Microwave non-destructive testing has received extensive attention in recent years owing to its advantages of non-contact inspection, no need for a coupling medium or sensors attached on th...

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A review of microwave testing of glass fibre-reinforced polymer composites

Imaging probes which work at relatively low microwave frequencies in the ultrahigh frequency band (UHF) facilitate imaging through high loss dielectric materials and yield manageable imaging system complexity. However, acquiring images with resolutions on the order of few millimeters at frequencies within the UHF band requires super-resolution imaging methods and probes. In this paper, a compact near-field imaging probe working in the UHF band is proposed for high-resolution noninvasive imaging applications. The proposed probe is based on an electrically small loop loaded with a miniaturized spiral resonator. The probe is analyzed in this paper using numerical electromagnetic simulations and its equivalent circuit model is consequently developed. A prototype of the proposed probe operating at 426 MHz is fabricated and its spatial resolution as well as sensitivity are experimentally verified. A spatial resolution of around $\lambda $ /140 is demonstrated in simulations and verified experimentally. In order to highlight the utility of the probe for practical applications, images of many test samples including multilayer dielectric composite structures and corroded metallic substrate are acquired using the fabricated prototype. It will be shown that the images obtained using the proposed probe is comparable with those produced using an open-ended rectangular waveguide operating at 24 GHz. The salient features of the proposed probe include its high sensitivity and resolution while operating at relatively low frequency.

High-Resolution UHF Near-Field Imaging Probe

The structure of a high-resolution microwave material sensor is presented based on the injection locking phenomenon. For a sample under test, the variation of center frequency and lock range of the oscillator with respect to a reference sample are measured to indicate the resonance frequency and quality factor of the resonator. A low power level and low phase noise injection locking can significantly reduce the lock range of the oscillator. Therefore, even small variations in the loop filter of the oscillator change the response and can be determined with a high sensitivity. The unknown sample is characterized based on the measured center frequency and quality factor of the resonator. An output power control (OPC) system increases the dynamic range of the sensor with respect to the dielectric constant of samples. Moreover, a method of material detection in the microwave frequency is proposed based on data from the OPC to eliminate the frequency analyzer from the system for Internet of Things (IoT) applications. As an example for validation, a 1-GHz sensor is fabricated and the measurement results are presented.

High-Resolution Dielectric Sensor Based on Injection-Locked Oscillators

Near-field microwave imaging is increasingly finding utility for non-invasive inspection of various critical components in a wide range of industries. Conventionally, microwave imaging is performed using probes of linear electric field polarization such as a rectangular aperture probe excited with its dominant mode. Consequently, detecting polarized targets, e.g., cracks and highly conductive linear inserts, becomes challenging and highly dependent upon the orientation of the target with respect to the irradiating electric field vector. In practice, the orientation of the target could be unknown a priori. Hence, selecting a single suitable polarization is not always feasible. To address this challenge, a dual-polarized Teflon-loaded circular aperture-based microwave probe is proposed herein for imaging applications at 24 GHz. The proposed probe irradiates the target with two orthogonal polarizations and allows reconstructing the image using the average of the reflected signals due to these polarizations. The scattering parameters and electric field distribution of the proposed probe are analyzed using numerical electromagnetic simulations. A prototype of the proposed probe is fabricated and its characteristics are measured experimentally. The efficacy of the proposed imaging probe is demonstrated by acquiring phase and magnitude images of highly polarized targets. The images obtained using the proposed probe are compared to those obtained using a conventional open-ended rectangular waveguide probe. It is shown that unlike the conventional rectangular waveguide probe, the proposed probe could effectively detect polarized targets irrespective of their orientation.

Dual-Polarized Microwave Imaging Probe

Microwave imaging of thick composite structures using circular aperture probe

Near-field microwave imaging and subsurface sensing have shown significant potential in a wide range of applications, including corrosion mapping and composite structure inspection. Most of the conventional near-field imaging systems developed so far utilize an antenna (probe) scanned over the structure-under-test (SUT). The near-field interaction between the microwave signal and the SUT reveals the sought-after information about the interior of the irradiated structure. Previous systems commonly used open-ended rectangular waveguides as the near-field imaging probe. In this paper, the utility of circular waveguide probes for near-field imaging will be discussed in detail and compared with the conventional rectangular probe. The motivation behind utilizing the circular probe is the fact that its aperture field distribution yields lower side lobes. Herein, the reflection properties, near-field characteristics, sensitivity, and spatial resolution of both probes are contrasted using simulations and measurements. Furthermore, imaging results of various targets, such as flat-bottom holes, notches, as well as corrosion-under-paint samples acquired using circular and rectangular probes are presented. It will be shown that the circular probes offer key advantages in terms of resolution as well as sensitivity compared with the conventional open-ended rectangular waveguides.

Near-Field Microwave Imaging Using Open-Ended Circular Waveguide Probes

Thick multi-layer dielectric composite materials are integral part of many industries like aerospace, marine, petrochemical, etc. It is very important to ensure that these materials are of good quality and free from any kind of defects since these materials are used for critical applications. Several Non-destructive testing (NDT) methods have been applied to test and evaluate any flaws in these materials. In this paper, a waveguide based microwave NDT technique is used to interrogate the composite materials. A custom made Teflon loaded circular waveguide is employed for the inspection of defects in a test sample. The multi-layer composite structure and the waveguide are modeled in a numerical electromagnetic simulation software. The sensitivity of the circular probe to air inclusions in the composite is checked at 24 GHz by analysing the electric field variation at the defected layers. In addition to this, the response of the circular waveguide to the composite structure defects is compared to that of a conventional air filled rectangular waveguide at the same frequency point. The numerical simulation results are validated by imaging a practical five-layer honeycomb composite sample. The measured results are compared with the images produced using both the waveguide probes. It is observed that the circular probe is better in terms of resolution compared to the conventional rectangular waveguide.

Applied microwave imaging of composite structures using open-ended circular waveguide

Near-field microwave imaging has shown great potential for wide range of subsurface sensing and imaging applications including corrosion mapping and composite structure inspection. Most of the systems developed so far use an antenna (probe) scanned over the structure-under-test (SUT). The near-field interaction between the microwave signal and the SUT reveals the sought-after information about the interior of the irradiated structure. Almost entirely, previous systems used open-end waveguides as the imaging sensor. In this paper, the utility of circular waveguides for near-field imaging will be discussed in detail. Herein, both the sensitivity and spatial resolution of such a near-field probe will be investigated using simulations and measurements.

Near-field microwave imaging using open-ended circular waveguides

Ultrasonic testing (UT) techniques are extensively used for nondestructive testing (NDT) of material structures in various industries. In particular, phased array UT systems have proven significant industrial utility for a wide range of inspection applications including corrosion and material loss mapping in metals. On the other hand, near-field microwave NDT is emerging as powerful inspection modality for similar applications. Consequently, it becomes imperative to benchmark the capabilities of these modalities on common test specimen. In this paper, a practical evaluation of the capabilities of these two techniques for surface defect imaging is presented and discussed. A near-field microwave imaging system operating at 24 GHz as well as a 5 MHz phased array UT system are introduced and described. A varying set of test specimen which includes surface defects such as flat bottom holes, slots, and corrosion-under-paint are imaged using both modalities. The resulting C-scan images of the two systems are reported and compared. It is shown that, even when the utilized microwave frequency is relatively low, the microwave system produces images with comparable resolution to those produced with the UT phased array system.

Surface defects mapping using microwaves and ultrasonic phased array imaging

Field Performance of Bitumen Emulsion Cold Central Plant Recycling (CCPR) Mixture with Same Day and Delayed Overlay Compared with Traditional Rehabilitation Procedures

Mohammad R. Ramzi

Papers

Near-Field Microwave Imaging Using Open-Ended Circular Waveguide Probes

Applied microwave imaging of composite structures using open-ended circular waveguide

Near-field microwave imaging using open-ended circular waveguides

Surface defects mapping using microwaves and ultrasonic phased array imaging

Field Performance of Bitumen Emulsion Cold Central Plant Recycling (CCPR) Mixture with Same Day and Delayed Overlay Compared with Traditional Rehabilitation Procedures