Showing papers in "Ndt & E International in 2015"

Review of pulsed thermal NDT: Physical principles, theory and data processing

Abstract: This paper summarizes the basics of pulsed thermal nondestructive testing (TNDT) including theoretical solutions, data processing algorithms and practical implementation. Typical defects are discussed along with 1D analytical and multi-dimensional numerical solutions. Special emphasis is focused on defect characterization by the use of inverse solutions. A list of TNDT terms is provided. Applications of active TNDT, mainly in the aerospace industry, are discussed briefly, and some trends in the further development of this technique are described.

Electrical conductivity measurement of ferromagnetic metallic materials using pulsed eddy current method

Abstract: Pulsed eddy current testing (PECT) method for electrical conductivity measurement of ferromagnetic metallic materials is proposed. Based on time-domain analytical solutions to the PECT model of ferromagnetic plates, the conductivity and permeability are determined via an inverse problem established with the calculated and measured values of induced voltage. PECT method for conductivity measurement is verified by the four-point probe method on three carbon steel plates. In addition, the effects of the amplitude of pulsed excitation current and the lift-off of probe coils on measurement results are studied. PECT is an innovative, non-contacting method with good repeatability for electrical conductivity measurement.

Weld defect detection using PPM EMAT generated shear horizontal ultrasound

Abstract: Austenitic welds are inspected using PPM EMAT generated shear horizontal (SH) waves. Results are compared to measurements taken using a 1D piezoelectric phased array using the total focusing method (TFM). For the first time there is clear experimental evidence of the SH wave method demonstrating higher sensitivity to defect detection. SH waves suffer less beam steering in a weld than either compression or SV waves, which can miss defects due to weld microstructure anisotropy and attenuation. All defects were identified from every side of the weld/plate using the SH waves, but this was not possible using the piezoelectric transducer.

Automatic weld defect detection method based on Kalman filtering for real-time radiographic inspection of spiral pipe

Abstract: A method based on Kalman filtering is proposed for weld defect detection in real-time radiographic NDT of spiral pipes. The existence of the image noises and the inhomogeneity of the background contrast induce numerous false alarms. In this paper, the trajectory continuity of the defects in the image sequence is detected by Kalman filtering for the identification of true defects. Potential defect regions without continuous motion are considered as false alarms and are eliminated. Experiments are performed to demonstrate the adaptability of the proposed method. The robustness of the method is also verified under unstable detection velocity.

Determination of the applicability and limits of void and delamination detection in concrete structures using infrared thermography

Abstract: Based on the results of the infrared thermography of 51 artificially created defects – voids and delaminations – in concrete, it was shown that it is possible to detect defects at depths that are equal or less than the defect size D using the thermal contrast method. By applying the phase contrast method, an increase of 50% in the maximum depth for a given defect size D was achieved. Delaminations containing thin air gaps were detected with the same success as much larger voids of the same cross section.

Nonlinear ultrasonic evaluation of the fatigue damage of adhesive joints

Abstract: An experimental method based on the nonlinear ultrasonic technique is presented to evaluate fatigue damage of an adhesive joint. In this paper, specimens made from AZ31 magnesium–aluminum alloy bonded through an epoxy layer are subjected to a fatigue load. The ultrasonic harmonics generated due to damage within the adhesive layer are measured; and the acoustic nonlinearity parameter (ANP) based on the fundamental and second harmonics is determined. The results show that the normalized ANP increases with the fatigue cycles. Furthermore, a theoretical model with different interfacial compression and tension stiffness is proposed to interpret the generation of second harmonics.

Quantitative non-destructive evaluation of wall thinning defect in double-layer pipe of nuclear power plants using pulsed ECT method

Abstract: In this paper, an inversion algorithm for three-dimensional profile reconstruction of wall thinning defect in a double-layer region of a typical coolant pipe of nuclear power plants from pulsed eddy current testing (PECT) signals has been proposed and experimentally validated, based on a fast simulator of PECT signals and a deterministic optimization strategy. First, the fast simulator developed by authors for PECT signal prediction based on a Fourier-series scheme in addition with interpolation and database approaches is briefly described as a base for the inversion of PECT signals. Then, the formula of the conjugate graduate inversion algorithm for sizing three-dimensional wall thinning from PECT signals is deduced in detail based on that for crack like reconstruction using single frequency ECT signals. The three-dimensional local wall thinning is modeled as a group of planar defects with different length and depth which are reconstructed from two-dimensional scanning PECT signals through inverse analysis. Through conducting PECT experiment for double-layer coolant tube test-piece and reconstructing wall thinning profile from the measured signals, the efficiency and the robustness of the proposed inversion algorithm are demonstrated.

Enhancement of thermographic images as tool for structural analysis in earthquake engineering

Abstract: In this paper, we present an application of a reconstruction method to thermographic images employed to analyze the response of a masonry structure under seismic actions. At first the theory of linear multivariate sampling Kantorovich operators is presented. By means of the above operators, we are able to reconstruct images taken from thermographic survey of masonry walls, and enhance their quality. Digital image processing of reconstructed images allows us to identify the mutual arrangement of the blocks (made of stones and/or bricks) and mortar joints inside the wall portion analyzed, and therefore the texture of the masonry. Subsequently, the texture has been used to estimate the equivalent elastic properties of the masonry by means of homogenization techniques. Finally a real-world case-study is analyzed, taking into account the mechanical properties estimated from reconstructed thermographic images and evaluating the structural response in terms of modal analysis.

A novel approach of accurately evaluating residual stress and microstructure of welded electrical steels

Abstract: In the present research work the determination of residual stress distribution in welded non-oriented electrical steel samples is discussed. Tungsten inert gas was used for the welding method. Residual stress was directly determined through deformation measurements and appropriate math calculations. Two methods were used: the magnetic, non-destructive method of Barkhausen noise and the semi-destructive method of X-ray diffraction. In order to evaluate the accuracy and reliability of the magnetic method applied, the steel samples were subjected to both compressive and tensile stresses and the magnetic noise values were correlated to residual stress values through an appropriate calibration curve. The results were then verified by the XRD method and were further evaluated by examining the microstructure and the mechanical properties of the as received and welded samples through scanning electron microscopy and hardness measurements, respectively. It was found that the deviation between the two methods was within acceptable limits, thus implying potential applicability of the MBN method in non-destructive testing of materials.

Investigation into eddy current pulsed thermography for rolling contact fatigue detection and characterization

Abstract: This paper reports on the use of eddy current pulsed thermography (ECPT) for detection and characterization of rolling contact fatigue (RCF). Detection mechanisms with eddy currents and heat propagation effects were discussed with RCF modeled as a simple angled defect. Two different angled defects were studied through numerical simulations and experimentally by using uniform magnetic field (UMF) excited by Helmholtz coils. Finally, a rail sample with RCF defects was inspected using UMF excitation. It is shown that ECPT with UMF excitation provides an efficient and robust method to detect angled defects, compared with nonuniform magnetic field (NUMF) excitation.

Weak magnetic flux leakage: A possible method for studying pipeline defects located either inside or outside the structures

Abstract: Magnetic leakage distribution results from linear defects of oil–gas pipelines in a weak magnetic field, which is modeled by the magnetic dipole theory. The analysis is useful for the identification of defects located either inside or outside the pipelines. The results indicate that the radial signals of inside–outside defects can be clearly distinguished, and the axial signals are basically the same in a weak magnetic field. The theoretical and the experimental results are very consistent.

In-situ monitoring of fatigue crack growth using high frequency guided waves

Abstract: The development of fatigue cracks at fastener holes represents a common maintenance problem for aircraft. High frequency guided ultrasonic waves allow for the monitoring of critical areas without direct access to the defect location. During cyclic loading of tensile, aluminum specimens fatigue crack growth at the side of a fastener hole was monitored. The changes in the energy ratio of the baseline subtracted reflected guided wave signal due to the fatigue damage were monitored from a stand-off distance using standard ultrasonic pulse–echo measurement equipment. Good sensitivity for the detection and monitoring of fatigue crack growth was found.

Torsional mode magnetostrictive patch transducer array employing a modified planar solenoid array coil for pipe inspection

Abstract: In recent investigations, the application of the transducers used in plates has been extended to pipe inspection with circumferential arrays because of the similar particle displacement patterns of some modes between plate and pipe, such as SH and torsional modes. Motivated by the configuration of magnetostrictive patch transducer (MPT), in which a planar solenoid array (PSA) coil was adopted to generate SH waves in plates, we proposed a MPTs array employing a modified planar solenoid array (MPSA) coil, i.e. MPSA coil-MPTs array, for generating and receiving the torsional mode in pipes. The configuration of the MPSA coil modified from PSA coil is suitable to be arranged on the pipe surface and can generate high-power waves. In the suggested configuration of MPT, because the direction of the actuating dynamic magnetic field produced by the MPSA coil is orthogonal to the direction of the static magnetic field produced by the permanent magnets in the magnetostrictive patch, the shear deformation induced by the magnetostriction in the patch will be delivered to the pipe, thus generating the torsional mode. First, it was experimentally verified that the fundamental torsional mode, T(0,1), could be effectively generated and received in pipes with the developed MPSA coil-MPTs array. Then, we studied the relationship between the frequency response characteristic of the developed MPSA coil-MPTs array and the interval defined by the distance of adjacent solenoids in the MPSA coil. Simultaneously, the circumferential crack defects in pipe were successfully identified. Finally, the performance of the MPSA coil-MPTs array was compared with that of previous meander coil-MPTs array. The comparison results demonstrated that the developed MPSA coil-MPTs array could enhance the power of the torsional mode.

Application of regularized deconvolution technique for predicting pavement thin layer thicknesses from ground penetrating radar data

Abstract: In this paper, regularized deconvolution is utilized to analyze GPR signal collected from thin asphalt pavement overlays of various mixtures and thicknesses on a test site. By applying regularized deconvolution and the L-curve method, the overlapped interface was identified in the signal. The thickness of the thin layer was predicted with maximum error of 4.2%, which is less than 1.5 mm, a value well below the layer tolerance during construction. The study shows that the algorithm based on regularized deconvolution is a simple and effective approach for processing GPR data collected from thin pavement layers to predict their thickness.

Use of gamma rays in the inspection of steel wire ropes in suspension bridges

Abstract: Regularity visual inspections are performed on steel wire ropes of suspension bridges. However, because the steel wire ropes were coated with plastic materials, inspectors could not visually detect the deterioration conditions of the ropes. In this paper, radiation tests and electromagnetic testing were compared. The gamma rays used in the radiation tests were employed to develop two assessment techniques, namely the exposure time formula and sensitivity assessment of steel wire ropes. Actual tests showed that such techniques can be adopted to evaluate the defects of steel wire ropes and help engineers improve the safety of suspension bridges.

Extended synthetic aperture focusing technique for ultrasonic imaging of concrete

Abstract: Quantitative nondestructive characterization of defects and inclusions in portland cement concrete structures are realized in this paper via extended reconstructions for linear array ultrasound systems. This is accomplished through generalization of traditional Kirchhoff-based synthetic aperture focusing technique migration to mitigate the effects of limited aperture and handle multiple scans as a single virtual array with increased effective aperture. Pearson’s correlation is utilized to account for uncertainty in relative position of individual measurement and mitigate the need for robotic precision when placing adjacent scans. The robustness of the method is demonstrated on artificially generated data as well as in-situ measurements for assessment of internal portland cement concrete characteristics such as inclusions and cracks.

Estimating the self-healing capability of cementitious composites through non-destructive electrical-based monitoring

Abstract: Self-healing evaluation of cementitious composites was made by non-destructive test (NDT) methods (electrical impedance [EI], rapid chloride permeability test [RCPT] and resonant frequency [RF]). Correlations among results obtained from different NDT methods were reported. Conclusions revealed that EI testing is easy to perform, takes very limited time and looks promising for self-healing assessment although the method itself seems to be remarkably influenced by anything modifying the ionic state of specimens. A solid exponential relationship exists between EI and RCPT measurements, but results from RF tests do not correlate with EI and RCPT results due to different parameters affecting individual tests.

Noncontact fatigue crack visualization using nonlinear ultrasonic modulation

Abstract: This paper presents a complete noncontact fatigue crack visualization technique based on nonlinear ultrasonic wave modulation and investigates the main source of nonlinear modulation generation. Two distinctive frequency input signals are created by two air-coupled transducers and the corresponding ultrasonic responses are scanned using a 3D laser Doppler vibrometer. The effectiveness of the proposed technique is tested using aluminum plates with different stages of fatigue crack formation such as micro and macro-cracks. Furthermore, the main source of nonlinear modulation is discussed based on the visualization results and the microscopic images.

Nonlinear ultrasonic characterization of precipitation in 17-4PH stainless steel

Abstract: This research is part of a broader effort to develop a nondestructive evaluation technique to monitor radiation damage in reactor pressure vessel steels, the main contributor being copper-rich precipitates. In this work, 17-4PH stainless steel is thermally aged to study the effects of copper precipitates on the acoustic nonlinearity parameter. Nonlinear ultrasonic measurements using Rayleigh waves are performed on isothermally aged 17-4PH. Results showed a decrease in the acoustic nonlinearity parameter with increasing aging time, consistent with evidence of copper precipitation from hardness, thermo-electric power, transmission electron microscopy, and atom probe tomography measurements.

Elimination of liftoff effect using a model-based method for eddy current characterization of a plate

Abstract: Model-based inversion method has been studied extensively for characterization of a metal plate in eddy current testing. However, few reports cover liftoff elimination. In this work, model-based inversion method is evaluated in terms of liftoff reduction. For better inversion accuracy, a complex yet accurate procedure is presented to do calibrations of coil parameters before use. The results from simulations and experiments demonstrate that model-based inversion method has an exceptional ability to compensate influence of large liftoff variations by considering it as an unknown variable to be determined.

Dynamic thermal tomography: Recent improvements and applications

Abstract: The concept of “dynamic thermal tomography” (DTT) was suggested in the 1980s. At that time, there was a wave of interest in the tomographic analysis of materials by active thermal nondestructive testing (TNDT). Unlike particles and quanta of electromagnetic radiation, thermal energy propagates in solids by diffusion. Therefore, a purely geometrical approach, that is characteristic of computed X-ray tomography, is replaced in DTT with the analysis of the evolution of temperature versus time. DTT is based on the fact that, in one-sided TNDT, deeper material layers are characterized by longer time delays of the thermal response. The DTT algorithm is relatively stable when used in the inspection of certain materials. Thermal waves experience damping by amplitude and retardation in time. This limits the detection depth and produces certain artifacts that can be suppressed by thresholding maxigrams. DTT can also be considered as a specific way of data presentation that has proven to be useful in many practical cases, including surface and volumetric thermal stimulation of both metals and non-metals. Thermal tomograms appear similar to binary maps of defects, thus enabling more reliable defect detection in comparison to conventional IR thermograms. In this paper, a “reference-free” approach to DTT is proposed being based on some mathematical manipulations with a front-surface temperature response. Also, the possibility of using the DTT principles for processing the results of ultrasonic infrared thermography is demonstrated.

Creep degradation characterization of titanium alloy using nonlinear ultrasonic technique

Abstract: A through transmission nonlinear ultrasonic measurement has been proposed to characterize the creep degradation of titanium alloy that was conducted by creep tests at temperature of 600 °C. The experimental results show a change of “N”-like shape of the acoustic nonlinearity versus the creep loading time, which reveal based on metallographic studies that the variation of acoustic nonlinearity is closely related to the microstructure evolutions. An analytical model calculation has revealed a good agreement with the measured result, which indicates that the precipitate–dislocation interaction is likely the dominant mechanism responsible for the change of acoustic nonlinearity in the crept materials.

A new X-ray backscatter imaging technique for non-destructive testing of aerospace materials

Abstract: This paper presents a new X-ray backscatter technique (XBT) for non-destructive imaging of aerospace materials with only a single-sided access. It uses a special twisted slit collimator to inspect the whole object by changing the viewing direction of the X-ray backscatter camera. For the first time, the X-ray backscatter measurements were conducted using high-energy (>500 keV) X-ray sources. Experiments were performed on thick complex structured aluminium components, stringers and honeycomb structures to validate the applicability of the present technique to image small changes in the material properties and also to detect low-density material inclusions. In order to reduce the inspection time from hours to several seconds and to improve the image quality of the X-ray backscatter image, the backscattered signals were measured using a digital detector array with high spatial resolution (200 µm). The influence of the energy of the X-ray source and the slit width of the camera on the X-ray backscatter image were also investigated. In the proposed technique, the whole object is irradiated by an un-collimated X-ray beam resulting in a low image acquisition time of 3 min that facilitates the use of XBT for the real time NDT&E of aerospace materials.

Evaluation of mean grain size using the multi-scale ultrasonic attenuation coefficient

Abstract: An evaluation model based on the multi-scale ultrasonic attenuation coefficient was developed to control both systematic error and random error. AISI 304 stainless steel was used to validate the presented model. Wavelet transformation was used to obtain the variation of ultrasonic signal over time and scale. Particle swarm optimization was utilized to correlate the coefficient with grain sizes. The model shows the attenuation of all scales increased with the grain size, and ultrasound attenuates faster on smaller scales. Compared with the ultrasonic velocity method and the traditional attenuation method, the proposed method has less systematic error and random error.

Mode separation and characterization of torsional guided wave signals reflected from defects using chirplet transform

Abstract: The sensor configuration of a magnetostrictive guided-wave system can be described as a single continuous transducing element which makes it difficult to separate the individual modes from the reflected signal. In this work, we develop the mode decomposition technique employing chirplet transform, which is able to separate the individual modes from dispersive and multimodal waveforms measured using the magnetostrictive sensor, and estimate the time–frequency centers and individual energies of the reflection, which would be used to locate and characterize defects. The reflection coefficients are calculated using the modal energies of the separated modes. Experimental results on a carbon steel pipe are presented, which show that the accurate and quantitative defect characterization could be enabled using the proposed technique.

Multi sensor data fusion approach for automatic honeycomb detection in concrete

Abstract: We present a systematic approach for fusion of multi-sensory nondestructive testing data. Our data set consists of impact-echo, ultrasonic pulse echo and ground penetrating radar data collected on a large-scale concrete specimen with built-in honeycombing defects. From each data set, the most significant signatures of honeycombs were extracted in the form of features. We applied two simple data fusion algorithms to the data: Dempster’s rule of combination and the Hadamard product. The performance of the fusion rules versus the single-sensor testing was evaluated. The fusion rules exhibit a slight improvement of false alarm rate over the best single sensor.

2D finite element modeling of the non-collinear mixing method for detection and characterization of closed cracks

Abstract: The non-collinear mixing technique is applied for detection and characterization of closed cracks. The method is based on the nonlinear interaction of two shear waves generated with an oblique incidence, which leads to the scattering of a longitudinal wave. A Finite Element model is used to demonstrate its application to a closed crack. Contact acoustic nonlinearity is modeled using unilateral contact law with Coulomb׳s friction. The method is shown to be effective and promising when applied to a closed crack. Scattering of the longitudinal wave also enables us to image the crack, giving its position and size.

An NDT guided wave technique for the identification of corrosion defects at support locations

Abstract: Due to the large number of pipe supports over a piping run, a rapid reliable NDT system is needed to identify hidden corrosion defects at a pipe-support interface. To accomplish this, a system using Periodic Permanent Magnet (PPM) Electromagnetic Acoustic Transducers (EMAT׳s) to generate dispersive SH1 guided waves is implemented. For this study, both the effect of the support contact and a corrosion type defect are evaluated independently through finite element models and experiments utilizing a flat plate approximation. It was found that utilizing the SH1 plate wave near the inflection point or ‘knee’ of the dispersion curve yields a high sensitivity to gradual wall loss defects while experiencing a minimal effect from the support contact.

Multi-sensor image fusion at signal level for improved near-surface crack detection

Abstract: This study aims at improving the detection of near-surface defects in magnetizable and conductive specimens by combining the measurements of eddy current, magnetic flux leakage and thermography testing Different signal processing methods for data normalization are proposed to enable data fusion at the pixel level These methods are applied to a test specimen which contains 10 variably-sized defects We quantitatively evaluate the performances of a total of 29 detection methods with respect to false alarm reduction at a fixed level of true positive rate We report that false positive rate could be reduced from 165% down to 028% by the best multi-sensor method compared to the best single-sensor performance on the smallest defect, when 50% found flaw pixels are required for successful detection

Evaluation of the intergranular corrosion in austenitic stainless steel using collinear wave mixing method

Abstract: Failures due to intergranular corrosion in components of austenitic stainless steel have always been a tough problem in engineering practice. In this paper, the collinear wave mixing was investigated to evaluate the intergranular corrosion in austenitic stainless steel. An acoustic nonlinearity parameter related to the bispectrum, the propagation distance and the amplitudes of fundamental waves in measured signal is proposed. Nonlinear acoustic measurements were conducted on four tubes with different degree of intergranular corrosion. The experimental results demonstrated that the proposed acoustic nonlinearity parameter is sensitive to intergranular corrosion in samples, and is well correlated with the degree of damage.