Defects In Metallic Plates Research Articles

Large-area inspection of defects in metal plates using multi-mode guided acoustic waves and sparse sensor networks

Various types of defects can be induced during the manufacturing or operation of engineering structures. For effective detection and characterization of the defects in large engineering structures, this paper proposes a large-area inspection technique that combines multi-mode guided acoustic waves with sparse sensor networks. The basic sparse sensor network employed in this study is composed of one transmitter and three receivers, distributed in a square lattice on the test plates. Multi-mode guided waves were excited and acquired by means of commercial single-element sensors of the network. To experimentally demonstrate the proposed technique, four different types of defects were simulated in aluminum test plates, including aluminum tape-based material addition, drilled material loss, indented deformation, and thermal embrittlement. For the evaluation of defects, acoustic response of each defect was analyzed based on the combination of linear vs. nonlinear acoustic characteristics, dependence on the type of the guided acoustic mode, and the directionality of the acoustic response on the network. Results indicate that each of the four representative defects can be uniquely identified (classified) and quantified using the proposed technique.

Analytical model and optimal focal position selection for oblique point-focusing shear horizontal guided wave EMAT

As an important application in nondestructive testing (NDT) technology, ultrasonic guided wave testing is widely used in the detection of defects in metal plates, pipes, and bridges. In the detection of defects in metal plates, electromagnetic acoustic transducers (EMATs) can easily excite shear horizontal (SH) guided waves, and a couplant is not required, in contrast to piezoelectric ultrasonic transducers. However, the low energy conversion efficiency of an EMAT has always been its main drawback. Therefore, in this work, we propose a new oblique point-focusing SH (OPFSH) EMAT configuration and obtain two analytical models for both continuous sine pulses and burst signals. The proposed transducer can achieve high-sensitivity detection of defects in the focal area. Moreover, the influence of the focal position on the signal intensity is also studied, and an optimal focal position selection method is demonstrated through simulation and experimental methods. By performing finite element calculations of the wave fields at different focal positions, we find that the optimal focal points are approximately distributed along a line. Therefore, the optimal focal position should be selected carefully to achieve the best OPFSH-EMAT focusing performance.

Application of common-path speckle interferometer with unlimited minimal shearing amount to characterization of irregularly shaped notch

Non-destructive evaluation (NDE) techniques are widely used to detect and characterize different defects in engineering structures. However, the highly sensitive real-time quantitative imaging of irregularly shaped defects in large-area metallic structures remains challenging if conventional NDE approaches are used. In this paper, a speckle interferometer with unlimited minimal shearing amount – based on a simple imaging setup without the need to tilt any optical components – is explored for the application of detection and characterization of complex-shaped defects in metallic plates. This technology is able to directly yield strain measurements, allowing the defects to be detected by identifying induced strain concentrations under stress loading. Acoustic waves are used as stress loading for this imaging system with the advantage of a deeper penetration depth and a larger area of inspection as compared to conventional loading methods, e.g. thermal loading. The spatial carrier technique is introduced to quantitatively reconstruct the phase distribution, which contains information of the strain measurement from a single recording. To achieve the near real-time imaging of defects with relative insensitivity against noise, the image pairs are continuously recorded and correlated using the sequential subtraction method. The performance of the imaging system was tested with experimental data. The results demonstrate that the imaging system can generate high-quality images to provide good detection and characterization properties regarding irregularly shaped notches as well as accurate sizing of different notch dimensions. This work introduces a promising practical NDE tool for the near real-time, non-contacting and quantitative detection of subsurface defects with complex geometries in large-area metallic structures.

Imaging of material defects with a radio-frequency atomic magnetometer.

Non-destructive inductive testing of defects in metal plates using the magnetic resonance signal of a radio-frequency atomic magnetometer is demonstrated. The shape and amplitude of the spatial profile of the signal features, which represent structural defects, are explored. By comparing numerical and experimental results on a series of benchmark aluminium plates, we show correspondence between the properties of the secondary field and those of the magnetometer signal. In particular, we show that two components of the secondary field are mapped onto the amplitude and phase of the atomic magnetometer signal. Hence, a magnetic field measurement with the atomic magnetometer, although scalar in its nature, provides semi-vectorial information on the secondary field. Moreover, we demonstrate a robust process for determining defect dimensions, which is not limited by the size of the sensor. We prove that the amplitude and phase contrast of the observed profiles enables us to reliably measure defect depth.

Measurement of shallow defects in metal plates using inter-digital transducer-based laser-scanning vibrometer

Scanning laser Doppler vibrometry (SLDV) is a promising method for visualizing defects in plate-like structures. The Lamb wave-based SLDV method using continuous excitations has been recently developed in nondestructive evaluation (NDE). In addition, various signal processing techniques have been developed to enhance damage detection capability for the method. To visualize defects, the A0 mode is commonly used because of its sensitivity to the thickness variations, i.e. the wave speed changes according to the thickness of a plate. The A0 mode is easily generated by lead zirconate titanate (PZT) transducers. However, for thick plates, the A0 mode is found to be less sensitive to thickness variations compared to thin structures. To address this issue, a mode-tuned inter-digital transducer (IDT) is adopted to efficiently generate and utilize the symmetric modes for damage detection and visualization of thick plates. Based on the wavelength of the S0 mode using the dispersion curve of the plate, the IDT is designed and fabricated for efficient symmetric mode generation. To evaluate the capability of the mode-tuned IDT-based SLDV, a 6-mm-thick plate (40 mm × 40 mm) with defects of 0.3 mm, 0.6 mm, 0.9 mm, and 1.2 mm depths is used. The S0 mode is selected for its wavenumber sensitivity and the degree of separation between modes. It is demonstrated that the detectability of shallow defects in a thick plate is improved compared to that when using the A0 mode.

NDT of Cavity Defect in Metallic Plate Using dc Potential Drop Method

Abstract A zoning testing method is proposed for the nondestructive testing of cavity defects in metallic foam plates based on the principle of dc potential drop testing. The dc is injected from the two ending points of the diagonal line while the inspection signal is measured from the nodal potential of nodes at the midline of the target region. The proposed method is numerically validated in terms of its signal sensitivity to the size and the location of the defect as well as its signal interference–resistant capability to defects located in the surrounding regions. Furthermore, experimental validation is conducted, and the result shows that the proposed zoning testing method is suitable for the detection of cavity defects in practical cases.

Structural Health Monitoring (SHM) of Civil Structures

As newer and more reliable ways of construction were developed, civilization began to spread out further and retain functional infrastructure for longer periods of time.[...]

Sizing Subsurface Defects in Metallic Plates by Square Pulse Thermography Using an Oriented Gradient Map

We developed a new approach for sizing subsurface defects in the square pulse thermography of metallic plates by employing the oriented gradient of histograms. To size defects with high accuracies is still a challenge in infrared (IR) thermography today. Especially for blurry defects, accurate sizing of them is difficult with existing methods. The oriented gradient of histograms, which is used in the successful probability of boundary (Pb) contour detector in natural image processing literature, is employed in this work to improve the sizing accuracy in square pulse thermography. Experiments on a corroded steel plate with flat blind holes have verified the effectiveness of the proposed approach to size defects. Experimental results show that the proposed approach can size distinct and blurry defects with high accuracies. Comparison research is also implemented between the proposed approach and other sizing methods. The comparison results show that the proposed approach is superior to existing methods.

Multi-Mode Electromagnetic Ultrasonic Lamb Wave Tomography Imaging for Variable-Depth Defects in Metal Plates.

This paper proposes a new cross-hole tomography imaging (CTI) method for variable-depth defects in metal plates based on multi-mode electromagnetic ultrasonic Lamb waves (LWs). The dispersion characteristics determine that different modes of LWs are sensitive to different thicknesses of metal plates. In this work, the sensitivities to thickness variation of A0- and S0-mode LWs are theoretically studied. The principles and procedures for the cooperation of A0- and S0-mode LW CTI are proposed. Moreover, the experimental LW imaging system on an aluminum plate with a variable-depth defect is set up, based on A0- and S0-mode EMAT (electromagnetic acoustic transducer) arrays. For comparison, the traditional single-mode LW CTI method is used in the same experimental platform. The imaging results show that the computed thickness distribution by the proposed multi-mode method more accurately reflects the actual thickness variation of the defect, while neither the S0 nor the A0 single-mode method was able to distinguish thickness variation in the defect region. Moreover, the quantification of the defect’s thickness variation is more accurate with the multi-mode method. Therefore, theoretical and practical results prove that the variable-depth defect in metal plates can be successfully quantified and visualized by the proposed multi-mode electromagnetic ultrasonic LW CTI method.

A New Omni-Directional EMAT for Ultrasonic Lamb Wave Tomography Imaging of Metallic Plate Defects

This paper proposes a new omni-directional electromagnetic acoustic transducer (EMAT) for the ultrasonic Lamb wave (ULW) tomography imaging (TI) of defects in metallic plates. The proposed EMAT is composed of a permanent magnet and a coil with a contra-flexure structure. This new EMAT coil structure is used for omni-directional ULW transmission and reception and ULW TI for the first time. The theoretical background and the working principles of this EMAT are presented and analyzed. The experimental results of its use on a 3 mm thick aluminum plate indicate that the EMAT with a contra-flexure coil (CFC) can transmit and receive a pure single A0 mode ULW with a high signal-to-noise ratio (SNR). Thus, the extraction of the projection data used for ULW TI may be performed accurately. The circumferential consistency of the projection data is only slightly influenced by the distortion of the eddy current field that is induced by the new CFC with an irregular shape. When the new EMAT array is used for ULW TI using the cross-hole method and SIRT arithmetic, a desirable imaging quality can be achieved, and the estimated size of an artificial corrosion defect agreed well with its actual value. The relation between the reconstruction resolution and the number of the new EMATs used is analyzed. More TI experiments are carried out when the aluminum plate defect is in two different locations relative to the EMAT array, for the further investigation of the performances of the new EMATs.

A New Approach to Evaluate Defects in Metallic Plates Based on Computing with Words and Fuzzy Entropy

Eddy-Current Techniques for non–destructive testing and evaluation of conducting material are one of the most known application oriented field in electromagnetics research. According to the Italian PRIN Project (proj. 2009TCLKNF_002), the Authors, in this work, propose a novel approach based on soft computing domain to characterize defects in metallic plates in terms of depth and shape starting from a set of experimental measurements. The problem is solved by means of a classification system based on computation with words and fuzzy entropy extracting information on the specimen under test from the measurements carried out in Non Destructive Tests Laboratory of “Mediterranea” University. Whereas, fuzzy entropy minimization module is based on traditional fuzzy inference system due to intrinsic characteristics of data the main advantage of the proposed approach is the introduction of computation with words in order to improve the data characterization.

A simple forward direct problem solver for eddy current non-destructive inspection of aluminum plates using uniform field probes

In this paper we show that a simple algorithm used to model the eddy current inspection of an aluminum plate can be used to preview the acquired voltage signals. Thus, the algorithm is suitable to work as a forward problem solver to determine the expected measurement signal obtained with a uniform excitation field probe including a giant magnetoresistor sensor. The algorithm is based on a conformal transformation and is able to preview the shape of the electrical current lines when a metallic plate with a superficial straight crack is subject to a sinusoidal excitation field with constant amplitude and orientation in a bounded zone around the sensor element. Simple and fast algorithms as the one presented in this paper are of paramount importance for testing inversion algorithms used to characterize defects in metallic plates.

The AMDE project: 3D volumetric anomalies reconstruction by eddy current testing

The AMDE project (“Applicazioni di Metodi per Diagnostica Elettromagnetica”, in English “Applied Methods for Electromagnetic Evaluation”) is an Italian two-years national granted research project on electromagnetic non-destructive evaluation. This project involves five different research units located in Italy, about twenty researchers and ten among Ph.D. students and post-doc. Here we focus on the contribution of the AMDE project to the reconstruction of 3D volumetric defects in metallic plates as those used in aeronautical and aerospace industry. In particular, in this work we propose an efficient numerical model for evaluating the field due to volumetric anomalies and an inversion method based on the Total Variation Regularization. Numerical examples confirm the capability of the proposed strategy to reconstruct 3D volumetric defects.

Multi-frequency identification of defects in conducting media

An eddy currents based procedure for the 3D image reconstruction of defects in metallic plates from multi-frequency data is presented. In particular, we exploit the collection of data at different probe positions as well as at different excitation frequencies in order to improve the amount of information content, the accuracy of the inverse methodology and its robustness against the experimental noise. The identification tool we developed, exploits the geometric A − χ formulation for the solution of the eddy-current forward problem together with a full nonlinear iterative inversion algorithm based on the total variation regularization.

Fuzzy computation for classifying defects in metallic plates

Fuzzy computation for classifying defects in metallic plates

Image Reconstruction of Defects in Metallic Plates Using a Multifrequency Detector System and a Discrete Geometric Approach

We present an inversion procedure for the image reconstruction of defects in metallic plates, using a multifrequency eddy-current system. The solution of the eddy-current forward problem is achieved by means of a discrete geometric approach, while the inverse problem is resolved with an iterative linearization algorithm based on sensitivity data. In particular, we propose a suitable measurement point on the region under test using a probe coil exited by means a multifrequency signal, in order to improve the amount of usable data and the accuracy of the inverse procedure

FEM time domain analysis for the detection of depth and thickness of cylindrical defects in metallic plates

In this work, we will present a strategy for the evaluation of depth and thickness of defects like holes and cracks in metallic plates via the analysis of the response in frequency of a suitable eddy-current detection system. In order to obtain the data for the inversion in real time, the analysis in frequency domain is deduced by the analysis of the response of the system to an exciting rectangular voltage pulse of suitable length, according to the size of defect to detect. By finite element method numerical simulations in time domain of the detection system applied to known defect configurations, we have found some interesting information for the inverse problem solution. In this paper, we present the theory of the detection technique above mentioned and results about numerical and experimental analysis.

A novel approach for detecting and classifying defects in metallic plates

In the field of nondestructive testing on defect identification in metallic plates, the shape reconstruction is still an open question. State-of-the-art technologies indeed enable the operator to locate the position of a defect but not its shape. The aim of this paper is to make a contribution to the solution of this side of the problem suggesting a novel methodology based on a neurofuzzy approach. Sugeno's neurofuzzy inferences have been carried out for this purpose, as a first step in this direction. Fuzzy entropy was then exploited to measure how far is a given defect from a well-known depth. A sort of classification based on the depth of a defect has been performed this way.