Detection of Surface Cracks in Metals using Microwave and Millimeter-Wave Nondestructive Testing Techniques-A Review

Integrity Assessment of Metallic Structures Requires Inspection Tools Capable of Detecting and Evaluating Cracks Reliably. to This End, Many Microwave and Millimeter-Wave Nondestructive Testing and Evaluation (NDT&E) Methods Have Been Developed and Applied Successfully in the Past. Detection of Fatigue Cracks with Widths Less Than 5 Μ M using Noncontact Microwave-Based Inspection Methods Was Demonstrated in the 1970s. Since their Introduction, These Methods Have Evolved Considerably Toward Enhancing the Detection Sensitivity and Resolution. Undertaking Key Application Challenges Has Attracted Considerable Attention in the Past Three Decades and Led to the Development of the Near-Field Techniques for Crack Detection. to Address a Need that Cannot Be Fulfilled by Other NDT&E Modalities, Innovative Noncontact Microwave and Millimeter-Wave NDT&E Methods Were Devised Recently to Detect Cracks of Arbitrary Orientations under Thick Dielectric Structures. While the Reported Methods Share the Same Underlying Physical Principles, They Vary Considerably in Terms of the Devised Probes/sensors and the Application Procedure. Consequently, their Sensitivity and Resolution as Well as their Limitations Vary. This Article Reviews the Various Crack Detection Methods Developed To-Date and Compares Them in Terms of Common Performance Metrics. This Comprehensive Review is Augmented with Experimental Comparisons and Benchmarking Aimed to Benefit NDT&E Practitioners and Researchers Alike

Improved Grounded Coplanar Waveguide-To-Multilayer Substrate Integrated Waveguide Transition for Efficient Feeding of an Antipodal Vivaldi Antenna for Imaging Applications

This paper presents a novel transition junction between a grounded coplanar waveguide (GCPW) and a multilayer substrate integrated waveguide (SIW) that is intended as a feed for an SIW antipodal Vivaldi antenna. Compared to other similar transitions, this transition has better matching and lower insertion loss. More specifically, the junction between GCPW and multilayer SIW is designed to improve matching between the two-layer feed line and the multilayer SIW, thereby reducing unwanted radiation from the transition and improving the radiation pattern of the SIW antipodal Vivaldi antenna. These improvements are illustrated through simulation and measurement of a 34.5-37.5 GHz antipodal Vivaldi antenna that is fed by a SIW feed connected to a GCPW via the improved junction

A Comprehensive Bi-Static Amplitude Compensated Range Migration Algorithm (AC-RMA)

In this paper, a comprehensive form of the range migration algorithm (RMA) is analytically derived for reconstructing the reflectivity function using synthetic aperture imaging techniques. Specifically, amplitude compensation, in addition to the typical phase compensation, is included in the development of the matched filter of the RMA, with the result herein referred to as the amplitude compensated RMA (AC-RMA). To illustrate the improvements offered by the AC-RMA, simulation and measurement (at Ka-band, 26.5 - 40 GHz) are performed to reconstruct the reflectivity function of a target using both RMA and AC-RMA algorithms. The results prove that the AC-RMA is a robust algorithm that can successfully reconstruct the reflectivity function of a target with higher accuracy, regardless of its dielectric properties, including scenarios with low contrast between the dielectric properties of the background and target in the presence of noise. This approach is also independent of the bandwidth of the imaging system and is applicable to multilayer media as well. In addition, while the formulation of the AC-RMA is more complicated than the traditional (phase compensation only) RMA, the processing time necessary for images created with the AC-RMA is just 1.2 times greater than that of the traditional RMA processing time

Health Monitoring of RAM-Coated Structures by Active Microwave Thermography

In this article, the application of active microwave thermography (AMT) is investigated as an efficient nondestructive technique (NDT) for health monitoring of structures coated with radio frequency absorbing materials (RAMs). The uniqueness of AMT for this particular application is the use of microwave excitation, which acts as a highly efficient thermal source due to the inherent absorptive electromagnetic properties of RAM-coated structures. In addition, the microwave-induced heating takes place subsurface within the absorbing material of an RAM-coated structure, as opposed to the sole surface heating of conventional thermography. Specifically, this work focuses on detection of delamination, a common defect in carbon fiber reinforced polymer (CFRP) structures, via AMT. To this end, a formulation and analysis method based on electromagnetic/thermal modeling is presented, which enables estimation of the output of an AMT inspection of an RAM-coated CFRP structure suffering from delamination. The effect of the structure\u27s constitutive properties, the microwave excitation parameters, and the delamination size and depth on its detectability is investigated. In addition, several simulated and experimental results are provided, which verifies the accuracy of the proposed approach and shows the high efficiency of AMT for detection of delamination in such structures