Nondestructive Testing Applications Research Articles

A robust adaptive-thresholding primal-dual sparse recovery for THz imaging

Terahertz time-domain spectroscopy (THz-TDS) system is a powerful tool in material spectral analysis and non-destructive testing (NDT). However, the data acquisition is time-consuming due to the point-by-point scanning process, which leads to limitations in THz imaging applications. In this paper, a robust adaptive-thresholding primal-dual sparse recovery (APSR) method is proposed to reconstruct high-resolution THz images and reduce the acquisition time. A sparsity averaging dictionary, which is a concatenate of multiple bases, is applied to improve the robustness and generalization. A reweighted scheme is also adopted to enhance the sparse solution. As a crucial consideration in the sparse recovery method, we propose a robust adaptive threshold estimator based on the median absolute deviation in the sense that the final threshold determines the reconstruction quality and the convergence rate. Meanwhile, together with the Nesterov acceleration technique, a large threshold can be applied at the beginning of the iteration to help accelerate the convergence. Numerical experiments demonstrate that the APSR method can reconstruct THz images with robustness even with low SNR (of 30 dB) and low sampling rate (of 30%) compared to conventional methods. In the real THz-TDS system, our proposed sparse imaging method can successfully recover a composite object with much fewer THz spectra and reduces the acquisition time to 30% at the cost of a relative error of 2.5%, demonstrating the efficiency of our proposed method and the penetrating ability of THz technology in the NDT applications.

Acoustic attenuation coefficient measurement in extreme environments: Experimental characterization of uncertainty in multiple-reflection method

Measuring acoustic properties in extreme environments, involving hazardous chemical samples or temperature above 50ºC or below 0ºC, is challenging yet crucial for understanding waveform interactions with materials, with applications in fields like non-destructive testing (NDT) and medical applications. Multiple-reflection methods (MRM), using a buffer-rod between transducer and sample, offer a solution. Yet, the impact of variables like contact force, buffer-rod size, and surface roughness on the accuracy and precision of MRM measurements remains unvalidated. To address this, we introduce a system and experiments to quantify the accuracy of measurements of acoustic attenuation coefficient. Results demonstrate the substantial impact of contact force on system accuracy and precision, reflected by a decrease in the measured attenuation coefficient at 0.5 MHz from 3.8 to 1.5dB/cm with increasing contact force. Effects of buffer-rod width were less pronounced and supported the theory that a cylindrical buffer-rod with twice the radius of the transducer is sufficient. Interestingly, surface roughness, despite being over 150 times smaller than the wavelength of the acoustic signal, influences measurement accuracy. This study illuminates the crucial role of system factors on measurement accuracy and uncertainty, which will aid optimization of measurements for diverse practical applications.

Development of a lead-free, high-frequency ultrasound transducer with broad bandwidth and enhanced pulse-echo response, employing β-Ni(OH)2/PVDF-TrFE piezoelectric composite

Piezoelectric composite-based high-frequency ultrasound transducers (USTs) have gained significant prominence as a leading and effective tool in non-invasive medical diagnosis, non-destructive testing and diverse industrial applications. In this study, we present the fabrication of a lead-free, high-frequency ultrasound transducer (P-UST), having a very simple and unique assembly procedure. To achieve this, we employed PVDF-TrFE based piezoelectric composite thin film (NTr5) integrated with a transition metal hydroxide (β-Ni(OH)2 nanoplates) and demonstrated its outstanding performance, thereby showcasing its potential in various applications. The composite exhibited exceptional β-phase crystallization, reaching impressive maximum value of 90.04%, owing to the favorable interaction between electrostatic ions and dipoles, along with the presence of strong surface polarization. Upon analysis, remarkable piezoelectric (d33∼50.22 pm/V) and ferroelectric properties were observed in the composite material, accompanied by significant enhancement in surface roughness. Here, the piezoelectric property's impact on UST output is well-explained using Maxwell's equation. P-UST, being a versatile system, is capable of generating and receiving pulse signals. By conducting a pulse-echo test on an aluminium block with ultrasound gel, we achieved a peak-to-peak output voltage of 5.19 V, surpassing the output voltage magnitude of any previously reported UST in this field. The Fast Fourier transformed data of the voltage output revealed an impressive −6 dB bandwidth of 51.22 MHz (102.4%) with a high central frequency of 50.01 MHz. Hence, our P-UST's exceptionally high central frequency promises significantly enhanced ultrasound image resolution, which expands the potential for highly accurate medical assessments for advanced healthcare outcomes and improving patient well-being.

All-optical Compton scattering at shallow interaction angles

All-optical Compton sources combine laser-wakefield accelerators and intense scattering pulses to generate ultrashort bursts of backscattered radiation. The scattering pulse plays the role of a small-period undulator ( ${\sim }1\,\mathrm {\mu }{\rm m}$ ) in which relativistic electrons oscillate and emit X-ray radiation. To date, most of the working laser-plasma accelerators operate preferably at energies of a few hundreds of megaelectronvolts and the Compton sources developed so far produce radiation in the range from hundreds of kiloelectronvolts to a few megaelectronvolts. However, for such applications as medical imaging and tomography the relevant energy range is 10–100 keV. In this article, we discuss different scattering geometries for the generation of X-rays in this range. Through numerical simulations, we study the influence of electron beam parameters on the backscattered photons. We find that the spectral bandwidth remains constant for beams of the same emittance regardless of the scattering geometry. A shallow interaction angle of $30^{\circ }$ or less seems particularly promising for imaging applications given parameters of existing laser-plasma accelerators. Finally, we discuss the influence of the radiation properties for potential applications in medical imaging and non-destructive testing.

Improving the reliability of NiTi endodontic files using eddy currents, thermography and fatigue tests: A multimethod approach

Non-Destructive Testing (NDT) by Eddy Currents (EC) allows for determining material properties and detecting surface and subsurface defects that can harm structural integrity of electrically conductive components. However, despite being a standard technique and with many years of accumulated experience, there are some conditions where difficulties persist, such as detecting small defects in complex components with small dimensions and conicity (e.g. endodontic files used in a clinical environment).This work aimed to design, numerically simulate, produce, and experimentally validate different configurations of EC probes for defect detection in endodontic files. For the application of NDTs, an artificial defect was introduced in the apical region of a commercial endodontic file with 4% taper and a tip’s diameter equal to 0.20 mm. The experimental results showed that most of the methodologies developed for the inspection of files through NDTs proved unable to detect the defective region, primarily due to its taper and the special Pauli paramagnetic properties of the NiTi alloy. Nevertheless, the customized EC toroidal probe operating in differential bridge mode could detect the artificial defect in the endodontic file. In parallel, a thermographic inspection of the defective instrument was made. As a result, it was possible to detect the defect by thermography, although with low Signal-to-Noise Ratio (SNR) due to its small dimensions.Finally, commercial endodontic files manufactured by Electrical Discharge Machining (EDM) were tested and compared with the results obtained for conventional manufactured endodontic files. The endodontic instruments were placed in rotation in an experimental setup designed to simulate a root canal with a radius of curvature equal to 5 mm applied along a 45° angle. The experimental results showed that the latest generation of endodontic files, manufactured using EDM process, are significantly more resistant to fatigue fracture when compared to the Controlled Memory (CM) machined files.

Defect-Band Splitting of a One-Dimensional Phononic Crystal with Double Defects for Bending-Wave Excitation

Extensive prior research has delved into the localization of elastic wave energy through defect modes within phononic crystals (PnCs). The amalgamation of defective PnCs with piezoelectric materials has opened new avenues for conceptual innovations catering to energy harvesters, wave filters, and ultrasonic receivers. A recent departure from this conventional paradigm involves designing an ultrasonic actuator that excites elastic waves. However, previous efforts have mostly focused on single-defect scenarios for bending-wave excitation. To push the boundaries, this research takes a step forward by extending PnC design to include double piezoelectric defects. This advancement allows ultrasonic actuators to effectively operate across multiple frequencies. An analytical model originally developed for a single-defect situation via Euler–Bernoulli beam theory is adapted to fit within the framework of a double-defect set-up, predicting wave-excitation performance. Furthermore, a comprehensive study is executed to analyze how changes in input voltage configurations impact the output responses. The ultimate goal is to create ultrasonic transducers that could have practical applications in nondestructive testing for monitoring structural health and in ultrasonic imaging for medical purposes.

Machine Learning Applications in Nondestructive Testing of Concrete Structures

AbstractMachine Learning bears great potential for data‐driven solutions in the field of nondestructive testing (NDT) of concrete structures. The analysis of the data collected with NDT methods, such as ultrasonics, impact‐echo and ground penetrating radar, can be complex and requires experience. The expected benefit of Machine Learning applications in this context goes beyond the increase of efficiency obtained by automating the analysis process. While traditional analysis approaches are usually solely based on key features according to the basic principles, Machine Learning algorithms can consider the full data content and reveal hidden correlations. For an organized approach to Machine Learning on NDT data, an analysis tool has been developed. The tool provides a graphical user interface to manage and label training/test data and interactively define the Deep Neural Network architecture. In particular, Convolutional Neural Networks, as proven successful in various image recognition tasks, are implemented. The Machine Learning concepts are demonstrated in show cases, comprising ultrasonic as well as impact‐echo applications. In particular, the relevance of a targeted preprocessing is addressed, comparing the effectiveness of time‐, frequency‐ and joint‐time‐frequency‐representations.

Microwave Real-Time and High-Resolution Imaging System Development for NDT Applications: A Chronology

In the past four decades, microwave imaging has experienced a renaissance. This has happened because of new technology development, improved and robust image reconstruction algorithms, expansion of imaging modalities, and operational friendliness, particularly for nondestructive testing (NDT) applications (e.g., high-resolution, real-time, portable systems). Efforts to push the envelope forward continues. This paper provides a brief background on some of the key works that laid the foundation for these advancements. Next, the focus of the paper shifts to the author’s research group’s contributions to the field since the early 1990s to the present day in developing new imaging methodologies and tools specifically for NDT applications. This paper gives a chronology of these specific developments and illustrates how the desire to address NDT needs and requirements have kept these developmental processes moving forward.

The Application of Optical Nondestructive Testing for Fresh Berry Fruits

The Application of Optical Nondestructive Testing for Fresh Berry Fruits

Antisymmetric Lamb Wave Simulation Study Based on Electromagnetic Acoustic Transducer with Periodic Permanent Magnets.

Due to its multi-mode and dispersion characteristics, Lamb waves cause interference to signal processing, which profoundly limits their application in nondestructive testing. To resolve this issue, firstly, based on the traditional EMAT, a horizontal polarization periodic permanent magnet electromagnetic acoustic transducer (HP-PPM-EMAT) was proposed. A 2-D finite element model was then developed to compare magnetic flux density, Lorentz force, and signal strength between the traditional EMAT and the HP-PPM-EMAT. The simulation results show that the HP-PPM-EMAT enhances the A0 mode Lamb wave (A0 wave) and suppresses the S0 mode Lamb wave (S0 wave). Finally, the influence of structural parameters of the HP-PPM-EMAT on the total displacement amplitude ratio of A0 and S0 was investigated using orthogonal test theory, and the width of magnet units was improved based on the orthogonal test. The results show that the total displacement amplitude ratio of A0 to S0 of the improved HP-PPM-EMAT can be improved by a factor of 7.74 compared with that of the traditional Lamb wave EMAT, which can produce higher-purity A0 mode Lamb waves.

Application of Ultrasonic Tomography to detect defects in Concrete Structures

Concrete is a material widely used in civil construction for structural elements. Due to this, there is an increasing concern about the knowledge of its deterioration state and safety. This stimulates research for methods that allow the evaluation of finished structures. Through the application of Nondestructive Testing (NDT) methods, it is possible to proceed with an investigation of the concrete structures. The application of NDT can contribute to problems diagnosis and quality control of concrete structures. Ultrasound Tomography (UT) tests are an attractive and feasible research strategy. UT allows mapping of the internal section of the object from projections of the same. The tomography uses an antenna, which in turn utilizes a series of dry contact point transducers that emit shear waves into the concrete structures. From the data collected by the equipment, 3D images of the element's reflected interfaces are generated. The present study aims to detect different anomalies that can be found in reinforced concrete slabs, verifying the potential of ultrasonic tomography for the detection of these anomalies. The study was performed by the technical team of the Structural Tests and Materials Laboratory at the Federal University of Rio Grande do Sul (LEME / UFRGS).

Application of Non-destructive Testing in Quality Control of Manufactured Aluminium Metal Matrix Composite Components for the Automotive Industry

The manufacturing of automotive components using aluminium metal matrix composites (Al-MMCs) is progressively gaining a relevant place in the automotive industry. Due to the increasing competition in the automobile sector, there is a great need to supply efficient, stable, and high-quality products. Generally, following automotive standards is voluntary, which means that businesses and other organizations are not legally obliged to apply them. Nevertheless, in certain cases standards may facilitate compliance with legal requirements, such as those contained in European directives and regulations. IATF 16949:2016 is the current version of the IATF 16949 Standard and outlines the requirements an organization must maintain in their quality system for IATF 16949 certification. This new application of manufacturing Al-MMCs requires additional procedures to guarantee the quality of the components manufactured. Because the manufacturing process may generate various defects, such as cracks, voids, lack of fusion, inclusions, porosities, and other metallurgical aspects, non-destructive testing (NDT) techniques should be used for guaranteeing the quality of the components manufactured. The development of reliable NDT methods for aluminium metal matrix composite parts is thus a major challenge. This aspect is related to how NDT procedures need to be optimised or developed to address the singular structures of the processes, such as complex geometry, anisotropic material properties, special internal structures, and the presence of nanoparticles. The main objective of this work is to summarize and present the relevant potential of different NDT techniques used in quality control of manufactured Al-MMC components in the automotive industry, to be explorer in the European FLAMINGo project (Fabrication of Lightweight Aluminium Metal Matrix Composites and Validation in Green Vehicles). This study is very significant because the NDT techniques present different probabilities of detection of the defects, principally of the microdefects.

Mechanically tunable multi-band terahertz absorber based on overlapping graphene nanoribbon arrays

With promising applications in broadband communication, terahertz (THz) imaging, biochemical sensing, nondestructive testing and other fields, THz absorbers have become a hot research topic in recent years. Although a number of graphene-based multi-band THz absorbers with tunable performances have been reported, the tuning methods are mostly limited to electrical tuning. Here, we report a mechanically tunable multi-band THz absorber, which is based on overlapping graphene nanoribbon arrays. By adjusting the horizontal position of the upper cover plate with graphene nanoribbon array, the frequencies of the absorption peaks can be tuned in a wide range. In addition to the absorption peaks originating from even-order lateral Fabry-Pérot resonance (LFPR) modes, there are also absorption peaks originating from inductor-capacitor (LC) oscillations. An equivalent LC circuit model is developed and theoretical results match well with simulated values. The proposed mechanically tunable THz absorber may provide a reference for tunable THz absorbers that can be applied in various fields.

Thermoacoustic phased-array radiators – Theory, characteristics, and applications

Standard non-invasive acoustic non-destructive testing (NDT) methods like testing in contact or using immersion tank arrangements are limited since the probes need to be in direct contact with the test piece or using a fluid as coupling media. Air-coupled ultrasonic testing avoids these limitations and enables new testing methods. Two up-and-coming transducer technologies are recognized as promising: transducers using ferroelectrets and transmitters based on the thermoacoustic principle. Thermoacoustic radiators based on thin-film technology offer a resonant-free generation of ultrasound. The resonance-free operation enables the wideband generation of acoustics waves and promises advantages for existing NDT methods. Using specially shaped substrates and electrodes enables the fabrication of planar and focused thermoacoustic radiators. This contribution presents the latest results of our ongoing research regarding thermoacoustic radiators, which is the thermoacoustic phased-array. Phased-array transducers offer the on-the-fly change of the steering angle and the position of the focal point without moving or modifying the transducer itself. Combining both technologies, thermoacoustic emitter and the phasedarray principle broaden the spectrum of classical NDT applications and modern approaches like the excitation of guided waves or structural health monitoring. In detail, we modelled the sound field of a self-fabricated thermoacoustic phased-array radiator, characterised the emitter elementwise and tested several application scenarios (e.g., transmission, beam steering, focusing and the excitation of guided waves).

On monitoring fretting fatigue damage in solid railway axles by acoustic emission with unsupervised machine learning and comparison to non-destructive testing techniques

Railway axles are safety-critical components of the rolling stock and the consequences of possible in-service failures can have dramatic effects. Although this element is traditionally designed against such failures, the initiation and propagation of service cracks are still occasionally observed, requiring an effective application of non-destructive testing and structural health monitoring approaches. This paper investigates the application of structural health monitoring by acoustic emission to the case of solid railway axles subject to fretting fatigue damage. A full-scale test was performed on a specimen in which artificial notches were suitably manufactured in order to cause the initiation and evolution of fretting fatigue damage up to the stage of relevant propagating fatigue cracks. During the test, both periodical phased array ultrasonic inspections and continuous acquisition of acoustic emission data have been carried out. Moreover, at the end of the test, the specimen was inspected, analyzed and evaluated by visual inspection and magnetic particles testing, while acoustic emission raw data were post-processed by a special unsupervised machine learning algorithm based on an Artificial Neural Network. It is demonstrated that the proposed methodology is very effective to detect the onset of crack initiation in a non-invasive and safe way.

Image restoration algorithm for terahertz FMCW radar imaging.

The terahertz frequency modulation continuous-wave (THz FMCW) imaging technology has been widely used in non-destructive testing applications. However, THz FMCW real-aperture radar usually has a small depth of field and poor lateral resolution, thus restricting the high-precision imaging application. This paper proposes a 150-220GHz FMCW Bessel beam imaging system, effectively doubling the depth of field and unifying the lateral resolution compared to the Gaussian beam quasi-optical system. Moreover, a THz image restoration algorithm based on local gradients and convolution kernel priors is proposed to eliminate further the convolution effect introduced by the Bessel beam, thereby enhancing the lateral resolution to 2mm. It effectively improves the image under-restoration or over-restoration caused by the mismatch between the ideal and actual point spread function. The imaging results of the resolution test target and semiconductor device verify the advantages of the proposed system and algorithm.

Accuracy of Non-Destructive Estimation of Length of Soil Nails

The effective length of soil nails is one of the critical parameters ensuring the reinforcing effect, and its accurate estimation is of great significance for the safety of the slope and deep foundation pit supporting projects. Traditional quality insurance methods, such as nail pullout tests, suffer from a series of drawbacks including being destructive, high cost, and time-consuming. In contrast, non-destructive testing (NDT) has been increasingly applied in various engineering fields in the past decades given its advantages of not damaging the material and easy operation. However, the current application of NDT in soil nail length measurement is relatively limited, and its accuracy and reliability are yet to be quantitatively evaluated. This paper introduces three methods for estimating soil nail length based on guided wave theory and collects 116 sets of NDT data for nail length. The accuracy of the NDT in soil nail prediction is statistically analyzed using the model bias method. The results show that those methods can accurately predict the nail length with an average error of less than 3% and a low dispersion of 12%. The accuracy of the NDT methods is not affected by the hammer type or estimation method. Furthermore, this paper proposes a model calibration to the original NDT method, which improves the model’s average accuracy by 3% and reduces dispersion by 4% without increasing computational complexity. Finally, the probability distributions of the model biases are characterized. This study can provide practical tools for fast estimation of in situ nail length, which is of high significance to supporting slopes and deep foundation pits.

Metasurface Optics With On-Axis Polarization Control for Terahertz Sensing Applications

Non-destructive testing and imaging of materials with unknown properties are important applications of terahertz technology. Different contrast forming methods are available to obtain diverse information on the imaged region, including intensity, color, phase and polarization. Polarimetric imaging is relatively under-investigated owing to the difficulty in its implementation with complex optical arrangements. In this paper we present the design and monolithic fabrication of an anisotropic metasurface that incorporates several beam forming functions in a single layer with high efficiency thus simplifying instrument construction. The probe beam generated by the metasurface consists of an orthogonally polarized pair of collinear Bessel beams that create a well-defined on-axis change of polarization. The metasurface is integrated into a polarimetric imaging microscope for 3D beam profiling and sensing experiments at 2.52 THz. An analytical model to describe the on-axis polarization variation was found to be in excellent agreement with both finite-difference time-domain (FDTD) simulations and the experimental results. The setup was used to measure the refractive index and diattenuation of homogeneous sample regions in a proof-of-principle application. We anticipate this technology will find future application in non-destructive testing and polarimetric imaging of polymer composites and 3D profilometry of reflective surfaces.

Evaluation and New Innovations in Digital Radiography for NDT Purposes

The identification of dental and maxillofacial sores has essentially improved with the utilization of digital radiography. A sort of x-ray imaging called digital radiography utilizes digital X-ray sensors instead of regular visual film. Time reserve funds from staying away from substance handling and the ability to move and improve photographs digitally while exposing labourers to less radiation are benefits. The high-energy non-destructive testing (NDT) applications are making progress thanks to the headways in digital radiography innovation. Digital radiography has various advantages, including the ability to make pictures in a flash and to introduce an excellent picture on the PC screen. This study investigated how a U-net profound learning semantic division model acted comparable to two picture quality boundaries: motion toward clamour proportion (SNR) and contrast-to-commotion proportion (CNR). The exposure factors used to make the information pictures, for example, kilo voltage, mill ampere, and exposure time, affected the nature of the radiography pictures that were delivered. The discoveries of this study feature the meaning of making a preparation dataset that is offset as per the quality factors that were examined to work on the usefulness of profound learning division models for NDT digital X-ray radiography applications.

Validated and Deployable AI/ML for NDT Data Diagnostics

While artificial intelligence/machine learning (AI/ML) methods have shown promise for the analysis of image and signal data, applications using nondestructive testing (NDT) for managing the safety of systems must meet a high level of quantified capability. Engineering decisions require technique validation with statistical bounds on performance to enable integration into critical analyses, such as life management and risk analysis. The Air Force Research Laboratory (AFRL) has pursued several projects to apply a hybrid approach that integrates AI/ML methods with heuristic and model-based algorithms to assist inspectors in accomplishing complex NDT evaluations. Three such examples are described in this article, including a method that was validated through a probability of detection (POD) study and deployed by the Department of the Air Force (DAF) in 2004 (Lindgren et al. 2005). Key lessons learned include the importance of considering the wide variability present in NDT applications upfront and maintaining a critical role for human inspectors to ensure NDT data quality and address outlier indications.