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.

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 permittivity and nanoscale level relating to imaginary part of permittivity. The findings can contribute to improved design of carbon fibre composites for electromagnetic applications, like shielding, curing and non-destructive inspection.

A review of microwave testing of glass fibre-reinforced polymer composites. Nondestructive testing and evaluation.

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 microwave transmission line technique is presented as an effective method for evaluation of honey purity for the first time. The electrical permittivity is an intrinsic parameter of a material that can be used as a purity index. 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. 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.

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.

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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 openended 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 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.