Showing papers in "Journal of Nondestructive Evaluation in 2013"

Relation Between Magnetic Barkhausen Noise and Hardness for Jominy Quench Tests in SAE 4140 and 6150 Steels

Abstract: The nondestructive Magnetic Barkhausen Noise (MBN) technique was applied for the evaluation of SAE 4140 and SAE 6150 steels after a Jominy end-quench test. Microstructures were also characterized by SEM (Scanning Electron Microscope) and hardness tests. MBN measurements were performed on the same sample regions at three excitation frequencies. Different parameters of the measured signals (signal peak position and height, and Root mean square) were calculated. A relationship between mechanical hardness and MBN parameters was found for both materials, with the best correlation coefficient being found in low excitation frequency range.

Nondestructive Monitoring of Ageing of Alkali Resistant Glass Fiber Reinforced Cement (GRC)

Abstract: Glass fiber reinforced cement (GRC) is a composite material made of portland cement mortar and alkali resistant (AR) fibers. AR fibers are added to portland cement to give the material additional flexural strength and toughness. However, ageing deteriorates the fibers and as a result the improvement in the mechanical properties resulted from the fiber addition disappears as the structure becomes old. The aim of this paper is monitoring GRC ageing by nondestructive evaluation (NDE) techniques. Two different NDE techniques—(1) nonlinear impact resonant acoustic spectroscopy analysis and (2) propagating ultrasonic guided waves—are used for this purpose. Both techniques revealed a reduction of the nonlinear behavior in the GRC material with ageing. Specimens are then loaded to failure to obtain their strength and stiffness. Compared to the un-aged specimens, the aged specimens are found to exhibit more linear behavior, have more stiffness but less toughness. Finally, undisturbed fragments on the fracture surface from mechanical tests are inspected under the electron microscope, to understand the fundamental mechanisms that cause the change in the GRC behavior with ageing.

Long Range Detection of Defects in Composite Plates Using Lamb Waves Generated and Detected by Ultrasonic Phased Array Probes

Abstract: This article presents a technique for the generation and detection of Lamb waves guided along large plate-like structures made from various types of materials (metal, polymer, fibre-reinforced composite, etc.). A multi-element matrix ultrasonic probe is driven using the well-known phased array principle, for launching and detecting pure Lamb modes in/from specific directions along the plate, which are arbitrary for isotropic materials and limited to specific directions for anisotropic materials, e.g. principal directions or directions for which both phase and group velocities are collinear. The probe is gel-coupled to the tested specimen and allows quick inspection of large area from its fixed position, even of zones with limited access. The technique, which takes into account the frequency dispersive effects, is different than SHM-like (Structural Health Monitoring) inspection, since all transmitting or receiving elements are grouped together in a localized area defined by the active surface of the probe, and not permanently attached to the tested structure. The use of a multi-element probe, for long range Lamb waves-based inspection, is also distinctive from that usually performed, which consists of very local inspection of a material by steering the ultrasonic beam below and nearby the probe. A prototype is presented, as well as measurements of its performances in terms of modal selectivity and directivity. Finally the detection and localisation of a through-thickness hole in a large aluminium plate, of a delamination-like defect in a carbon epoxy composite plate and of an impact damage on a stiffened composite curved plate are shown.

Experimental Study on Ultrasonic Monitoring of Splitting Failure in Reinforced Concrete

Abstract: This paper investigates inspection of reinforced concrete elements sensitive to the splitting failure. The behaviour of a reinforced concrete specimen subjected to a tensile stress is considered. The damage detection procedure is based on the ultrasonic wave propagation technique. The piezoelectric transducers are located on both ends of the specimen and the measurements are taken periodically during the incrementally increased loading. The features of measured signals in time and frequency domains as well as wavelet transforms before and after the splitting failure are studied. The experimental results show that proposed method can be used for monitoring of damage evolution in concrete elements. The method makes possible detection of the moment just before the splitting failure occurs.

Characterization of Wire Rope Defects with Gray Level Co-occurrence Matrix of Magnetic Flux Leakage Images

Abstract: Magnetic flux leakage (MFL) techniques are used extensively for non-intrusively detecting and characterizing wire rope defects. Traditionally, MFL signals are captured with induction coil sensors. However, the output of coil sensors is related to the wire rope speed, and they can only provide the axial distribution along the wire rope. Hall sensors array are designed due to the limitation of coil sensors. In this paper, a Hall sensors array was designed to capture the MFL signals both axially and circumferentially. 30-channel data from Hall sensors are processed to compose a MFL image. A digital image process technique is introduced to preprocess the MFL image, the MFL images from different types of defects show different texture characteristics. Gray level co-occurrence matrix is utilized for feature extraction of the texture in the MFL image. Five typical eigenvalues (contrast, correlation, energy, homogeneity and entropy) are used as the inputs of back propagation (BP) networks. After training with typical samples, the BP networks show good performance in the quantitative recognition of different defects. The result of this work shows that texture analysis method for MFL image is suitable for feature extraction and quantitative detection of wire rope defects.

Evaluation of Concrete Distributed Cracks by Ultrasonic Travel Time Shift Under an External Mechanical Perturbation: Study of Indirect and Semi-direct Transmission Configurations

Abstract: Techniques based on non-linear acoustics have been proven sensitive to micro-defects in heterogeneous materials, such as concrete, but their implementation on-site is very restrictive. Ultrasonic travel time shift, a technique where a high frequency ultrasonic wave probes the medium while a low frequency elastic wave disturbs it to create a “time delay”, is a new promising technique that may be used efficiently on-site. This technique is based on nonlinear behaviour of concrete. Moreover, this technique offers the possibility of evaluating linear parameter, such as ultrasonic pulse velocity of direct waves. The scope of this paper is to study the applicability of the technique at different level of concrete damage and define its advantages, limitations in order to optimize its use. Because of the large dimensions, the geometry and limited access to various faces of existing structures, the transducers often have to be set on the same side. Two types of configurations, the indirect transmission (with incident waves at 90∘) and the semi-direct transmission (with incident waves at 45∘), were studied with cement base samples at different levels of damage (generated by freeze-thaw cycles). Up to now, test results have shown that time-shift is more sensitive when used in an indirect configuration of transmission rather than in a semi-direct configuration. Overall, the non-linear time-shift technique is much more sensitive to the initiation of cracking than linear indicators and its versatility (different indicators for different levels of damage) is of interest for rapid testing of structures.

Detection of Fibre Waviness Using Ultrasonic Array Scattering Data

Abstract: Composite materials owe their success to the ability to favour mechanical properties in specific directions whilst minimising the weight of components. Although the composite manufacturing process has been progressively improved, subtle defects such as fibre waviness are still commonplace. Any localised departure of a ply from the desired lay-up direction is known to adversely affect strength. Therefore, manufacturers and end users are interested in detecting defects such as fibre waviness at various stages during prototyping and as part of the manufacturing process. In this paper, an ultrasonic array is used to both image the composite and extract information that characterises the scattering of the interior structure. The scattering information is encoded in the scattering matrix: defined as the far field amplitude of scattered signals from a defect as a function of the incident and scattering angles. A method for extracting the scattering matrix from experimental array data over a spatially localised region is presented. Ultimately this could lead to the ability to map the distribution of scattering behaviour within the composite. The method is demonstrated on composite samples containing various levels of waviness. It is also shown that use of the differences in the scattering matrices can offer the possibility to statistically differentiate wavinesses of different nature and severity.

Determination of the Emissivity of Wood for Inspection by Infrared Thermography

Abstract: Infrared thermography (IRT) is a non-contact technique for visualizing the surface temperature of objects based on the radiation emitted. Recently, this non-destructive technique (NDT) has begun to be used for the diagnosis and evaluation of wood structures. In the process of transforming radiant energy into temperature, which is performed by thermographic devices, one of the most significant properties that affects the calculation of temperature is the emissivity of the material being analyzed. Prior knowledge of the emissivity is essential for a thermographic inspection because it quantitatively determines the amount of energy that is actually produced by the material. In this research study, the emissivity of different types of wood has been determined, and the influences of temperature and surface finish of the samples on the emissivity values have been analyzed.

Moisture Distribution in Partially Saturated Concrete Studied by Impedance Spectroscopy

Abstract: The moisture content and its spatial distribution has a great influence on the durability properties of concrete structures. Several non-destructive techniques have been used for the determination of the total water content, but moisture distribution is difficult to determine. In this paper impedance spectroscopy is used to study the water distribution in concrete samples with controlled and homogeneously distributed moisture contents. The technique is suitable for the determination of water distribution inside the sample, using the appropriate equivalent circuits. It is shown that using the selected drying procedures there is no change in the solid phase of the samples, although the technique can only be used for the qualitative study of variations in the solid phase when samples are too thick. The results of this work show that for a wide range of concrete percentages of saturation, from full to 18 % saturation, practically all the pores keep at least a thin layer of electrolyte covering their walls, since the capacitance measurement results are practically independent of the saturation degree.

Quantitative Determination of Density and Mass of Polymeric Materials Using Microfocus Computed Tomography

Abstract: A density calibration of homogeneous polymeric materials in the range of 0.9 to 2.2 g/cm3 with micro Computed Tomography (CT) scanning was devised and its accuracy, repeatability and potential sources of error were investigated. The density of unknown materials could be determined successfully in many cases with this calibration. However, in some cases the experimental values deviated significantly from the actual values. These deviations could be attributed mainly to chemical compositional differences compared to the calibration materials. Dual energy CT could be used to determine whether a material is within the range of atomic composition of the calibration materials, in which case the calibration function is accurate. If a material is outside the chemical composition range determined by dual energy CT, the calibration is not applicable and accurate density cannot be determined.

Study on the Effect of Elastic Stress and Microstructure of Low Carbon Steels on Barkhausen Noise

Abstract: The present study investigates the effect of elastic stress and microstructure on Barkhausen noise in low carbon steels subjected to different heat treatments. Barkhausen noise in an as-received test piece and a test piece heated at 450 ∘C for 1.5 hours was found to increase with increasing elastic stress. However, in a test piece heated at 700 ∘C for 10 hours, Barkhausen noise was observed to saturate with increasing elastic stress following an initial increase. To clarify the reason for this saturation behavior, magnetization measurements were carried out and the microstructure and texture of the test pieces were evaluated using microscopy and electron backscatter diffraction. The results indicated that for the test piece heated at 700 ∘C for 10 hours, a drastic change in the microstructure occurred compared to that for the other test pieces. From the experimental and analytical results, it was concluded that for the former test piece, Barkhausen noise saturated under a low elastic stress due to the globularization of pearlite, which caused 90∘ domain walls to become 180∘ domain walls when a low elastic stress is applied.

Defect Characterization Using Pulsed Thermography

Abstract: Infrared Thermography is one of the advanced NDE methods that is becoming attractive due to its ability to inspect non invasively large areas in short times and provide full field images in a non contact nature. While initially the method started as a qualitative technique for defect detection alone, with the advent of lock in and pulsed techniques, quantitative defect detection was made possible. While these techniques have been applied in case of ceramics and composites for defect quantification, defect sizing and quantification using these methods have not been systematically attempted in case of stainless steels. A systematic study has been undertaken to evaluate the accuracy of these empirical Pulsed Thermography (PT) techniques for defect size and depth estimation in type 316 L austenitic stainless steels. Theoretical modelling based on finite difference analysis using Thermo Calc 6L software has also been done and the results compared.

Numerical and Experimental Study of Crack Depth Measurement in Concrete Using Diffuse Ultrasound

Abstract: This paper presents a combined numerical and experimental study on the diffuse ultrasonic measurement technique for determining the depth of surface breaking cracks in concrete. A finite element (FE) model for the dissipative diffusion in a two-dimensional domain with a surface breaking crack is developed using a commercial FE package; for this purpose, the dissipation term is eliminated by a simple change of variables. Three concrete blocks with a crack depth between 25.4 mm to 101.6 mm are prepared. Diffuse ultrasonic measurements are performed on uncracked and cracked concrete blocks, from which the diffuse energy evolution curves are obtained. The basic material parameters of the hardened concrete, i.e. the diffusivity and dissipation, are retrieved, which are needed for the numerical simulations. The crack depths are then determined by comparing the experimental and numerical arrival times of the average diffuse ultrasonic energy. Various geometrical configurations that arise in real-world concrete structures are simulated including an inclined crack, a partially closed crack, two parallel cracks, and a crack with an underlying reinforcement bar. The objective is to investigate the possible limitations of the diffuse ultrasonic measurement technique when implemented in real concrete structures. Finally, it is shown that the time of flight (TOF) of the average diffuse ultrasonic energy constitutes the theoretical basis of the present diffuse ultrasonic measurement of macroscopic cracks and therefore the present diffuse ultrasonic method forms another kind of TOF technique.

Detection of the Subsurface Cracks in a Stainless Steel Plate Using Pulsed Eddy Current

Abstract: The nondestructive method to detect subsurface defects is limited because conventional eddy current are concentrated near to the surfaces adjacent to the excitation coil. The PEC technique enables detection of cracks buried deeper under the surface with relatively small current density. In the present study, an attempt has been made to investigate detection of subsurface cracks using a specially designed double-D differential probe. The tested sample is a SS304 with a thickness of 5 mm; small EDM notches were machined in the test sample at different depths from the surface to simulate the sub surface cracks in a pipe. The designed PEC probe has two excitation coils and two detecting Hall-sensors. The difference between two sensors is the resultant PEC signal. The cracks under the surface were detected using peak amplitude of the detected pulse; in addition, for a clear understanding of the crack depth, the Fourier transform is applied. In time domain, the peak amplitude of the detected pulse is decreased, and in the frequency domain, the magnitude of the lower frequency component has been increased with an increase in the crack depth. The experimental results have indicated that the proposed differential probe has the potential to detect the sub surface cracks in a stainless steel structure.

Microstructural Development of Hydrating Portland Cement Paste at Early Ages Investigated with Non-destructive Methods and Numerical Simulation

Abstract: Microstructure development of hydrating cement paste at early ages is not only an indicator of the reactivity of cement, but also a factor on the workability of fresh concrete. In this study, the microstructure development of hydrating cement paste at early ages is investigated with non-destructive methods including ultrasound P-wave propagation velocity measurement and non-contact electric resistivity tests, together with conventional needle penetration depth and calorimetry tests. The hydration process and microstructural development of the cement paste is modeled with the three-dimensional computer model CEMHYD3D. Evolution of microstructural parameters including the volumetric fraction of phases and their percolation status are analyzed by using the results of the numerical simulation. Microstructural mechanisms of the two non-destructive techniques (ultrasound pulse propagation and electric resistivity measurements) are discussed. The main findings of this study are that the velocity of ultrasound P-wave propagation in hydrating cement paste is a function of the propagation routes in the material and inter-particle forces. The electric resistivity is controlled by the ionic concentrations in the pore solution during the early hours and later by the connectivity of pores. A model for the development of ultrasound P-wave propagation velocity is also proposed.

Quantification of the Uncertainty of Pattern Recognition Approaches Applied to Acoustic Emission Signals

Abstract: Acoustic emission analysis is a nondestructive technique frequently used to assess the integrity of fiber reinforced plastics. Pattern recognition techniques have shown great potential to identify microscopic failure mechanisms in plate-like structures. Because every assignment of an acoustic emission signal to a respective failure mechanism is possibly associated with an error, one key question is the reliability of the assignment method. It is useful to distinguish between the uncertainty of the assignment and the false assignment of an acoustic emission signal to a group of signals. The first is owed to statistical effects and the reliability of the classification method itself. The second is caused by false conclusions or disputable assumptions on the source mechanisms. The present study will focus on the first aspect. For this purpose, we propose a model based algorithm that estimates the uncertainty of a feature based pattern recognition approach based on cluster validity indices. Further, we demonstrate the application of the algorithm to experimental acoustic emission data obtained from a double cantilever beam specimens with unidirectional layup of carbon fiber reinforced polymer. Based on previous investigation we use a pattern recognition approach to distinguish between different failure mechanisms like matrix cracking, interfacial failure and fiber breakage based on the frequency features of the acoustic emission signals. We consider the influence of dispersion and attenuation effects during propagation of Lamb-waves on the extracted acoustic emission features. This is done by investigating the influence of source-sensor distance by test sources like pencil lead breaks and piezoelectric pulsers. Using the model based algorithm it is possible to calculate the uncertainty of the pattern recognition results as a function of source-sensor distance. It is found that dispersion effects of Lamb-waves do not seriously affect the distinction between microscopic failure mechanisms for source-sensor distances up to 375 mm. We demonstrate that the spatial distribution of acoustic emission sources has a larger impact on the uncertainty of assignment than the absolute source-sensor distance. Applying the proposed algorithm to the current experimental setup, we obtain an uncertainty of classification below 7 % for source-sensor distances below 375 mm. Attenuation is quantified to be 0.165 dB/mm for the A 0-mode and 0.047 dB/mm for the S 0-mode. Within the source-sensor distance of 375 mm this causes severe attenuation of the signal amplitude and thus prohibits detection of weak acoustic emission signals long before the uncertainty of the classification method reaches 10 %.

Nondestructive Porosity Assessment of CFRP Composites with Spectral Analysis of Backscattered Laser-Induced Ultrasonic Pulses

Abstract: The laser-ultrasonic method for nondestructive quantitative local porosity assessment for CFRP composites is proposed and realized experimentally for only one available flat surface of a specimen or a product. This method combines the laser thermoelastic generation and the high-sensitivity piezoelectric detection of broadband pulses of longitudinal ultrasonic waves and does not require the detection of the backwall echo ultrasonic signal. The generation and the detection of ultrasonic pulses is carried out with the specially designed laser-ultrasonic transducer, which allows one to obtain both the temporal profile and the frequency spectrum of a part of the ultrasonic signal backscattered by gas voids in a composite specimen. The frequency spectrum of backscattered ultrasonic pulses is analyzed for three sets of CFRP specimens with different epoxy matrix fractions and porosity. The empirical relation between porosity of CFRP specimens and the spectral power (structural noise power) of ultrasonic signals backscattered by voids is obtained for porosity values up to 0.15. This relation allows one to evaluate the local porosity from measured structural noise power both for CFRP specimens and products fabricated from the same composite material. The proposed laser-ultrasonic setup demonstrates a basis for a system of CFRP porosity assessment in field conditions. It can be very useful especially for nondestructive detection of structural changes of composite materials that will allow evaluation of products during their life time.

Leakage Locating in Underground High Pressure Gas Pipe by Acoustic Emission Method

Abstract: Detecting gas leakage accurately in urban high-pressure natural gas pipelines is a great problem all over the world. Gas leakage may lead to pollution and severe environmental damages; therefore, in order to minimize the leakage problem, it is necessary to maintain pipelines. Acoustic emission is a technique to detect leakage in urban pipelines. Leakage in high-pressure pipes radiates acoustic emission signals that are transferred through the pipe walls. This paper proposes a leakage detection algorithm combined with wavelet transform, filtering and cross-correlation techniques to locate leakage source in urban gas pipes. The major noise of acoustic emission signals is removed through the wavelet transform and filtering technique. After removing the noise, the time difference between the signals recorded at two sensors is precisely computed using cross-correlation function. Experiments are carried out with continuous leakage source and a linear array of two sensors positioned in two sides of the leakage source. To study the accuracy of the proposed algorithm, several tests were carried out changing the source-sensor distance, and a percentage error less than 5 % was found in the leakage detection.

SAFT and TOFD—A Comparative Study of Two Defect Sizing Techniques on a Reactor Pressure Vessel Mock-up

Abstract: Defect sizing is required for a quantitative assessment of the quality and reliability of safety relevant components and materials using ultrasonic non-destructive testing. The SAFT (Synthetic Aperture Focussing Technique) and the TOFD technique (Time Of Flight Diffraction) are such promising sizing candidates, extracting more information from the raw ultrasound echo data and the corresponding crack tip response. In this work the phased array technique is used to inspect a clad mock-up model of a pressure vessel section. The full scale model contains artificial test reflectors which are located in the weld and in the cladding region as well. The defects—representing typical flaws at a very early stage—are analysed with different frequencies, beam angles and directions of incidence. For the reconstruction of reflector indications a SAFT algorithm is applied to the phased array measurement results. Additionally the reflectors are analysed by means of the TOFD technique, using different beam angles at the same time. Both analysis methods are performed using different directions of incidence considering the complex cladding structure underneath the inner surface of the mock-up model. A direct comparison of the SAFT and TOFD techniques shows that, besides the clarity of the results, the detection and sizing capabilities of SAFT are far better.

Evaluation of Railroad Wheel Steel with Lamellar Duplex Microstructures Using Diffuse Ultrasonic Backscatter

Abstract: The effects of lamellar duplex microstructure within grains that contain alternating phases of cementite and ferrite on ultrasonic scattering in railroad wheel steel are evaluated using a diffuse ultrasonic backscatter technique. A new singly scattered response (SSR) model that considers the lamellar duplex microstructure within grains is developed based on a previous SSR model. The results show that the amplitude of ultrasonic scattering decreases with decreasing lamellar space. Corresponding experiments are performed with 10 MHz and 15 MHz focused transducers by scanning both unquenched and quenched wheels. The experimental results show that the ultrasonic scattering amplitudes drop dramatically near the quenched tread surface, a result which is attributed to the creation of duplex microstructure (pearlite phase) within grains due to the quenching process. The lamellar spacing within grains increases progressively from the tread surface to the deeper locations due to the non-uniform cooling rate. The distribution of lamellar spacing within grains as a function of depth is quantified with the modified SSR model. Good agreement with optical microscopy is observed. The diffuse ultrasonic backscatter technique exhibits strong sensitivity to microstructure changes, an outcome that may be applicable for quality control during manufacturing.

Corrosion Monitoring Strategies—Choice Between Area and Point Measurements

Abstract: There is increasing interest in the use of permanently installed monitoring systems to track the progress of corrosion. There is a choice between point thickness measurement systems, those that monitor the average thickness over a modest area, and large area monitoring systems that will detect relatively severe, localised corrosion. The relative merits of the different types of monitoring system are assessed for cases with different likely fractions of the total surface area affected by corrosion. If the wall loss is expected to be fairly uniform over the component surface then a small number of point sensors is the most attractive solution, whereas if the loss is likely to be highly localised at an unpredictable location and potentially severe, a large area monitoring system is most suitable. In the intermediate case of modest loss over a significant fraction of the surface, the choice is more complex. If the requirement is to detect relatively large wall loss that may be localised then the number of point sensors required increases substantially and the average thickness area monitoring systems appear to be more attractive. However, it is shown that corrosion distributions are frequently exponential, and this can lead to large maximum pit depths when the average wall loss is at the margin of detectability with current commercially available systems.

An Efficient Procedure for Identifying the Prediction Model Between Residual Stress and Barkhausen Noise

Abstract: Residual stress of case-hardened steel samples is predicted in this paper with the linear multivariable regression model. The development of the prediction model is based on the huge set of features calculated from the Barkhausen noise measurement signal among which the most suitable ones are chosen. The selection uses a genetic algorithm with leave-multiple-out cross-validation in the objective function. The original feature set contains collinear features that make the selection task even more complex. Thus a feature elimination procedure based on the successive projections algorithm is studied in this paper. Also the standard genetic algorithm is slightly modified to better serve the feature selection task. The obtained results are good showing that the proposed procedures suit well for residual stress predictions. Also the applied feature elimination procedure is applicable and can be safely used to reduce the dimensionality of the selection problem.

Probabilistic Fatigue Life Prediction and Structural Reliability Evaluation of Turbine Rotors Integrating an Automated Ultrasonic Inspection System

Abstract: The paper presents a general method and procedure for fatigue reliability assessment integrating automated ultrasonic non-destructive inspections. The basic structure of an automated ultrasonic inspection system is presented. Fatigue reliability assessment methodology is developed using uncertainty quantification models for detection, sizing, and fatigue model parameters. The probability of detection model is based on a classical log-linear model coupling the actual flaw size with the ultrasonic inspection reported size. Using probabilistic modeling, the distribution of the actual flaw size is derived. Reliability assessment procedure using ultrasonic inspection data is suggested. A steam turbine rotor example with realistic ultrasonic inspection data is presented to demonstrate the overall method. Calculations and interpretations of assessment results based on risk recommendations for industrial applications are given.

Ultrasonic Backscattering in Cubic Polycrystals with Ellipsoidal Grains and Texture

Abstract: An ultrasonic model for backscattering from polycrystalline microstructure is developed for polycrystals with uniaxial texture and elongated cubic crystallites. The uniaxial texture or crystallographic orientation of the grains is described by a modified Gaussian orientation distribution function (ODF) with a texture parameter. Macroscopically such a textured polycrystalline medium exhibits hexagonal symmetry. The preferred texture direction and elongation are independently defined in a global system. The dependence of backscattering coefficients and their directional ratios on both texture and grain anisotropy are discussed. Attenuation coefficients in the high frequency range for arbitrary wave propagation direction are obtained and then the ratios in the three axis directions are studied. The model is compared with experimental data available in the literature for Al rolled alloys and shows good agreement when accounting for both texture and grain anisotropy effects.

Nondestructive Evaluation of Welding Residual Stresses in Dissimilar Welded Pipes

Abstract: This paper investigates the nondestructive capability of ultrasonic waves in residual stress evaluation of dissimilar welded pipes. Longitudinal critically refracted (L CR ) waves are employed to measure the residual stresses in a pipe-pipe joint of stainless steel 304 and carbon steel A106. Measuring the acoustoelastic constant is usually accomplished through the tensile test which needs cutting the tested material to extract tensile test specimens. However, cutting the tested pipe to complement the nondestructive ultrasonic measurement was not considered here. Instead, a dissimilar welded plate with the same welding specification, joint geometry, thickness and the same dissimilar materials is used to extract tensile test samples. The measured acoustoelastic constant of the plate along with the measured time of flight of the L CR wave on the pipe, are utilized for ultrasonic stress measurement. A finite element model of welding process validated by hole-drilling method is used to verify the ultrasonic results. The results show good agreement between finite element and ultrasonic measurements in the pipe measured without any destructive process.

Damage Localization in Plates Using Mode Conversion Characteristics of Ultrasonic Guided Waves

Abstract: The present study provides a concise description of wave propagation in cellular sandwich panels. A novel approach of damage detection based on mode conversion is proposed which can be useful for detecting relatively small damages in cellular sandwich structures using high frequency guided waves. The new methodology applies the continuous wavelet transform (CWT) and the cosine formula to extract the damage location from the time signal of displacements. Experiments conducted on a honeycomb and a metallic hollow sphere sandwich plate highlight the feasibility of the novel technique.

Magnetic Nondestructive Test for Resistance Spot Welds Using Magnetic Flux Penetration and Eddy Current Methods

Abstract: Resistance spot welding technologies are widely used in industry. A highly reliable monitoring method is needed to effectively weld and create a robust structure. We developed a combined technique using magnetic flux penetration and an eddy current test (ECT). The magnetic measuring system consists of a pair of magnetic probes having an induction coil and detection coil, a lock-in amplifier, a current source, and a personal computer. The magnetic flux penetration through both surfaces at the weld was measured at low frequency. The ECT was performed at each surface with multiple frequencies. The magnetic flux penetration method showed good correlation with the destructive shear test because of the change in permeability due to the formation of the nugget. The ECT method reflected the depth profile of the nugget and was effective for determining a defective product.

Characterizing Particle Flow by Acoustic Emission

Abstract: A simple theoretical model which captures the statistical nature of an acoustic emission (AE) signal generated by a stream of solid particles impinging on a flat solid surface is presented. It rests on well-known results dating back to the fundamental work by Hertz, which assumes the impact to be elastic, the particle to be spherical, and the surface on which the particle bounces perfectly flat. The average power of the signal is evaluated in two limits. In the first one, the frequency content of the signal is identical to that of the forces developed during impact. In the second limit, only the low frequency part of the source function contributes to the spectrum of the recorded signal, the transfer function of the measuring system acting as a low-pass filter. In this case, it is shown that the result has an immediate physical interpretation, although its practical relevance is still limited. Additional progress is accomplished by experimentally proving that the statistical average of the particles’ momentum may be replaced with the average flow velocity, a quantity that may be directly measured in real life situations. It is suggested that this simplified version of the theoretical result may provide a useful tool to characterize particle flow in systems of industrial interest.

Quantitative Evaluation of Corrosion in a Thin Small-Bore Piping System Using Bobbin-Type Magnetic Camera

Abstract: Nondestructive testing (NDT) methods have been developed to ensure the integrity of heat exchanger pipes. NDT systems can be used to not only locate cracks on the pipes but also evaluate the size and position of the cracks. A bobbin-type magnetic camera (BMC), an electromagnetic NDT system, was developed to inspect both the inner and outer diameters (ID and OD, respectively) for stress corrosion cracking on a small-bore piping system with a high spatial resolution at high speeds. In this paper, an algorithm that discriminates the ID cracks from the OD cracks and estimates the crack volume is proposed. Artificial ID and OD hole-type cracks which have diameter of 1–4.5 mm and depth of 0.3–27 mm were used to imitate the corrosions on a 1.27 mm thickness and 16.56 mm inner diameter copper alloy pipe to verify the proposed algorithm. The inspection results of the BMC with respect to excited frequencies from 1–9 kHz are presented. 100 % of the ID and OD cracks were discriminated, and their volumes were estimated with a standard deviation of 1.132 mm3 for volume from 1.00–9.01 mm3.

Novel Coupled Electric Field Method for Defect Characterization in Eddy Current Non-destructive Testing Systems

Abstract: This article presents a defect modeling in eddy current non-destructive testing systems by using a new developed method called coupled electric field. It permits to improve qualitatively several models developed so far by many authors using coupled circuit methods that consider the defect only as loss of material. However, a defect can occur with a finite conductivity such as impurity, small burns and micro-solder. For this reason, this investigation consists of extending the coupled circuit method to the modeling of this kind of defects. The proposed approach consists of firstly considering the defect as an electric conductive volume and secondly changing the state variable presenting the electric current by the electric field one. This procedure permits expressing explicitly the impedance variation caused by the presence of an axi-symmetrical defect according to its characteristics. The comparison between the impedance variations calculated using finite elements method and the proposed one demonstrates a very good concordance. After this validation, the study covers also the influence of the defect shape and position on encircling probe impedance. This method is interesting since it permits a fully characterization of this kind of defects and facilitates the inversion process. Moreover, using a 3D finite element observation, this fast tool of simulation can be adapted for a fast phenomenological modeling of asymmetrical configurations.