Abstract
Non-destructive tests working at lower microwave frequencies have large advantages of dielectric material penetrability, lower equipment cost, and lower implementation complexity. However, the resolution will become worse as the work frequencies become lower. Relying on designing the structure of high field confinement, this study realizes a simple complementary spiral resonators (CSRs)-based near-field probe for microwave non-destructive testing (NDT) and imaging around 390 MHz (λ = 769 mm) whereby very high resolution (λ/308, 2.5 mm) is achieved. By applying an ingenious structure where a short microstrip is connected to a microstrip ring to feed the CSR, the probe, that is a single-port microwave planar circuit, does not need any extra matching circuits, which has more application potential in sensor arraying compared with other microwave probes. The variation of the electric field distribution with the standoff distance (SOD) between the material under test and the probe are analyzed to reveal the operation mechanisms behind the improved sensitivity and resolution of the proposed probe. Besides, the detection abilities of the tiny defects in metal and non-metal materials are demonstrated by the related experiments. The smallest detectable crack and via in the non-metal materials and the metal materials are of a λ/1538 (0.5 mm) width, a λ/513 (1.5 mm) diameter, a λ/3846 (0.2 mm) width and a λ/513 (1.5 mm) diameter, respectively. Moreover, to further evaluate the performance of the proposed probe, the defects under skin layer in the multilayer composite materials and the defects under corrosion in the carbon steel are inspected and imaged. Due to lower work frequency, high resolution, outstanding detection abilities of tiny defects, and large potentials in sensor arraying, the proposed probe would be a good candidate for microwave NDT and imaging.
Highlights
Microwave near-field imaging techniques are widely applied in non-destructive testing (NDT)
The resolution of open-ended waveguides is limited by their apertures, and smaller apertures mean higher working frequencies which would weaken the ability of penetrating into the materials under test, and bring high equipment costs and implementation complexity [2]
We propose a much simpler single-port compact probe working around 390 MHz
Summary
Microwave near-field imaging techniques are widely applied in non-destructive testing (NDT). Non-resonant microwave imaging generally uses open-ended waveguides as probes [4,5,6,7,8,9,10,11] and has been successfully implemented to detect cracks and corrosion on metal surfaces [4,5,6]. A loop antenna around SRs is needed to excite it, and a necessary matching circuit is used to match the probe to the 50 Ω coaxial feed line This probe could achieve 5 mm (λ/140) spatial resolution at 433 MHz in defect imaging of composite materials, but its resolution is limited to the separation of the electric field peaks [2]. A microstrip ring with a short 50 Ω microstrip, is utilized to feed the CSR to make the proposed probe a compact microwave single-port planar circuit. The abilities of the proposed probe for detecting defects under a skin layer and corrosion are demonstrated
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