Abstract
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.
Highlights
Microwaves are commonly used for telecommunications and food processing, while extensive attention has been given to their potential in material characterisation and non-destructive testing
The electric permittivity is an intrinsic parameter of a material that describes the interaction with the electromagnetic field, and microwaves are highly sensitive to its variation
A number of microwave techniques have been used for permittivity measurement: resonance methods, transmission line technique, free space methods and open-ended rectangular waveguide/coaxial probe techniques
Summary
Microwaves are commonly used for telecommunications and food processing, while extensive attention has been given to their potential in material characterisation (e.g., porosity evaluation [1,2] and moisture measurement [3]) and non-destructive testing (e.g., corrosion detection [4], crack detection [5,6,7], thickness variation [8], delamination detection [9,10] and impact damage inspection [11,12,13,14]). A number of microwave techniques have been used for permittivity measurement: resonance methods, transmission line technique, free space methods and open-ended rectangular waveguide/coaxial probe techniques. In the free space method, the sample and the sample holder should be placed between two horn antennae, and special attention must be paid to the sample geometry and location In these three methods, cutting and machining of test samples is required. Considering the time for contacting the mechanical workshop, task scheduling, actual machining work and delivery, the total time could be a couple of days This kind of mechanical work is labour intensive and sample cutting from the examined component may not be permissible due to coatings [17] and could undermine structural integrity. Compared with the coaxial open-ended probe, the open-ended rectangular waveguide is more capable for measurements of low-permittivity materials
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