NDT, Volume 3, Issue 1 (March 2025) – 6 articles

The thermoplastic composite pipe (TCP) manufacturing process introduces defects that impact performance, such as voids, misalignment, and delamination. Consequently, there is an increasing demand for effective non-destructive testing (NDT) techniques to assess the influence of these manufacturing defects on TCP. The objective is to identify and quantify internal defects at a microscale, thereby improving quality control. A combination of methods, including NDT, has been employed to achieve this goal. The density method is used to determine the void volume fraction. Microscopy and void analysis are performed on pristine samples using optical micrography and scanning electron microscopy (SEM), while advanced techniques like X-ray computer tomography (XCT) and ultrasonic inspections are also applied. The interlayer between the reinforced and inner layers showed good consolidation, though a discontinuity was noted. Microscopy results confirmed solid wall construction, with SEM aligning with the XY axis slice, showing predominant fibre orientation around ±45° and ±90°, and deducing the placement orientation to be ±60°. Comparing immersion, 2D microscopy, and XCT methods provided a comparative approach, even though they could not yield precise void content values. The analysis revealed a void content range of 0–2.2%, with good agreement between microscopy and Archimedes’ methods. Based on XCT and microscopy results, an increase in void diameter at constant volume increases elongation and reduces sphericity. Both methods also indicated that most voids constitute a minority of the total void fraction. To mitigate manufacturing defects, understanding the material’s processing window is essential, which can be achieved through comprehensive material characterization of TCP materials. Full article

The non-destructive geophysical testing method ground penetrating radar (GPR), along with satellite image and air photo assessment, a review of the existing literature sources, and Holocaust survivor testimony, was used to document the location of potential mass graves in Alytus, Lithuania. In World War II, six million Jews were murdered, as were as many as five million other victims of Nazi Germany’s orchestrated persecution. In the summer of 1941, 8030 Jews (4.70 percent of Lithuania’s Jewish population) lived in Alytus County, where the town of Alytus is located. It is estimated that over 8000 Jews were murdered in Alytus County, including nearly the entire Jewish population of the town of Alytus. The murder of Jews from Alytus County accounts for approximately 4.2% of the total number of Lithuanian Jews killed in the Holocaust. Survivor testimony indicates that several thousand Jews from both the town and county were murdered and buried in the Vidzgiris Forest about 1000 m from the town center. In 2022, field reconnaissance at locations in the forest, which appeared to be disturbed in a 1944 German Luftwaffe air photograph, indicated that these disturbances were associated with natural geomorphic processes and not the Holocaust. Analysis of GPR data that was collected using a pulseEKKO Pro 500-megahertz groundpenetrating radar (GPR) system in 2022 in the vicinity of monuments erected in the forest to memorialize mass graves indicates that no mass graves were directly associated with these monuments. The 1944 air photograph contained two roads that traversed through and abruptly ended in the forest, which was the impetus for detailed field reconnaissance in that area. A segment of a 150 m long linear surface feature found in the forest was assessed using GPR, and based on the profile that was generated, it was determined that this feature is possibly a segment of a much more extensive mass grave. Testimony of a Holocaust survivor stated that as many as three burial trenches exist in this portion of the forest. Additional research using non-destructive GPR technology, air photograph and satellite image assessment, and the existing literature and testimony-based data are required for the Vidzgiris Forest to better define these and other potential mass graves and other Holocaust-related features. Full article

Radar polarimetric imaging for non-destructive testing is a powerful and flexible tool that can be used to enhance the detection of internal structures. In this study, reinforced autoclaved aerated concrete (RAAC) is measured using a polarimetric system in three different acquisition modes—two downward-looking and one sideways-looking configurations, each at a different height. Each acquisition mode is compared and new polarisation states are created using the principle of polarisation synthesis. Images of the internal structures are created using a 3D imaging algorithm, which are used for the analysis. The comparison between acquisition modes demonstrates that using a higher lift-off and polarisation synthesis could offer more flexible operation in the field, allowing the use of handheld detectors and drone-based systems for inaccessible areas. Additionally, the sideways-looking data captured both horizontal and vertical reinforcement and were detected within a single polarisation channel; this configuration also has reduced clutter from the air–concrete boundary, providing a viable option for single polarisation systems. Full article

This review examines the essential application of non-destructive testing (NDT) techniques in assessing the integrity and damage of composite materials used in aerospace engineering, focusing on polymer matrix composites (PMCs), metal matrix composites (MMCs), and ceramic matrix composites (CMCs). As these materials increasingly replace traditional metallic and alloy components due to their advantageous properties, such as light weight, high strength, and corrosion resistance, ensuring their structural integrity becomes paramount. Here, various NDT techniques were described in detail, including ultrasonic, radiographic, and acoustic emission, among others, highlighting their significance in identifying and evaluating damages that are often invisible, yet critical, to parts safety. It stresses the need for innovation in NDT technologies to keep pace with the evolving complexity of composite materials and their applications. The review underscores the ongoing challenges and developments in NDT, advocating for enhanced techniques that provide accurate, reliable, and timely assessments to ensure the safety and durability of aerospace components. This comprehensive analysis not only illustrates current capabilities but also directs future research pathways for improving NDT methodologies in aerospace material engineering. Full article

The calibration of a universal testing machine (UTM) verifies the accuracy of the system instruments responsible for obtaining force and displacement measurements. This process involves comparing the instrument to equipment that has already been calibrated to a known traceable standard. The limit of accuracy is then certified and the traceability of the measurements is determined. There are several internationally recognized standards that are used to calibrate the cross-head speed and displacement (ASTM E2658 and E2309, respectively), strain and load rate (ASTM E2309), measurement of tension, compression (ASTM E4) and dynamic force (ASTM E467). The current study aims to monitor the calibration status of UTMs through the implementation of acoustic methods. A methodology is developed whereby a reference sample is initially identified with suitable material properties, enabling it to be used continuously. The sample is used simultaneously with acoustic instruments to check its natural frequencies, which enables the monitoring of the UTM calibration status. An algorithm is developed that enables the user to interact with the system, thus forming an interface and helping the user to check the calibration status of the equipment. The entire system is validated to check if the equipment and the inbuilt algorithm can predict the calibration status of the machine. It was found that the geometric constraints imposed on the sample influence the output from the algorithm, and hence correct values should be fed to the system. Our sample never lost its elastic characteristics through continuous use, demonstrating that it can be used to continuously monitor the machine’s status. Full article

Temperature has a significant impact on the operational lifetime of electronic components, as excessive heat can lead to accelerated degradation and ultimately failure. In safety-critical applications, it is important that real-time monitoring is employed to reduce the risk of system failures and maintain the safety, reliability, and integrity of the connected systems. In the case of printed circuit boards (PCBs), it is often not feasible to install enough sensors to adequately cover all of the temperature sensitive components. In this study, we present a novel method for the temperature monitoring of PCBs using ultrasonic guided waves and machine learning techniques. Our approach utilizes a small number of low-cost, unobtrusive piezoelectric wafer active sensors (PWAS) sensors for propagating ultrasonic guided waves across a PCB. Through interaction with board features, the temperature of components can be predicted using multi-output regression algorithms. Our technique has been applied to three different PCBs, each with five hotspot positions, achieving an RMSE of <3.5 °C and $R^2$ > 0.95 in all three cases. Full article