Showing papers in "Journal of Nondestructive Evaluation in 2015"

GDXray: The Database of X-ray Images for Nondestructive Testing

Abstract: In this paper, we present a new dataset consisting of 19,407 X-ray images. The images are organized in a public database called $$\mathbb {GDX}$$ ray that can be used free of charge, but for research and educational purposes only. The database includes five groups of X-ray images: castings, welds, baggage, natural objects and settings. Each group has several series, and each series several X-ray images. Most of the series are annotated or labeled. In such cases, the coordinates of the bounding boxes of the objects of interest or the labels of the images are available in standard text files. The size of $$\mathbb {GDX}$$ ray is 3.5 GB and it can be downloaded from our website. We believe that $$\mathbb {GDX}$$ ray represents a relevant contribution to the X-ray testing community. On the one hand, students, researchers and engineers can use these X-ray images to develop, test and evaluate image analysis and computer vision algorithms without purchasing expensive X-ray equipment. On the other hand, these images can be used as a benchmark in order to test and compare the performance of different approaches on the same data. Moreover, the database can be used in the training programs of human inspectors.

Review of Second Harmonic Generation Measurement Techniques for Material State Determination in Metals

Abstract: This paper presents a comprehensive review of the current state of knowledge of second harmonic generation (SHG) measurements, a subset of nonlinear ultrasonic nondestructive evaluation techniques. These SHG techniques exploit the material nonlinearity of metals in order to measure the acoustic nonlinearity parameter, $$\beta $$ . In these measurements, a second harmonic wave is generated from a propagating monochromatic elastic wave, due to the anharmonicity of the crystal lattice, as well as the presence of microstructural features such as dislocations and precipitates. This article provides a summary of models that relate the different microstructural contributions to $$\beta $$ , and provides details of the different SHG measurement and analysis techniques available, focusing on longitudinal and Rayleigh wave methods. The main focus of this paper is a critical review of the literature that utilizes these SHG methods for the nondestructive evaluation of plasticity, fatigue, thermal aging, creep, and radiation damage in metals.

Nonlinear Imaging Method Using Second Order Phase Symmetry Analysis and Inverse Filtering

Abstract: This paper presents a nonlinear imaging method based on nonlinear elastic guided waves, for the damage detection and localisation in a composite laminate. The proposed technique relies on the study of the structural nonlinear responses by means of a combination of second order phase symmetry analysis (PSA) with chirp excitation and inverse filtering (IF) method. PSA was used to exploit the invariant properties of the propagating elastic waves with the phase angle of the pulse compressed chirp signals, in order to characterise the second order nonlinear behaviour of the medium. Then, the IF approach was applied to a library of second order nonlinear responses to obtain a two-dimensional image of the damage. The experimental tests carried out on an impact damage composite sample were compared to standard C-scan. The results showed that the present technique allowed achieving the optimal focalisation of the nonlinear source in the spatial and time domain, by taking advantage of multiple scattering and a small number of receiver sensors.

Evaluation of Williamson–Hall Strain and Stress Distribution in ZnO Nanowires Prepared Using Aliphatic Alcohol

Abstract: Synthesis of ZnO nanowires (NWs) using vapor phase transport (VPT) assisted with thermal evaporation of brass (CuZn) assisted by hotwire was presented. The effects of aliphatic alcohols such as methanol and ethanol as source of oxygen were investigated using field emission scanning electron microscope and X-ray diffraction (XRD). Signifi- cant changes in the morphology and structure of both ZnO NWs prepared using methanol (ZnO/M NWs) and ethanol (ZnO/E NWs) depicted the influence of aliphatic alcohols. Debye Scherer (DS), Williamson-Hall (W-H) and size-strain plot (SSP) analysis on the XRD peak broadening revealed that ZnO/M NWs revealed lower strain and stress value com- pared to ZnO/E NWs. ZnO/M NWs, which was preferential to � 002� crystallographic orientation found to be hexagonal isotropic crystalline nature whereas ZnO/E NWs preferential of � 101� crystallographic orientation is anisotropic crys- talline nature.

Finite Element Modelling of Cracks as Acoustic Emission Sources

Abstract: This paper presents results for a new acoustic emission crack source model based on a finite element modelling approach which calculates the dynamic displacement field during crack formation. The specimen modelled is statically loaded until conditions for crack growth as defined by a failure criterion are fulfilled. Subsequently, crack growth is modelled by local degradation of the material stiffness utilizing a cohesive zone element approach. The displacements due to crack growth generate the acoustic emission signal and allow detailed examination of the principles of acoustic emission sources operation. Subsequent to crack growth signal propagation is modeled. The signal propagation is modeled superimposed on the static displacement field. The presented model comprises a multi-scale and multi-physics approach to consider the signal propagation from source to sensor, the piezoelectric conversion of the elastic wave to an electric signal and the interaction to the acquisition electronics. Validation of the modeling approach is done by investigating the acoustic emission signals of micromechanical experiments. Using a specifically developed load stage, carbon fiber filament failure and matrix cracking can be prepared as model sources. A comparison of the experimental signals to the modeled signals shows good quantitative agreement in signal amplitude and frequency content. A comparison between the present modeling work and analytical theories demonstrates the substantial differences not considered in previous modeling work of acoustic emission sources.

Ultrasonic Evaluation of Residual Hoop Stress in Forged and Cast Railroads Wheels—Differences

Abstract: Paper describes the assessment of hoop residual stresses in the rim of monoblock railroad wheels using ultrasonic methods. Dangerous tensile stress builds up during service as a result of heat loads caused by braking with braking blocks/brake shoes. Results of experiments on two types of monoblock wheels are described: on rolled-forged wheels used in Europe and on cast wheels manufactured and used in North America. Stresses in forged wheels are evaluated with birefringence technique. Investigations carried out in various countries, using different ultrasonic equipment, proved that an ultrasonic technique can provide valuable information concerning stress values in wheels during manufacturing and in service. However, they also showed that due to different microstructures in the rim material and differences in wheel plate design, forged and cast wheels present unique problems for ultrasonic stress evaluation. The aim of this paper is to emphasize these differences and to illustrate how they influence ultrasonic readings. The second ultrasonic technique to evaluate stress is proposed to standard cast wheels – technique based on measurements of time of flight of subsurface (surface-skimming) longitudinal waves propagating in hoop direction on both rim faces. Presented data are based on detailed measurements of acoustic properties of monoblock wheel materials and on earlier experiments performed on forged and cast wheels subjected to inductive heating, braking in a test stand or on track.

Investigation of Clamping Effect on the Welding Sub-surface Residual Stress and Deformation by Using the Ultrasonic Stress Measurement and Finite Element Method

Abstract: In this study, sub-surface residual stress and deformations, produced by the welding process, are investigated by using ultrasonic stress measurement method and finite element (FE) simulation. The FE analysis is employed to evaluate the residual stresses and deformations caused by the tungsten inert gas welding of 304L stainless steel plates. Residual stresses obtained from the FE analysis are then used to validate results of the ultrasonic stress measurement method, which is fulfilled by employing longitudinal critically refracted ( $$L_{CR})$$ waves. By using four different frequencies of ultrasonic probes, the sub-surface residual stress fields are mapped in four different depths of the examined material. Two different plates are welded with and without the use of clamp to investigate the clamping effect on the residual stress and deformation. By employing the through-thickness measurement of residual stresses, the clamping effect on the sub-surface distribution of the residual stresses is also studied. As a result, the $$L_{CR}$$ ultrasonic method is accurate enough to distinguish the surface and sub-surface residual stresses in the clamped and non-clamped welding plate. Consequently, the longitudinal residual stresses have been increased by using the clamp during the welding of stainless steel plates. However, using the clamp significantly influences the amount and distribution of longitudinal residual stress in the base metal. Regarding the welding deformation results, it has been concluded that employing the clamp considerably decreases the deformations of the stainless steel plates.

Thermographic Non-destructive Evaluation of Carbon Fiber-Reinforced Polymer Plates After Tensile Testing

Abstract: Infrared thermography (IT) is a safe non-destructive evaluation technique that has a fast inspection rate and is generally contactless. It is used for diagnostics and monitoring in several fields including composite materials. In this paper carbon fiber-reinforced polymer plates submitted to tensile testing are inspected using IT. More specifically, carbon/polyether ether ketone panels made of random-oriented strands by compression moulding are submitted to tensile testing and then inspected using three different IT active approaches. The first two approaches use optical sources however with different scanning modes. The first active approach tested is a static surface scanning inspection in reflection mode. The second one is a dynamic line scanning technique where the energy source and camera are in movement with regards to the test sample. The last active IT approach tested uses a mechanical source (ultrasound excitation) to transfer heat to the sample being tested. This last approach is commonly called vibrothermography. Results obtained were then compared to results obtained by micro computed tomography inspection and microscopy. Results revealed voids associated with resin-rich regions as well as cracks.

Detection of Localized Damage in Water Wall Tubes of Thermal Power Plants Using GMR Sensor Array Based Magnetic Flux Leakage Technique

Abstract: A flexible giant magneto-resistive (GMR) sensor array based magnetic flux leakage technique has been developed for nondestructive inspection of carbon steel water wall tubes (outer diameter 64.0 mm and wall thickness 6.3 mm) used in a thermal power plant. Two saddle coils are used for magnetisation of the tube and a flexible GMR sensor array is used for detection of leakage fields from localized damage in the tube. 3D-nonlinear finite element modeling has been performed to optimize the magnetizing current, inter-coil spacing of the saddle coils and location of the GMR sensor array within the saddle coils. The performance of the technique has been evaluated by measuring the axial component of leakage flux from outer diameter (OD) erosion and inner diameter (ID) corrosion type flaws artificially machined on the water wall tube. Studies reveal that the proposed technique detects both OD erosion and ID corrosion damage in the water wall tubes.

A Fully Non-contact, Air-Coupled Ultrasonic Measurement of Surface Breaking Cracks in Concrete

Abstract: Previous researchers have shown that the surface wave transmission coefficient across a surface breaking crack in concrete can be used to estimate the depth of the crack. This study is the first to investigate a fully non-contact ultrasonic technique using air-coupled transducers which can reliably measure the surface wave transmission coefficient across a surface breaking crack in concrete. Using this setup, the transmission coefficient of Rayleigh surface waves is measured for three notches with different depths (0.5, 1, and 2 cm). Then, the relationship between the transmission coefficient and the normalized notch depth is experimentally reconstructed and compared with analytical and numerical results. It is demonstrated that the proposed fully non-contact technique is efficient and promising as a field application in civil infrastructure. The advantages of the proposed non-contact technique compared to existing methods are also discussed.

Review of Ultrasonic Wave Reflection Applied to Early-Age Concrete and Cementitious Materials

Abstract: The ultrasonic wave reflection method, as applied to characterize early age cement-based materials, is reviewed. The topic is first introduced with a historical review of method development. The theoretical basis of the reflection technique within the context of wave mechanics is then considered, followed by a description of experimental techniques and associated requirements of testing setups. Then fundamental background information about the method is described, including effects of ultrasonic wave mode (compression and shear waves) and buffer material (metal, ceramic and polymer) on the results and required experimental apparatus. Several test application tasks, across a range of cementitious material ages (hydration states), are summarized, including determination of global constitutive properties, estimation of setting time (initial and final sets), evolution of strength, and assessment of microstructure (e.g. flocculation state in setting material and porosity in solid material). The analyses of various techniques are presented in a unified manner for elastic, viscoelastic and poroelastic idealizations of the cementitious material.

Building Thermography: Detection of Delamination of Adhered Ceramic Claddings Using the Passive Approach

Abstract: Tile detachment is a common durability problem of adhered ceramic claddings causing safety risks. Passive infrared thermography using solar heat gain can be used to detect delamination leading to detachment, and is advantageous especially in the inspection of middle to high-rise buildings envelope although it is qualitative by nature. In this paper, findings of in situ thermographic inspections are comparatively analysed with findings of tapping control and surface moisture measurement, and with findings of thermal simulations to evaluate their efficiency. Comparative analyses showed that passive thermography can be used as a preliminary inspection technique to detect delamination and decide whether further inspection with advanced methods is required to implement maintenance operations.

Model-Based Design of Low Frequency Lamb Wave EMATs for Mode Selectivity

Abstract: A low-frequency, omni-directional A0 Lamb wave ElectroMagnetic Acoustic Transducer (EMAT) is developed for applications in guided wave tomography, operating at 50 kHz on a 10 mm thick steel plate. The key objective is to excite an acceptably pure A0 wave mode in relation to the S0 mode, which can also be present at this operating point and is desired to be suppressed by approximately 30 dB. For that, a parametric Finite Element (FE) model of the design concept is implemented in a commercially available FE software, where the bias magnetic field is calculated initially, then combined with the eddy current caused by the induction coil to produce a force. A numerical optimization process employing a genetic algorithm is set up and the EMAT design is optimized to yield an improved A0 mode selectivity. The parameters subjected to optimization are the magnet diameter and the magnet lift-off, which control the direction of the exciting force in the skin depth layer and therefore the mode selectivity. Although there are three possible electromagnetic acoustic interaction mechanisms, the optimisation considers only the Lorentz force, as its performance surface contains a clear optimum and from the optimised design a physical prototype is built. The FE model is validated against measurements on an aluminium plate for the Lorentz force excitation mechanism and on a steel plate for both the Lorentz and magnetisation force. For the steel plate, it is found that only considering the Lorentz force leads to a significant overestimation of the mode selectivity, as the S0 amplitude is underestimated by the Lorentz force, but the A0 amplitude remains mainly uninfluenced. Further, it has been found that additionally including the magnetisation force into the optimisation leads to a better mode selectivity, however, the optimisation drives the optimum to a minimum magnet diameter and therefore reduces the EMAT sensitivity. In a numerical study robustness is shown for fairly large variations of the magnet lift-off and the magnetic permeability. Based on the findings, a two-step model-based design approach is proposed whereby only the Lorentz force is used in the first step for the optimisation and then in a second step, a realistic estimate of the mode selectivity of the optimised design can be obtained by additionally considering the magnetisation force.

Clustering Based Multi Sensor Data Fusion for Honeycomb Detection in Concrete

Abstract: We use three clustering algorithms to aggregate a three-modal non-destructive testing data set into defect and not-defect groups. Our data set consist of impact-echo, ultrasound (US) and ground penetrating radar data collected on a large concrete slab with embedded simulated honeycombing defects. US performs best in defect discriminating and sizing, however the false positive rate is still high. We fuse the data set using K-Means, Fuzzy C-Means and DBSCAN clustering at feature-level. We discern that DBSCAN improves the detectability up to 10 %. A discussion of its advantages over commonly used K-Means and Fuzzy C-Means clustering are provided.

Acoustic Emission Characterisation of Failure Modes in Hemp/Epoxy and Glass/Epoxy Composite Laminates

Abstract: Acoustic emission (AE) can be used for online monitoring of composite structures. One of the main issues of using composite structures in various applications is to understand and characterize their failure mechanisms. In this paper, advanced signal processing technique is used to discriminate the failure mechanisms in hemp/epoxy and glass/epoxy composite laminates. Hemp/Epoxy laminates are made from 4 layers of chopped non-woven fiber mats and glass/epoxy cross ply laminates [0/90/90/0] $$_{3s}$$ are made from 12 layers of unidirectional glass fiber mats. ASTM standard specimens of size 100 $$\times $$ 20 $$\times $$ 4 mm were cut from the laminates. These specimens are subjected to flexural loading with AE monitoring. The choice of the AE signals for failure mode characterisation were obtained using parametric analysis of AE data such as counts rate and cumulative counts at different levels of loading. Wavelet decomposition was performed on the chosen AE signals to identify the percentage of energy and frequency content of each level which correlate the different failure modes. The studies reveal that wavelet decomposition of AE signals plays a significant role in discriminating the dominance of the different failure modes with respect to the different stages of loading.

Influence of Specimen Velocity on the Leakage Signal in Magnetic Flux Leakage Type Nondestructive Testing

Abstract: We investigate the influence of the specimen velocity on the magnetic flux leakage with the aim of selecting the optimum sensor locations. Parametric numerical simulations where the specimen velocity was in the range [0.1–20] m/s were carried out. As the specimen velocity is increased, the magnetic field varies from being symmetrical to being asymmetric. For the radial magnetic induction $$B_z$$ , the position at which the maximum difference between the minimum and maximum signal moves from the centre of the bridge towards the direction of the specimen movement. For the axial magnetic induction $$B_y$$ , the specimen velocity influence is dependent on the sensor location and a signal-velocity independent region was discussed.

Damage Identification in Active Plates with Indices Based on Gaussian Confidence Ellipses Obtained of the Electromechanical Admittance

Abstract: Structural health monitoring (SHM) is an important area of study in the diagnosis of structural failures. Through SHM, structures can be diagnosed by electromechanical impedance (EMI) technique to detect changes in operative state. The signals of EMI obtained from piezoelectric transducers (PZT) are processed to quantify and classify damage severities by means of indices. These indices are studied and analyzed to recognize damage patterns in the electrical signals emitted by PZTs. In this research, an experimental analysis is carried out to identify structural damage in active plates with indices based on Gaussian confidence ellipses of the EM admittance. Three experimental tests are performed to assess the feasibility of the indices in the damage identification. A damage metric is depicted in each test together with the conductance (G) and susceptance (B), which were analyzed from different points of view. A probabilistic procedure is established to estimate Gaussian ellipses from a structure without damage with the aim to establish an alert baseline before that damage is induced. The results show that using the proposed methodology, the damage identification is a feasible procedure in the proposed case studies. Therefore, the presented analysis can be extrapolated to other applications to identify damage and so to test the feasibility of the methodology in other context.

Discussion of Derivability of Local Residual Stress Level from Magnetic Stray Field Measurement

Abstract: The NDT procedure dubbed ‘metal magnetic memory’ method and the related ISO 24497 standard has found wide industrial acceptance in some countries, mainly in Russia and China. The method has been claimed by some researchers (Roskosz and Bieniek in NDTE Wilson et al. in Sens Actuators A 135:381–387, 2007) as having potential for quantitative determination of local residual stress state in engineering structures, at least for some steel grades. This work presents a critical reexamination of a previous important study by Roskosz and Bieniek, who claimed to have found a direct relationship between local residual equivalent stress levels ranging from 0 to 50 MPa, and the stray field gradients in T/P24 steel sample placed in the Earth’s ambient magnetic field. We reconstruct their experiment in a magnetic finite element simulation, computing stray magnetic field and its tangential gradients along the axis of the sample. Different combinations of remanent induction and relative magnetic permeability levels have been modeled, and the influence of geometrical discontinuity is quantified. In order to validate magnetic finite element methodology, a new experiment is presented, along with its numerical counterpart. The magnetic finite element method allowed to obtain a good quantitative correlation with well-controlled stray field measurements. It is demonstrated, that the residual stress level of order of 50 MPa is not the only factor, on which the stray field measurement depends. The geometrical discontinuity and the remanent induction contribute to a higher extent to the field amplitudes. Consequently we prove, that a bidirectional correlation between the magnetic field gradient and local stress levels cannot be determined because of at least three concurrent inseparable factors on which the measured stray field and its spatial gradient depends.

Nondestructive Material Characterization in the Terahertz Band by Selective Extraction of Sample-Induced Echo Signals

Abstract: Terahertz time-domain spectroscopy (THz-TDS) allows broadband noninvasive measurement of the optical parameters of various materials in the THz domain. The measurement accuracy of these parameters is highly influenced by the difficulty in distinguishing THz signals from unwanted signals such as noise, signal fluctuation, and multiple echoes, which directly affects material identification and characterization efficiency. We introduce a novel method that provides effective extraction and separation of THz signals from such undesired effects. The proposed algorithm was assessed through experiments that presented enhancement in material parameter evaluation, such as the decomposition of the sample-induced echoes (SIEs) from the complex THz sample signal with near-zero extraction error. Improved precision (\(\pm \)0.05\(\,\upmu \)m) was achieved in the determination of the sample thickness compared to that of the mechanical method (\(\pm \)10\(\,\upmu \)m). Furthermore, we could infer from the component concentration measurement results of a compound sample (44.2 % decrease in the root mean square concentration error) that the material parameter calculation accuracy had improved, proposing a means to enhance the ultimate nondestructive material evaluation performance.

Wavelet Cycle Spinning Denoising of NDE Ultrasonic Signals Using a Random Selection of Shifts

Abstract: Wavelets are a powerful tool for signal and image denoising. Most of the denoising applications in different fields were based on the thresholding of the discrete wavelet transform (DWT) coefficients. Nevertheless, DWT transform is not a time or shift invariant transform and results depend on the selected shift. Improvements on the denoising performance can be obtained using the stationary wavelet transform (SWT) (also called shift-invariant or undecimated wavelet transform). Denoising using SWT has previously shown a robust and usually better performance than denoising using DWT but with a higher computational cost. In this paper, wavelet shrinkage schemes are applied for reducing noise in synthetic and experimental non-destructive evaluation ultrasonic A-scans, using DWT and a cycle-spinning implementation of SWT. A new denoising procedure, which we call random partial cycle spinning (RPCS), is presented. It is based on a cycle-spinning over a limited number of shifts that are selected in a random way. Wavelet denoising based on DWT, SWT and RPCS have been applied to the same sets of ultrasonic A-scans and their performances in terms of SNR are compared. In all cases three well known threshold selection rules (Universal, Minimax and Sure), with decomposition level dependent selection, have been used. It is shown that the new procedure provides a good robust denoising performance, without the DWT fluctuating performance, and close to SWT but with a much lower computational cost.

Classification of Welding Flaws in Gamma Radiography Images Based on Multi-scale Wavelet Packet Feature Extraction Using Support Vector Machine

Abstract: This paper has introduced a digital process aimed at automatically identify and classify flaws in the weld joints. Therefore, various algorithms are presented for computer aided detection (CAD) and classification. These algorithms include preprocessing algorithms using Gaussian pyramidal transform, contrast enhancement algorithm using contrast stretch and normalization method, noise reduction algorithm by blind image separation (BIS), measurement of the noise separation quality and image segmentation algorithm based on expectation-maximization (EM) method. Also, an algorithm for detection and classification of welding defects from radiographic images is presented. This algorithm is based on multi-scale wavelet packet (MWP) technique for feature extraction. Also, extraction of features from its transform domains is proposed to assist in achieving a higher classification rate. Moreover, the support vector machine (SVM) is applied for matching the extracted features. Consequently, classification error was computed for both normal and defect images. The obtained results confirm that much higher classification error was computed for defect image due to various objects may be in the image. Moreover, the accuracy of the considered algorithms is determined by statistical measurements. These algorithms have the potential for further improvement because of their simplicity and encouraging results. Therefore, it will motivate real-time flaws detection and classification for many CAD applications.

Understanding Multi-modal Non-destructive Testing Data Through the Evaluation of Twelve Deteriorating Reinforced Concrete Bridge Decks

Abstract: Non-destructive testing (NDT) to identify reinforced concrete deterioration is an important tool in the management and preservation of the nation’s bridge inventory. Presented is research conducted to better understand multi-modal NDT and its identification of reinforced concrete deterioration. The objectives were to better understand the relationships between the various NDT data, understand damage progression with time, and increase confidence in NDT. Statistical analyses were conducted of multi-modal NDT data collected on 12 bridge decks through the USA. The research identified that the NDT methods utilized are strongly interrelated and can help identify future deterioration.

Time-Dependent Passive Building Thermography for Detecting Delamination of Adhered Ceramic Cladding

Abstract: Time-dependent passive infrared thermography (td-PIRT) is an inspection technique that can be used to detect delamination. td-PIRT was used to inspect two buildings in Lisbon (Portugal) with adhered ceramic cladding to evaluate its potential to detect delamination problems in such claddings, which is a common problem and has safety risks. The td-PIRT data were analysed quantitatively by means of simple image subtraction (SIS), nonnegative matrix factorisation (NMF), and principal component analysis (PCA). Comparative evaluations showed that td-PIRT has a great potential to detect delamination problems in adhered ceramic cladding, depending on the analysis method selected. PCA and NMF were found to be efficient methods because of their good, consistent performance in revealing defective areas.

Porosity Characterization of Aluminium Castings by Using Particular Non-destructive Techniques

Abstract: In this paper is presented an analysis of a comparison of particular non-destructive techniques: radiographic, ultrasonic, eddy current and pulsed infrared thermography to detect flaws in pressure die casting. Furthermore, a new way of the image segmentation method for porosity detection in aluminium pressure die casting using results from X-ray analysis and public domain software is proposed. The results of the research allowed verifying the possibility of using the aforesaid methods in controlled technological parameters of the casting process, and thus permitted confirming the quality of the manufactured castings. It was found that the radiographic method was the most preferred means for evaluating the quality of products and for optimizing the technological process for aluminium pressure casting. It permits a fast analysis of casting defects in the finished products, and when combined with the computer image analysis (the porosity segmentation algorithm as described in the article) it allows drawing conclusions regarding their causes (e.g. based on the size and shape analysis of the pores detected). The ultrasonic testing method can also be used to evaluate the quality of the casting (to detect it but without the possibility of interpreting the size or source of origin). The results obtained via this method allow for proper control of die casting parameters.

Locating Events Using Time Reversal and Deconvolution: Experimental Application and Analysis

Abstract: Time reversal techniques are used in ocean acoustics, medical imaging, seismology, and non-destructive evaluation to backpropagate recorded signals to the source of origin. We demonstrate experimentally a technique which improves the temporal focus achieved at the source location by utilizing deconvolution. One experiment consists of propagating a signal from a transducer within a concrete block to a single receiver on the surface, and then applying time reversal or deconvolution to focus the energy back at the source location. Another two experiments are run to study the robust nature of deconvolution by investigating the effect of changing the stabilization constant used in the deconvolution and the impact multiple sources have upon deconvolutions’ focusing abilities. The results show that we are able to generate an improved temporal focus at the source transducer using deconvolution while maintaining the robust nature of time reversal. Additionally, deconvolution’s costs are negligible due to it being a preprocessing step to the recorded data. The technique can be applied for detailed investigation of the source mechanisms (e.g. cracks) but also for monitoring purposes.

Non-destructive Electrical Methods for Measuring the Physical Characteristics of Porous Materials

Abstract: The durability of reinforced concrete structures, built on the seafront, has been at the heart of recent concerns. Indeed, chloride transport across the porosity of concrete coating produces medium to long-term corrosion of reinforcements. This has a direct impact on the mechanical behavior and the ageing of the affected structures. The difficulty of in situ monitoring of these structures, continuously and non-destructively against infiltration of chlorides, is still topical. The work presented here aims mainly at correlating the electrical properties, i.e. electrical resistance, conductivity..., with the physical characteristics (porosity, tortuosity, ...), of granular materials using Archie’s law. The recommended experimental program consists in placing, different granular materials, i.e. sand, mortar, bricks, and concretes saturated with electrolytic solutions, in a PVC cell fitted with a pair of stainless steel electrodes connected to an electrical circuit, in order to measure the electrical resistances. The results obtained allow one to determine durability factors, such as the connected porosity and the coefficient of chloride diffusion through a simple measurement of the EC, and estimate the tortuosity parameter that governs transport in porous media.

A Methodology for Identifying Defects in the Magnetic Flux Leakage Method and Suggestions for Standard Specimens

Abstract: In magnetic flux leakage (MFL) testing technology, the MFL signals are thought to result from all defects and are used in their evaluation. The tested defects include two types of defects, concave and bump-shaped features, and recently described mechanisms in the MFL method indicate that the former defects produce positive MFL because of magnetic refraction and the latter ones produce negative magnetic fields because of self-magnetization regulation; consequently, these defects result in raised test signal waves and sunken test signal waves, respectively. Thereby, a new methodology for accurately identifying the defect type based on the mapping relation between the signal features and defect types is proposed. Both simulations and experiments with three representative defects (i.e., notch, protrusion and combination) were conducted to confirm their identification using this new methodology. Combined with MFL standards such as American Society for Testing and Materials (ASTM) E570-09 (Standard Practice for Flux Leakage Examination of Ferromagnetic Steel Tubular Products, 2009) and British Standards (BS) EN 10246-4 (Non-destructive Testing of Steel Tubes—Part 4: Automatic Full Peripheral Magnetic Transducer/Flux Leakage Testing of Seamless Ferromagnetic Steel Tubes for the Detection of Transverse Imperfections, 2007), suggestions for standard specimens with reference defects that consist of both types of defects are provided.

Application of Microwave for Remote NDT and Distinction of Biofouling and Wall Thinning Defects Inside a Metal Pipe

Abstract: Wall-thinning and biofouling/biofilm defects inside metal pipes are frequently reported, and each occurrence will cause a discontinuity along the metal pipe, which can be taken as a circular waveguide of microwave. If their existence and detailed distribution are unknown in advance, evaluated results may become confusing due to their possible coexistence and then complicate the correct interpretation of signals. To solve this problem, both S11 and S21 parameters are measured using a vector network analyzer and two newly designed probes. This experimental approach is based on the fact that similar reflections of different kinds of defects would have different wavelength changes and/or energy attenuation. Characteristic signals of these two kinds of defects are comparatively studied to distinguish them and generalize the remote microwave NDE application.

Nondestructive Testing Method for Curved Surfaces Based on the Multi-Gaussian Beam Model

Abstract: A nondestructive testing method for curved surfaces based on the multi-Gaussian beam model is proposed in this study. The multi-Gaussian beam model for composites coupled with water is introduced into an automatic ultrasonic testing system. A mathematical model of the received beam field is established to analyze the effect of different testing parameters on the receiving transducer. The normal vector can be calculated by variational methods to achieve the transmitting transducer’s position and orientation. The position and orientation of the receiving transducer can be achieved by coordinate transformation method. The simulation and experiments illustrate the validity and effectiveness of the proposed method. Experimental results indicate that the received signal is strong and measurement accuracy is high with the proposed method. The proposed method provides an effective solution to curved composite surface parts and would significantly benefit industrial development.

Implementation of Coda Wave Interferometry Using Taylor Series Expansion

Abstract: Coda waves are sensitive to small changes in scattering media, in which multiple scattering and repeatedly sampling occur due to the changes. Coda wave interferometry (CWI) is a technique to monitor the changes in media by comparing the coda waves before and after perturbation. Doublet technique and stretching technique are two commonly used approaches based on cross correlation when CWI analysis is performed. However these two methods are time-consuming due to the involvement of cross correlation calculation. This paper presents a new CWI calculation approach using Taylor series expansion. The computation time is significantly reduced compared to the cross correlation based methods. This method is demonstrated on a numerical example as well as an experimental example. Comparison of the results between the stretch technique and the method proposed is presented, and the influences of different factors on the CWI analysis are discussed.