Current Nondestructive Testing Research Articles

Inverse estimation method for internal defects based on surface stress of carbon-fiber-reinforced plastics using machine learning

Carbon-fiber-reinforced plastic (CFRP) is a composite material whose base material is plastic and reinforcement material is carbon fibers. CFRP is widely used in various fields for laminating prepregs. The laminated plate tends to sustain damage, such as delamination, fiber breakage, and base material breakage; hence, we must conduct high-precision and efficient nondestructive testing (NDT). Examples of NDT are ultrasonic examination, X-ray tomography, and infrared stress analysis. With most NDT methods, it is difficult to easily obtain detailed correct information of defects, such as their depth, position, and size. To solve this problem, we develop a machine-learning-aided inverse analysis model that predicts the spatial information of defects from the sum of the principal stresses on the surface calculated from the temperature change measured by infrared analysis, and we propose it as an alternative method to the existing damage analysis. Applying the proposed method to the simulated stress distributions of quasi-isotropic CFRP laminates with defects, the results showed over 99% success to recognize the detail information of defects. Additionally, we examine the properties of the dataset using a forward analysis model and a variational autoencoder. Our method with a convolutional neural network enables us to successfully estimate the information of defects at high speed. Experimental data can be applicable as well as the simulation results to our proposed method, and we believe our method will be a powerful supporting tool for the current NDT for CFRPs.

Mechanism of Magnetic Permeability Perturbation in Magnetizing-Based Eddy Current Nondestructive Testing.

DC magnetization is generally considered to suppress the usual local magnetic permeability variation and increase the penetration depth for magnetizing-based eddy current testing (MB-ECT) of ferromagnetic materials. In fact, such simple explanations lead to rough nondestructive evaluation and cause new neglected non-uniform magnetic characteristics. Hence, the “perturbation” of the internal magnetic field variation is analyzed using a magnetic dipole model and the mechanism of magnetic permeability perturbation in MB-ECT is revealed. The theoretical analysis and simulations show that a significant permeability perturbation always appears around a defect and presents opposite features with strong and weak magnetization. Furthermore, experimental results indicate that the hidden signal component arising from the local permeability perturbation is critical for both far-side surface and near-side surface defects in the MB-ECT method.

Influence of Core and Shield of Coil on Skin Depth in Eddy Current Testing

ABSTRACT In eddy current (EC) nondestructive testing, coil is usually wound on core or covered by shield to improve the sensitivity of defect detection and ability of anti-interference of the probe. However, when core or shield is used, the magnetic field will be redistributed, resulting in a change in the speed of EC attenuation in the depth direction. The purpose of this paper is to reveal the influence of core and shield on skin depth of EC. The results of the finite element analysis show that applying core or shield on coil results in smaller skin depth and the skin depth decreases as the core or shield approaches the test sample. In addition, when both core and shield are used, the reduction of skin depth is minimal if both core and shield are ferromagnetic. The simulation results are verified by experiment.

Stress influence to eddy current control of cracked aeronautical material

Eddy current testing is widely used for non-destructive evaluation of conductive materials where an induced current created by a variable magnetic field propagate on the material surface and sensed by a special sensor. The main purpose of this paper is to study the influence of the stress field near the crack tip especially in the singular and elaborated zones of aeronautical material aluminium 7075-T6 to eddy current non-destructive testing enhanced with a very particular type of probe to have better stresses sensitivity. This experimental part understands impedance variations due to the stress with high measurement reliability and shows that eddy current control is very sensitive to the stress and produces significant changes in magnetic parameters that mean stress should be considered in mechanical parts inspection.

MATHEMATICAL MODELING OF MULTISTRAND STAY CABLES EDDY CURRENT NON-DESTRUCTIVE TESTING

Some aspects of mathematical simulation of the eddy current non-destructive testing for ferromagnetic components are considered that allow simplifying the mathematical model. The influence of the motion of an inhomogeneous test object in an inhomogeneous field of an eddy current probe of finite dimensions is considered. The influence of the inhomogeneous magnetization of the object, the nonlinearity and ambiguity of the dependence of the magnetic flux density on the magnetic field intensity is described. An example of a mathematical model and simulation results is presented.

Analytical model of a T-core coil above a multi-layer conductor with hidden hole using the TREE method for nondestructive evaluation

PurposeIn eddy current nondestructive testing, a probe with a ferrite core such as an E-core coil is usually used to detect and locate defects such as cracks and corrosion in conductive material. However, the E-core coil has some disadvantages, such as large volume and difficulty in the process of winding the coils. This paper aims to present a novel T-core probe and its analytical model used for evaluating hidden holes in a multi- layer conductor.Design/methodology/approachBy using a cylindrical coordinate system, the solution domain is truncated in the radial direction. The magnetic vector potential of each region excited by a filamentary coil is derived, and the expansion coefficients of the solutions are obtained by matching the boundary and interface conditions between the regions. By using the truncated region eigenfunction expansion method, the final expression in closed form for the impedance of the multi-turn coil is worked out, and the impedance calculation is performed in Mathematica. For frequencies ranging from 100 Hz to 100 kHz, both the impedance changes of the T-core coil above the multi-layer conductor without a hidden hole and in the absence of the layered conductor were calculated, and the influence of a hidden hole in the multi-layer conducting structure on the impedance change was investigated.FindingsThe correctness of the analytical model of the T-core coil was verified by the finite element method and experiments. The proposed T-core coil has higher sensitivity than an air-core coil, and similar sensitivity and smaller size than an E-core coil.Originality/valueA new T-core coil probe and its accurate theoretical model for defect evaluation of conductor were presented; probe and analytical model can be used in probe design, detection process simulation or can be directly used in defect evaluation of multi-layer conductor.

A Pulsed Eddy Current Testing Sensor Made of Low-Cost Off-the-Shelf Components: Overview and Application to Pseudo-Noise Excitation

A low-cost device for eddy current non-destructive testing made of off-the-shelf components is described. The system consists of a dual H-bridge stepper motor driver, a coil wound in-house on an additively manufactured support, a tunneling magnetoresistance sensor, and a data generation/acquisition module. The system is compact, and it can flexibly generate rectangular pulses, square wave bursts and arbitrary binary excitation waveforms. Among these, in this paper pseudo-noise binary signals were used within a pulse-compression measurement protocol to increase the SNR and the inspection sensitivity with respect to pulsed excitation. A benchmark sample was analyzed, and all the defects were correctly located, demonstrating a good detection capability of the overall hardware-software solution proposed. It was also found that pulse-compression output signals can be further processed as standard pulsed eddy current data. Features can be extracted from them for defect classification purposes and the lift-off invariant point can be used for imaging. These results were achieved by assembling a low-cost handy device, which can be further improved in portability and performances using different off-the-shelf components and by integrating it with a single-board PC, paving the way for future developments in this field.

Integral equation fast solver with truncated and degenerated kernel for computing flaw signals in eddy current non-destructive testing

In this article, the kernel independent H-matrix method is applied as the mathematical framework. The dense matrix generated by the selected integral equation, without low frequency breakdown problem for solving 3D arbitrary shaped eddy current nondestructive evaluation (NDE) forward problems, is compressed by the truncated and degenerated (TD) kernel function method. It results in a significant reduction in computational burden with nearly linear complexity. The kernel functions (Green's function) are degenerated by Lagrange polynomials which leads to two advantages: firstly, the double integrals are separated in two single integrals, and secondly, they are represented in a factorized form with good accuracy control. Meanwhile, due to the nature of the kernel function with exponential decay in the lossy medium, it is truncated to improve the overall performance. The TD kernel function-based boundary element method (BEM) fits well for solving eddy current NDE problems over the adaptive cross approximation based BEM, especially for detections of flaws or slots in planar objects. Several numerical predictions calculated by the proposed method are compared with those achieved by others such as the experiment, analytical and semi-analytical methods from benchmarks. The robustness and efficiency are demonstrated with excellent agreements and reduced complexity.

A New GAN-Based Approach to Data Augmentation and Image Segmentation for Crack Detection in Thermal Imaging Tests

As a popular nondestructive testing (NDT) technique, thermal imaging test demonstrates competitive performance in crack detection, especially for detecting subsurface cracks. In thermal imaging test, the temperature of the crack area is higher than that of the non-crack area during the NDT process. By extracting the features of the thermal image sequences, the temperature curve of each spatial point is employed for crack detection. Nevertheless, the quality of thermal images is influenced by the noises due to the complex thermal environment in NDT. In this paper, a modified generative adversarial network (GAN) is employed to improve the image segmentation performance. To improve the feature extraction ability and alleviate the influence of noises, a penalty term is put forward in the loss function of the conventional GAN. A data preprocessing method is developed where the principle component analysis algorithm is adopted for feature extraction. The data argumentation technique is utilized to guarantee the quantity of the training samples. To validate its effectiveness in thermal imaging NDT, the modified GAN is applied to detect the cracks on the eddy current pulsed thermography NDT dataset.

Research Progress on Detection Technology of Composite Electrical Traction Steel Strip for Elevator

Aiming at the problems such as detection accuracy, damage identification, and performance evaluation in the inspection and testing of elevator composite traction steel strips, the related research results of nondestructive testing of elevator composite traction steel strips at home and abroad are comprehensively analyzed, and the future is summarized and proposed Research direction. First, the structural characteristics of composite traction steel strips for elevators in use today are introduced, and the advantages of composite traction steel strips for elevators on steel steel ropes are specifically analyzed. Second, the development of nondestructive testing technology is introduced, and the research and application of non-destructive testing of composite steel strips and steel steel ropes for elevators are emphasized. Among them, non-destructive testing of steel steel ropes mainly including AC electromagnetic method, permanent magnet method, magnetic flux leakage method and residual magnetism method. The detection methods of composite steel strip mainly include magnetic flux test method, life setting method and resistance test method. Finally, on the basis of comparing the current non-destructive testing methods, a differential field-weak non-destructive testing method of dual magnetic circuits is proposed, and the research trend of defect detection of elevator traction steel strips is pointed out.

Application of an Ultra-Compact Eddy-Current Transducer for Investigation of Defects in Welded Joints of High-Strength Steel

This article discusses the results of work on the creation of a flaw detection unit, which is focused on the study of high-strength steels. Welded joints of transformer tanks, which are made of steel grade 08G2B, were selected as the object of research. We chose this grade because of the ultra-low carbon content of this material. This opens up opportunities for the application of eddy current nondestructive testing methods. For practical measurements, a scanning device was designed and optimized. It includes a converter that uses the principles of eddy currents and includes three coils assembled on a ferrite core, grade 80NMZ. The device was tested on samples with model continuity defects (cracks and holes). Also, within the framework of the publication, modeling of corrosion defects in welded joints was carried out.

A novel TMR based triaxial eddy current test probe for any orientation crack detection

The paper proposes the development, the analysis, and the experimental validation of a novel probe for Eddy Current (EC) Non-Destructive Testing especially thought to warrant good signal to noise ratios in the detection of thin defects when low excitation currents are adopted and whatever the orientation of the probe respect to the orientation of the defect. The probe is based on a simple double coil excitation, that generate a single direction EC flow, and adopts three magnetic sensors that form a triaxial magnetic sensor. Other main contributions of this paper are: i) performance analysis of an EC triaxial probe carried out in simulation environment that considers the measurement uncertainty of the magnetic sensors; ii) an experimental campaign that allow evaluating the performance of the realized probe; iii) a comparison between simulated and experimental results that prove the goodness of the proposed approach.

INCREASING SENSITIVITY OF EDDY CURRENT NON-DESTRUCTIVE TESTING OF DELAMINATION IN CARBON-FIBER REINFORCED PLASTICS

The paper is concerned with increasing sensitivity of eddy current nondestructive testing of most dangerous delamination in carbon-fiber reinforced plastics (CFRP). Increased sensitivity is achieved by separate registration and comparison of eddy current signals obtained from a set of stratifications of carbon fibers with the same orientation. The separation of eddy current signals is possible due to pronounced anisotropy of the electrical conductivity of the layers dominant in the direction of the fibers of the corresponding layer. Eddy-current signals are registered by eddy current probes with maximum sensitivity in a given angular direction. Prior to the scan eddy current signals of the probe are leveled on a defect-free area. The influence of the working gap on the difference between the eddy current signals of the probe is suppressed by normalizing it according to one of the signals. The analysis of the registered signals from delamination has been performed using an approximate calculation model. The reliability of the obtained results has been confirmed by comparison with experimental results and calculations using the finite element method.

Evaluation of Cracks on the Welding of Austenitic Stainless Steel Using Experimental and Numerical Techniques

This paper deals with investigation and characterization of weld circumferential thin cracks in austenitic stainless steel (AISI 304) pipe with eddy current nondestructive testing technique (EC-NDT). During welding process, the heat source applied to the AISI 304 was not uniform, accompanied by a change of the physical property. To take into consideration this change, the relative magnetic permeability was considered as a gradiently changed variable in the weld and the heat affected zone (HAZ), which was generated by the Monte Carlo Method based on pseudo random number generation (PRNG). Numerical simulations were performed by means of MATLAB software using 2D finite element method to solve the problem. To verify, results from the modeling works were conducted and contrasted with findings from experimental ones. Indeed, the results of comparison agreed well. In addition, they show that considering this changing of this magnetic property allows distinguishing the thin cracks in the weld area.

Research on pulsed eddy current automatic detection technology for weld defects of pressure vessel cylinder

In order to solve the problems of low detection efficiency, high labor intensity and poor working environment of large pressure vessel cylinder, the pulsed eddy current (PEC) nondestructive testing technology is used to detect the defects in the welding seam of the cylinder. The PEC sensor composed of rectangular probe coil and Hall sensor is installed on the mobile inspection platform to test two simulated cylinder weld samples. Results show that the signal curve of sample 1 without defect is gentle and without mutation, while the signal of sample 2 with defect has three mutation signals. Then, the X-ray testing method is used to detect the sample, radiograph confirmed that there are no defects on sample 1 and three porosity defects in sample 2. Through the comparison and analysis of the above two kinds of detection results, the feasibility of the detection method for weld subsurface defects is verified.

Quantitative method for detecting internal and surface defects in wire rope

The detection of internal and external defects in steel wire rope is a very important task. Current nondestructive testing methods cannot distinguish between internal and surface defects, and cannot determine the quantitative characteristics of internal defects. This paper first analyzes the advantages and disadvantages of induction coil magnetic flux detection and Hall sensor-based magnetic flux leakage detection. The problem of simultaneous detection using these two methods is then solved, and a new quantitative detection method is proposed for internal and surface defects in wire rope. Finally, a calculation method that uses a two-step training algorithm to establish two sets of neural networks is proposed. Results from simulations and experiments verify that the proposed method can accurately distinguish between internal and surface defects in wire rope, and can also quantitatively detect the width, cross-sectional loss rate, and depth of the defects.

Investigation of DC Electromagnetic-Based Motion Induced Eddy Current on NDT for Crack Detection

Cracks caused by long-term cyclic loading and harsh working environment may seriously lead to the catastrophic industrial safety accidents. Therefore, a quantitative and accurate characterization for the cracks is necessary to ensure safety of the key component (e.g. rolling contact fatigue of high speed rail track in particular). This article investigates the motion induced eddy current (MIEC) of a direct current (DC) electromagnetic nondestructive testing (NDT) and proposes an approach to quantitative characterization the cracks in moving ferromagnetic material. The velocity effect as well as the distribution of MIEC of the ferromagnetic material under the DC electromagnetic probe are investigated firstly. Then, the relationships between the crack depth/width and detection signals are discovered. Finally, a quantitative characterization method of crack depth/width is proposed by experiment. The investigations indicate that for DC electromagnetic NDT, the dragging effect is related to the rate of the MIEC diffusion time, which is inversely proportional to the conductivity and permeability of metal, therefore, the higher speed, the more obvious the dragging effect is. The experiments show that the crack depth/width can be characterized quantitatively by the peak separation and differential value. The investigation in this article show that the DC electromagnetic based on MIEC can be used not only for the crack detection in moving ferromagnetic metals such as bearings, gears, etc., but also for the quantitative characterization of cracks when there is rapid relative motion between the detection devices and the ferromagnetic metals (e.g., rail, pipe, etc.).

Metal Detection of Wood Based on Thermal Signal Reconstruction Algorithm

In this paper, eddy current thermography is used to detect metal in wood materials, and thermal signal reconstruction (TSR) algorithm has been proposed to solve the problem of low resolution of metal detection. The basic principle of current nondestructive testing technologies for wood materials has been briefly reviewed, and the advantages and disadvantages have been analyzed. TSR algorithm can significantly enhance the contrast ration between metal and surrounding areas, different quantities of metal can be effective identified, and metal positions can be accurately realized. The experimental results show that the proposed eddy current thermography technology can quickly detect metal in wood materials and improve the efficiency and accuracy. The size and quantity of metal can be intuitively observed through thermal images.

An analytical solution to the problem of a cup-cored coil located over a conducting plate with a hole

Pot-core coils are utilised in eddy current non-destructive testing (NDT) to detect defects in conducting materials. In this paper, an analytical solution to the problem of a cup-cored coil placed over a three-layer plate with an inner hole is obtained. Using the truncated region eigenfunction expansion (TREE) method, expressions for the magnetic vector potential of the filamentary coil, and final formulae for the impedance of the cylindrical coil, are derived. These formulae make it possible to model conducting magnetic and non-magnetic plates with an internal hole, a surface hole, a subsurface hole and a through hole. The final expressions in a closed form are implemented in MATLAB. The calculation results are verified using finite element method (FEM) models created with COMSOL Multiphysics, Finite Element Method Magnetics (FEMM) and Ansys Maxwell software packages. Very good agreement for the coil impedance components is obtained within the whole studied frequency range of 1 kHz to 100 kHz.

Study on the characteristics of magneto acoustic emission for mild steel fatigue.

Fatigue life of materials or structures can be classified into three stages: fatigue hardening or softening, crack initiation and crack propagation, which includes two stages. Current mature non-destructive testing (NDT) methods can only detect macro or visible cracks in stage II crack propagation. In order to detect and evaluate the fatigue damage occurring before stage II crack propagation quickly and effectively, magneto acoustic emission (MAE) measurement was carried out on laboratory specimens with different numbers of fatigue cycles. With the accumulation of fatigue damage, the RMS of MAE decrease steadily on the whole, making MAE a promising non-destructive method for evaluating fatigue damage. To make MAE applicable in noisy environments, square waveform voltage were selected to excite magnetic fields, and 'T' type MAE signals with higher amplitude were produced. The variation of MAE with number of fatigue cycles at different excitation intensity indicated that the defects associated with fatigue damage have greater effects on the creation and annihilation of domain walls. The point where the MAE amplitude begins to increase instead of decrease with fatigue can be an indicator for the onset of stage II crack propagation. This article is part of the theme issue 'Advanced electromagnetic non-destructive evaluation and smart monitoring'.