liquid sample is placed in the cavity, the resonant responses are significantly changed caused by the material perturbation. From the measurement of standard liquid samples with known permittivity, it is well demonstrated that the electric permittivity can be accurately determined. For the honey-water mixtures with the added water content from 10% to 50% w/w, it is found that the resonance frequency decreases with increasing added water content, and its change conforms to the law of mixture. In addition, there is a linear relationship between the added water content and the real part of the permittivity, and a quadratic relationship is found between the real and imaginary parts of the permittivity. These findings agree well with the results of the mixtures with added water contents below 10%. Hence, both the resonant responses of the cavity and the permittivity obtained can be employed for the indication of adulteration and accurate evaluation of the water content. Further, it is shown that the full penetration of the liquid sample can be achieved. The microwave sensor presented here provides an alternative tool for rapid food evaluation.

The world's most consumed liquor-baijiu is characterised by measuring the perturbation in the resonance condition of a cylindrical microwave cavity as a function of alcohol content. Compared with other methods, the microwave sensor described here is compact, portable, low-cost and need of a small sample size (<0.1 mL) of liquid. The dielectric properties are obtained from the shift of the resonance frequency and changes in the quality factor. The measurement accuracy is validated by comparison with standard liquid samples, where high estimation accuracy (better than 6%) is achieved. The resonant responses and permittivity of baijiu samples are evaluated, where the alcohol content has a significant effect. Six other types of alcoholic drinks and ethanolwater mixtures were also measured for comparison purposes. From all the experimental results, it is demonstrated that the alcohol by volume can be readily deduced from the regressed functions obtained. In addition, the principal component analysis is successfully applied to the classification of liquids with varied alcohol contents.

In this paper, a detailed investigation of the electromagnetic (EM) properties of carbon fibre-reinforced polymer composites (CFRP) is presented. The electric permittivity, electrical conductivity, signal penetration and microwave absorption are discussed. Unidirectional composite laminate samples were characterised over X band (8-12 GHz) using the microwave transmission line technique. It is shown that the real part of the permittivity of the composites is not significantly anisotropic whereas the imaginary part is highly anisotropic. More microwave energy is reflected in the parallel case, while more energy is absorbed in the orthogonal case. These experimental results are studied at three geometric levels: macro-meso scale (laminate/lamina level) relating to lay-up and fibre direction dependence, microscale (fibre level) relating to the real part of the permittivity and nanoscale level relating to the imaginary part of the permittivity. The findings can contribute to improved design of carbon fibre composites for electromagnetic applications, like shielding, curing and non-destructive inspection.

In this paper, a new non-contact method for detection of impact damage in carbon fibre-reinforced polymer composites with a complementary split-ring resonator sensor is proposed. The resonance frequency is evaluated as an indicator of the presence of damage. The resonator is made on a printed circuit board, and in the experimental setup it is positioned close to the area of interest. Electromagnetic models are built, and from the resonant responses the appropriate frequency range used for the test is determined. The active sensing element in the resonator is found from the analysis of the magnetic field distribution. The parametric study performed shows that a larger frequency change occurs for a wider impacted region, which is of great use for practical applications. The proposed method is validated by the experimental results, where a frequency shift of 65 MHz was observed for a 0.36 mm deep dent.

Carbon fibre-reinforced polymer (CFRP) composites have been increasingly used by aerospace and other 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 coupling medium or sensors bonded to the object surface and relatively easy setup. This paper is intended to provide a comprehensive overview of the currently available microwave techniques appropriate for carbon fibre/polymer composites. The electromagnetic properties of carbon fibre composites associated with microwave testing are discussed first. Then, the microwave methods are categorised into self-sensing methods, near-field induction methods, near-field resonance methods, far-field sensing methods 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 and limitations in addition to research trends of microwave testing methods are presented.

A new convenient and non-destructive permittivity measurement method is presented. No physical cut of specimens is needed here for material characterisation. In the setup, the material under test is placed in the near-field region of a microwave open-ended waveguide. An electromagnetic model of the setup is built in the Computer Simulation Technology simulation software. Employing optimisation, the permittivity is obtained from the measured reflection coefficients \(\hbox {S}_{11}\). Using the same technique, the effect of the model size is investigated that could reduce the modelling effort for large structures. The efficiency of a traditional method (i.e., Newton) and an intelligent algorithm (i.e. particle swarm optimisation) for permittivity calculation is thoroughly studied and compared. The proposed methodology is validated by experimental data. It is demonstrated that the proposed method can provide more accurate permittivity results than the intrusive in-waveguide measurement. The proposed methodology can contribute to electromagnetic analysis, thickness measurement and non-destructive evaluation.

This article presents a comprehensive experimental study of impact damage detection for carbon fiber-reinforced polymer (CFRP) composites using an electromagnetic (EM) sensor with coupled spiral inductors (CSI). Two representative types of damage are detected and evaluated, i.e., barely visible impact damage (BVID) and delamination. A multifrequency inspection is performed, where the resultant images indicate the potential of the CSI sensor in the characterization of damage extent. The accuracy and efficiency of the CSI sensor are compared with the open-ended waveguide imaging, near-field microwave microscopy, microwave time-domain reflectometry, the complementary split-ring resonator, and ultrasonic scanning. Applications and limitations of these nondestructive testing (NDT) methods for identifying impact damage are discussed. There is a free edge effect on the electromagnetic signal, which is illustrated for the first time with the proposed EM technique. Detection of the air gap produced by inserting a thin piece into a machined subsurface groove is carried out. It is found that the developed CSI sensor is able to accurately resolve the location and extent of the air gap. The experimental results demonstrate that the sensor could offer an alternative relatively low cost method that can be fully automated for structural monitoring of aircraft and other composite structures.

The microwave transmission line technique is presented as an effective method for the evaluation of honey for the first time. The electrical permittivity is an intrinsic parameter of a material that can be used as an index of added water content. For the permittivity calculation, it is found that the combination of the characteristic matrix method and Tischer's model can offer the highest accuracy. A genetic algorithm is introduced to provide an initial approximate permittivity value and acquire the Cole-Cole parameters of the honeys examined. The accuracy provided by the methodology used in this study is superior to that offered by a commercially available probe. Operating at room temperature and a frequency range of 6 -8 GHz, the measurements demonstrate that the permittivity of honey increases with increased added water. A relationship between the added water content and the permittivity of honey-water mixture is established, which could be a powerful tool for detecting honey adulteration. (c) 2017 Elsevier Ltd. All rights reserved.

The structural integrity of composite structures can be monitored and evaluated by using microwave-based techniques. In this paper, five different types of microwave methods that can be used for carbon fibre composites are reviewed. Advantages, limitations and applications of these methods are described. Induction, resonance and time domain reflectometry are suitable for near-field non-contact applications, while thermography and antenna-based methods are advantageous for far-field inspections. Due to the conductive characteristic of carbon fibres, the near-field methods are preferred for better signal penetration over the microwave frequency range.

The structural integrity of wind turbine blades can be adversely affected by their structural dynamics, temperature extremes, lightning strikes, ultraviolet radiation from sunlight and airborne particulate matter such as hailstones and sand. If subsurface delamination occurs and is undetected then this can lead to fibre breakage and catastrophic failures in composite blades. In this paper we introduce a microwave scanning technique that detects such delamination in practical blade assemblies. Using an open-ended waveguide sensor, the electromagnetic signal reflected from the composite is found to have a phase profile that can detect changes in the composite cross section. Glass fibre T-joints are scanned and the results used to detect thickness variations (e.g., the presence of the web) and delamination. Results are compared across the 18–20 GHz frequency band. The dielectric permittivity of the composite system is measured and is used to estimate the stand-off distance and operating frequency of the sensor. This is critical to the system's ability to detect damage. When the sensor is close to the surface of the structure (standoff distance ≈ 5 mm), delamination down to 0.2 mm in width could be detected.

This paper presents a novel methodology for predicting the dielectric constant of three-dimensional woven glass fibre-reinforced composites. A well-established approach of deriving the effective dielectric constant is the dielectric mixing formulae (rule of mixtures based), which either provide a single value or offer upper and lower bounds. For composites with three-dimensional fibre architecture, an accurate model considering the three-dimensional effect is needed. Here, the anisotropic effect is revealed using electromagnetic simulation to extract the effective dielectric constant of a model material with unidirectional fibres, which are aligned or orthogonal to the electric field. The rule of mixtures based formulae are evaluated. The most suitable formula selected for each case is then extended to a general case with arbitrary fibre orientation and is further used to characterise the capacitor element of an electromagnetic model for 3D woven composites. The proposed method is compared to measurements to demonstrate the improved accuracy.

The paper presents a quantitative damage evaluation of carbon-fibre reinforced polymer (CFRP) plates using a non-contact electromagnetic (EM) sensor. The EM sensor with coupled spiral inductors (CSI) is employed here to detect both impact induced and simulated damage leading to an accurate evaluation of the location, depth and width of sub-surface defects. The effect of inspection frequency, standoff distance and signal power are also investigated leading to the development of an engineering circuit design tool that relates the set up and calibration of the sensor to its detection performance. It is found that the dynamic range of the transmission coefficient is the limiting factor in the original Salski CSI sensor and this problem is addressed by adding ferrite layers to reduce the reluctance of the magnetic circuit, improving damage sensing by 22%. The study leads to a further development of utilising an open ferrite yoke with a pair of encircling coils, which shows a 57% sensitivity improvement and clearer identification of air gaps (voids) and delamination in CFRP laminates. The proposed EM yoke design CSI sensor is low cost and could be assembled into an array for non-contact, in situ mechatronic scanning of aircraft composite wings.

The wind turbine is playing an increasingly important role in the generation of green electricity. The structural integrity and maintenance of the wind turbine blade is a critical issue in service due to the harsh operating environments. In this paper, the delamination in glass fibre composite joints of a wind turbine blade is detected using the non-contact microwave imaging with an open-ended waveguide. The existence and extent of the delaminated region could be found from the generated images. Thickness variation and the presence of the web can be identified as well. The optimum inspection frequency range is investigated, which offers a reference for future applications. By comparison, the phase profiling provided by the scanning shows better performance for the delamination detection rather than the magnitude profiling.

This paper presents a comprehensive experimental study of barely visible impact damage detection for carbon-fibre reinforced polymer (CFRP) composites using an electromagnetic sensor: coupled spiral inductors. A multi-frequency inspection is performed, where the resultant images indicate its great potential for the evaluation of damage propagation. The accuracy and efficiency are compared with the open-ended waveguide imaging, near-field microwave microscopy and microwave time-domain reflectometry. Advantages, disadvantages and applications of these methods used for impact damage detection are described. It is found that the phase can also be used for evaluation other than the magnitude. The experimental results demonstrate that the sensor could offer an alternative method for the structural monitoring and maintenance of aircraft composite structures.

This paper presents the application of electromagnetic coupled spiral inductors for the non-destructive evaluation of carbonfibre reinforced polymer (CFRP) plates. Three types of representative damage in carbon fibre composites are evaluated, i.e. barely visible impact damage (BVID), subsurface defects and internal micro-cracks. This work indicates that the size and location of the subsurface defects can be quantitatively assessed from the signal data acquisition.

Careful design of the magnetic bias circuit used in ferrite phase shifters can help to reduce space, weight and energy consumption. Low reluctance circuits must avoid air-gaps and this can be achieved using toroidal-shaped ferrite inserts positioned inside a rectangular waveguide. Here, these ferrite inserts are fabricated using a viscous plastic processing method that avoids machining and produces a continuous magnetic circuit. Finite-element methods are used to initially model the magnetostatic solution for the bias circuit before being used to analyse the microwave performance of a double toroid phase shifter. A closed-form technique is introduced to model the dielectric slab waveguide impedance transformer. The final insertion loss was <1 dB over the 9.5-10.3 GHz band and return loss of 20 dB was achievable. The phase shift calculation agrees to within 10% of the measured values.