Showing papers in "Ndt & E International in 2017"

A review of Ground Penetrating Radar application in civil engineering: A 30-year journey from Locating and Testing to Imaging and Diagnosis

Abstract: The GPR (Ground Penetrating Radar) conference in Hong Kong year 2016 marked the 30th anniversary of the initial meeting in Tifton, Georgia, USA on 1986. The conference has been being a bi-annual event and has been hosted by sixteen cities from four continents. Throughout these 30 years, researchers and practitioners witnessed the analog paper printout to digital era that enables very efficient collection, processing and 3D imaging of large amount of data required in GPR imaging in infrastructure. GPR has systematically progressed forward from “Locating and Testing” to “Imaging and Diagnosis” with the Holy Grail of ’Seeing the unseen’ becoming a reality. This paper reviews the latest development of the GPR’s primary infrastructure applications, namely buildings, pavements, bridges, tunnel liners, geotechnical and buried utilities. We review both the ability to assess structure as built character and the ability to indicate the state of deterioration. Finally, we outline the path to a more rigorous development in terms of standardization, accreditation, and procurement policy.

Automated detection of welding defects in pipelines from radiographic images DWDI

Abstract: This paper presents a method for the automatic detection and classification of defects in radiographic images of welded joints obtained by exposure technique of double wall double image (DWDI). The proposed method locates the weld bead on the DWDI radiographic images, segments discontinuities (potential defects) in the detected weld bead and extracts features of these discontinuities. These features are used in a feed-forward multilayer perceptron (MLP) with backpropagation learning algorithm to classify descontinuities in “defect and no-defect”. The classifier reached an accuracy of 88.6% and a F-score of 87.5% for the test data. A comparison of the results with the earlier studies using SWSI and DWSI radiographic images indicates that the proposed method is promising. This work contributes towards the improvement of the automatic detection of welding defects in DWDI radiographic image which results can be used by weld inspectors as a support in the preparation of technical reports.

Nonlinear Lamb wave-mixing technique for micro-crack detection in plates

Abstract: In engineering practice, failures due to fatigue cracks in metallic structures have always been difficult to predict. In our study, nonlinear Lamb wave-mixing was applied in the detection of micro-cracks in plates. The analysis of the nonlinear interaction of these waves with cracks of various lengths and widths was performed using finite-element simulations. The simulation results showed that the sideband at the sum frequency provide a sensitive means for micro-crack detection. Moreover, the sideband amplitudes show a monotonic increase with micro-crack length, but a decrease with micro-crack width. Experiments using Lamb wave-mixing were conducted on plates with fatigue cracks. The experimental results show that a proposed acoustic nonlinearity parameter related to the sideband at the sum frequency is sensitive to micro-cracks in plate, and is well correlated with the damage degree.

Circumferential guided wave EMAT system for pipeline screening using shear horizontal ultrasound

Abstract: The use of guided waves is now widespread in industrial NDT for locating metal loss in pipelines, that manifests as pitting, corrosion and general wall thinning. In this paper, a screening technique is assessed in terms of defect detection and defect sizing capability. Shear Horizontal (SH) guided waves propagate circumferentially around the pipe whilst the scanner is moved axially along the length. This type of tool is preferable to other methods, being applied to the exterior of the pipe, without requiring full circumferential access, and is able to operate through thin coatings (up to 1 mm thick). It is designed to provide a pipe screening tool for petrochemical pipelines both topside and subsea, particularly for detecting defects at pipe support areas. The system's efficacy in terms of detection and sizing of defects is considered via experimental measurements on artificially induced defects and in service corrosion patches, with results compared to finite element modelling of the interaction of the guided waves with artificial defects. Finite element modelling has been used to better understand the behaviour of different wave modes when they interact with defects, focusing on the mode conversions and reflections that occur.

Long pulse excitation thermographic non-destructive evaluation

Abstract: A comprehensive analysis of the defect detection performance of long pulse excitation thermographic NDE is presented. An analytical procedure for predicting the thermal image contrasts of defects of specified size and depth is developed and validated by extensive experimental studies of test pieces having a wide range of thermal properties. Results obtained using long pulse (~5 s) excitation are compared with those obtained using traditional flash excitation. The conditions necessary for the success of the long pulse method are explained and illustrated by both modelling and experimental results. Practical advantages of long pulse excitation are discussed.

Impact damaging of composites through online monitoring and non-destructive evaluation with infrared thermography ☆

Abstract: The aim of this work is to highlight the help offered by infrared thermography in the investigation of impact damaging of composites. In particular, infrared thermography is herein used with a twofold function: monitoring of impact tests and non-destructive evaluation of impacted specimens. Different types of composites are considered which involve changing of either the matrix from a thermoset to a thermoplastic one with also addition of a compatibilizing agent, or the reinforcement from carbon to glass. The obtained results show a different behaviour under impact of the different materials with fibres breakage only in thermoset matrix composites for the same impact energy. The presence of the compatibilizing agent in the thermoplastic matrix prevents the material from large deformation bringing it to behave more similar to a thermoset matrix based material. Post-processing of thermal images allows evaluation of the overall impact-affected zone.

Composite magnetic flux leakage detection method for pipelines using alternating magnetic field excitation

Abstract: Pipelines are an important transportation medium for petroleum and chemical products, but defects in the pipelines can present hidden dangers and affect the safe operation of the pipeline. The traditional pipeline magnetic flux leakage (MFL) scanning technique generally adopts the axial magnetization mode, which has increased the difficulty in detection and the possibility of missed detection of axial cracks. In this paper, a new composite MFL method using alternating magnetic field excitation is proposed for the detection of cracks in pipelines. The alternating magnetic field is first superimposed on the MFL magnetization field, which will form a parallel eddy current field perpendicular to the magnetization direction in the pipeline wall. The defects in the pipeline not only cause the flux leakage of the magnetization field, but also lead to the disturbance of the circumferential eddy current field. The disturbance signals can be picked up through a secondary induced magnetic field. Because the magnetic field and the eddy current field are orthogonal, the presented method can implement synchronous detection in two orthogonal directions to avoid missed detection caused by the crack orientation. A series of physical experiments are carried out in this paper. The results show that two orthogonal detection signals can be separated by a simple low pass filter. Therefore, with only one scan, the new detector can obtain the defect characteristics in the axial and circumferential directions to overcome the blind spot problem seen in traditional MFL detectors.

Optimization of the pulse-compression technique applied to the infrared thermography nondestructive evaluation

Abstract: Pulse-compression infrared thermography is an emerging nondestructive testing and evaluation technique. An analysis of the main issues hampering the full exploitation of this technique is presented from both theoretical and experimental point of view and various strategies are introduced to overcome these problems and optimize the defect detection performance. A comparison between conventional pulse-compression thermography procedures and the proposed one is reported, using an LED modulated with a Barker sequence as coded excitation, and a carbon fibre composite benchmark sample containing artificial defects at different depths. The experimental results show that the suggested signal processing procedure assures a higher SNR and hence an improved defect detection capability. In addition, a time-analysis of such signals allows the correlation between the depth of defects and heat diffusion time to be more clearly identified.

High temperature EMAT design for scanning or fixed point operation on magnetite coated steel

Abstract: Bulk thickness measurements were performed at elevated temperatures on magnetite coated low carbon steel pipe and aluminium samples, using a permanent magnet electromagnetic acoustic transducer (EMAT). The design presented here exploits the non-contact nature of EMATs to allow continuous operation at elevated temperatures without physical coupling, sample preparation (in the form of oxide scale removal), or active cooling of the EMAT. A non-linear change in signal amplitude was recorded as the magnetite coated sample was heated in a furnace, whereas a steady decrease in amplitude was observed in aluminium. For a magnetite coated pipe sample, after a dwell time of 3 h, a SNR of 33.4 dB was measured at 450 °C, whilst a SNR of 33.0 dB was found at 25 °C. No significant EMAT performance loss was observed after one month of continuous exposure to 450 °C. EMAT-sample lift-off performance was investigated at elevated temperature on magnetite coated steel; a single-shot SNR of 31 dB for 3.0 mm lift-off was recorded at 450 °C, highlighting the suitability of this design for scanning or continuous fixed point inspection at high temperature.

Pulsed eddy current thickness measurement using phase features immune to liftoff effect

Abstract: Conventionally, peak value and peak time are extracted from pulsed eddy current (PEC) response as features for thickness measurement. However, they suffer from liftoff variations. In this work, the phase of spectral PEC response from a Hall sensor are proposed to serve as robust features for thickness evaluation. The presented novel features are immune to liftoff effect, because phase signals remain nearly constant against liftoff variations. An analytical model was formulated, and simulations were performed to uncover the physics of the characteristics of the phase feature and establish the relationship between the phase feature and sample thickness. Experiments were carried out to validate the proposed phase feature. Eventually, the proposed phase feature was evaluated for accurate thickness measurement and some key factors were discussed.

Fourier-transform vibrothermography with frequency sweep excitation utilizing local defect resonances

Abstract: The defect activation and detection efficiency of low-energy vibrothermography can be further improved by periodically sweeping the ultrasonic excitation frequency and Fourier-transforming the temperature data at the modulation frequency. This is due to the resulting periodic heat production. While typical local defect resonance investigations make use of known resonance frequencies, this technique instead employs a wideband ultrasonic excitation signal. It can therefore be used to detect defects quickly without knowing the LDR frequency in advance, while still keeping the efficient defect activation effects provided by LDR. Otherwise undetectably low thermal defect signals are enhanced by means of Fourier filtering and the defects’ resonance frequencies are characterized by their phase values.

Nondestructive characterization of fiber orientation in short fiber reinforced polymer composites with X-ray vector radiography

Abstract: Short glass and carbon fiber reinforced polymer composites are used in many industrial fields such as in automotive and consumers industry. Their physical and mechanical properties are often superior to those of unfilled polymer components. One aspect being of utmost importance for these properties is the fiber orientation distribution. Here, we present X-ray vector radiography as a method to characterize fiber orientation in short fiber reinforced polymer components. The method is based on X-ray grating interferometry and takes advantage of X-ray scattering caused by the sample's microstructure. Therefore, micro-structural properties can be probed nondestructively without the need for high spatial resolution. Compared to standard X-ray imaging techniques, currently applied for fiber orientation studies, the presented method does not restrict the size of the sample under investigation and allows for much shorter measurement times. In contrast to existing methods, X-ray vector radiography allows the characterization of carbon fiber reinforced polymers despite the weak attenuation contrast between the fibers and the polymer matrix. As this method is also extendable into three dimensions it is a very attractive tool for complex component geometries and carries potential to be applied to materials other than short fiber reinforced polymers.

Passive RFID sensor systems for crack detection & characterization

Abstract: A significant requirement of low-cost sensor systems for defect detection and characterization is to bridge the gap of nondestructive testing & evaluation (NDT&E) and structural health monitoring (SHM). This paper introduces an ultra-high frequency (UHF) passive RFID sensor system for crack detection and potential structural monitoring. The working principles and major research challenges are presented for the new concept of NDT&E sensor systems. One novel approach using 3D antenna and kernel principal component analysis (PCA) is investigated and validated in conjunction with two case studies for evaluation of open and closed cracks. This proposed method can partly alleviate the nonlinearity from wireless channel and demonstrate the feasibility of the in-situ motoring through amplitude and phase of tag’s backscattered signals.

Interdigital lamb wave transducers for applications in structural health monitoring

Abstract: Interdigital transducers (IDTs), thanks to their multiple capabilities have the potential of increasing versatility of SHM systems. Migration of the IDT technology in SHM systems and devices is reviewed in this paper. A summary review of different types of IDTs is presented and their salient features are presented in terms of their applicability in the Lamb wave based SHM systems. Comprehensive review is provided concerning the implementation of the IDT capabilities towards the development of SHM systems. Experimental results obtained with prototype IDTs are provided for illustration. Finally, future development directions of the IDTs dedicated to SHM systems are outlined.

X-ray computed tomography for pore-related characterization and simulation of cement mortar matrix

Abstract: Application of X-ray computed tomography (CT) for pore-related characterization and simulation of cement mortar was explored in this paper. Based on visualization and quantitation of the three-dimensional (3D) morphological features (i.e., appearance, size, volume, shape, and distribution) of pores, the porosity, tortuosity-connectivity, permeability, and molecular diffusion of cement mortar were computed and simulated. The simulation of the permeability and molecular diffusion of cement mortar was carried out on the basis of Darcy's law and Fick's second law, respectively. The dynamical velocity field and concentration field were respectively visualized. It was found that the X-ray CT technique provided a promising non-destructive alternative for both qualitative and quantitative analysis of the pore-related characterizations of cementitious materials. Furthermore, it was possible to use the simulations to replace lab-intensive experimental tests to quickly obtain the permeability and diffusivity parameters of cement mortar for application in engineering design.

Feature guided wave inspection of bond line defects between a stiffener and a composite plate

Abstract: Adhesive bonding is widely used in aerospace composite structures. A continuous well-cured bond can offer good joint strength and improved fatigue and impact resistance, and is therefore crucial to the performance of the entire structure. This paper explores the feasibility of using feature guided waves (FGW) for rapid screening of the bond line between a stiffener and a carbon fiber reinforced polymer (CFRP) composite panel. Such FGWs are capable of focusing the wave energy along the stiffener and the bond layer, with limited radiation to the adjacent plate. The Semi-Analytical Finite Element (SAFE) approach is employed to understand the modal properties of FGWs that exist in the structure, and criteria are suggested to choose proper mode-frequency combination that is sensitive to adhesive defects. A shear horizontal type FGW mode is identified to be well suited, as it is easy to excite, and propagates with little dispersion and relatively low attenuation, while it retains sufficient energy around the bond layer. Both 3D Finite Element (FE) simulations and experiments are performed to study the interaction of the selected FGW mode with defects in the adhesive bond, and the results show excellent agreement. The reflection behavior and the wave-defect resonance phenomenon are investigated, which demonstrate the capability of the FGW for the bond line inspection.

EMAT pipe inspection technique using higher mode torsional guided wave T(0,2)

Abstract: We developed a pitch-catch system based on electromagnetic acoustic transducers (EMATs) for pipe inspection, which moves inside the pipe in the axial direction. The first higher mode, T(0,2), of the torsional guided wave is transmitted and detected, and variations of amplitude and phase are measured while moving the EMATs. Several aluminum pipes containing dish-shaped defects are inspected, and the amplitude and phase show enough detection sensitivity. It is found that the phase measurement has better potential as a tool for quantitative inspection. The applicability of the technique for steel pipe is also confirmed.

Multi-coil focused EMAT for characterisation of surface-breaking defects of arbitrary orientation

Abstract: Electromagnetic Acoustic Transducers (EMATs) are a useful ultrasonic tool for non-destructive evaluation in harsh environments due to their non-contact capabilities, and their ability to operate through certain coatings. This work presents a new Rayleigh wave EMAT transducer design, employing geometric focusing to improve the signal strength and detection precision of surface breaking defects. The design is robust and versatile, and can be used at frequencies centered around 1 MHz. Two coils are used in transmission mode, which allows the usage of frequency-based measurement of the defect depth. Using a 2 MHz driving signal, a focused beam spot with a width of 1.3±0.25 mm and a focal depth of 3.7±0.25 mm is measured, allowing for defect length measurements with an accuracy of±0.4 mm and detection of defects as small as 0.5 mm depth and 1 mm length. A set of four coils held under one magnet is used to find defects at orientations offset from normal to the ultrasound beam propagation direction. This EMAT has a range which allows detection of defects which propagate at angles from 16° to 170° relative to the propagation direction over the range of 0–180°, and the setup has the potential to be able to detect defects propagating at all angles relative to the wave propagation direction if two coils are alternately employed as generation coils.

Ultrasonic detection and sizing of compressed cracks in glass- and carbon-fibre reinforced plastic composites

Abstract: Nondestructive evaluation of compressed cracks is a major challenge. A quantitative study of the effect of crack-tip closure on the pulse-echo ultrasonic sizing of delaminations in fibre-reinforced polymer-matrix composites (FRP) is presented. In particular, this study focuses on the interaction of ultrasound with a closed crack or kissing disbond, and their effect on the ultrasonic inspectability of FRP laminates consisting of carbon and glass plies. The compression of laminar cracks in these two different laminate types is clearly detectable via both pulse-echo and through-transmission ultrasonic measurements, but the reflected ultrasonic pulses in the two material types exhibit markedly different behaviour. The glass-fibre laminates show a drop in the reflected signal for crack openings up to approximately half the crack growth load, whereas the corresponding carbon-fibre laminates show the expected increase in the reflected signal as the crack opens. The origins of the observed effect of crack closure on the reflection and transmission of ultrasound are analysed in detail to ascertain possible mechanisms responsible for these effects.

Numerical modeling of embedded solid waveguides using SAFE-PML approach using a commercially available finite element package

Abstract: Guided waves are attractive for long range inspections from a single generation position. However when the waveguide is embedded in another medium, the energy of the guided waves may leak into the surrounding material, causing significant reduction of the inspection distance. A number of analytical or numerical models were developed to understand the behavior of guided waves in embedded waveguides, among which one of the attractive methods was to combine the Semi-Analytical Finite Element (SAFE) method with Perfectly Matched Layer (PML). This paper presents a development to implement the SAFE-PML model in a commercially available Finite Element package. As no source code is required, the presented method will be attractive to a wide range of researchers in Non-Destructive Evaluation (NDE). The model is first demonstrated and validated in two cases with analytical solutions. Discussions have been carried out regarding the procedure to select proper modeling parameters. The potential of the model is also illustrated on an important application of guided waves along embedded pipelines.

Evaluation of sensitization in stainless steel 304 and 304L using nonlinear Rayleigh waves

Abstract: Austenitic stainless steels have a wide range of applications in the energy industry, but the corrosion resistance of these stainless steels can be reduced by sensitization, particularly in the heat affected zones in welds. Sensitization is the formation of chromium carbide precipitates along the grain boundaries, causing the formation of a zone of chromium depletion around the grain boundary. Since chromium is the primary alloying element that makes stainless steel corrosion resistant, this chromium depleted zone is susceptible to intergranular stress corrosion cracking (IGSCC). Sensitization occurs when a stainless steel is exposed to a high temperature for an extended time period, such as during welding. The objective of this research is to determine the sensitivity of nonlinear ultrasound to the presence of sensitization by using nonlinear Rayleigh waves to quantitatively track the sensitization of 304 and 304L stainless steels as a function of holding time at 675 °C. The effect of the carbon content of the alloys (304 versus 304L) to the sensitization process and the measured nonlinearity parameter, β are investigated. Annealing of these specimens isolates the effect of just sensitization, removing the presence of cold work which can also affect the material nonlinearity. Complementary electrochemical potentiodynamic reactivation (EPR) measurements and microscopy are used to confirm the absence or presence of sensitization. The results show that the acoustic nonlinearity parameter is sensitive to the presence of chromium carbide precipitates in sensitized austenitic stainless steels.

Detectability of corrosion damage with circumferential guided waves in reflection and transmission

Abstract: There is an increasing interest in high frequency short range guided waves to screen or monitor for corrosion. This contrasts with long range guided waves (LRGWs) which screen pipes for large patches of corrosion and have been successfully used in corrosion management for the past twenty years. The fundamental setup described in this paper uses circumferential guided waves, which are excited at a single location on a pipe and travel around the pipe wall and are detected at the same location. The study uses a finite element model assisted method to evaluate the detection capability of two short range circumferential guided wave setups which use both the reflected and transmitted signals. The setups themselves consist of either an axial array of transducers, for monitoring, or a single transducer which axially scans a pipe. Both setups have an array or scan pitch between either adjacent transducers or measurements. The detection capability of the fundamental Lamb wave modes (A0 and S0) in both reflection and transmission have been compared, as well as a hybrid shear horizontal wave setup, which uses the SH0 mode in reflection and the SH1 mode in transmission. A sensitivity analysis was conducted using two separate methods to determine the probability of detection (POD) for either the reflection or transmission signals. Both methods determine a POD for a specific defect, noise level, and array or scan pitch. Probability images are produced which map the POD for a range of defect sizes. For the parameters investigated in this study, it was found that in transmission large diameter defects have a higher detectability, whereas deep, narrow diameter defects are more detectable in reflection. A generalised overview of the sensitivity of short range guided waves is presented by combining both the reflection and transmission PODs. The data fused sensitivity of the S0 and SH hybrid modes are given as 0.6% and 0.75% cross sectional area (CSA) respectively, allowing for the comparison with LRGWs. The A0 mode was excluded from the POD analysis because it was much less sensitive than the other two modes.

Evaluation of the lift-off robustness of eddy current imaging techniques

Abstract: The development of lift-off invariant strategies is one of the main goals in Eddy Current Non-Destructive Testing research. In the present work, from the analysis of amplitude and phase signals of magnetic field sensors under Multi-Frequency Eddy Current excitation, two imaging procedures are analyzed and compared with respect to their ability to retrieve reliable results even in presence of huge changes of lift-off. A figure of merit based on the Signal to Noise Ratio evaluated on the 2D reconstructed images allows the comparison of the different strategies in terms of the quality of the image to show the defect. The numerical and the experimental tests realized show that the imaging procedure relying on the analysis of the phase-lag is quite insensitive to changes in the lift-off with respect to that based on the amplitude analysis. In detail the former guarantees good results even when the probe lift-off is randomly changed during the test with variations up to 3 mm, while the latter is able to tolerate only lift-off fluctuation lower than 1 mm.

Pulse-modulation eddy current probes for imaging of external corrosion in nonmagnetic pipes

Abstract: Since the nonmagnetic pipe is normally utilized in corrosive and hostile environment, it is prone to the external corrosion which occurs on the outer surface of the pipe and severely undermines the structural integrity and safety. Although Pulsed Eddy Current technique (PEC) is currently preferred for detection and evaluation of subsurface defects in tubular conductors, it is subject to technical drawbacks. In light of this, Pulse-modulation Eddy Current technique (PMEC) is intensively investigated in the paper for enhancement of the evaluation sensitivity to external corrosion and accuracy of corrosion imaging. Closed-form expressions of the PMEC response and its sensitivity to external corrosion in tubular conductors are formulated via the Extended Truncated Region Eigenfunction Expansion (ETREE) modeling. Following simulations for analysis and comparison of field signals and evaluation sensitivities of PMEC and PEC, experiments of PMEC for evaluation and imaging of external corrosion are carried out. Through theoretical and experimental investigation, it has been found that regarding the evaluation and imaging of external corrosion in nonmagnetic pipes, the PMEC-based probe have higher sensitivity and imaging accuracy than that based on PEC. The superiority of PMEC to PEC in inspection of tubular conductors is further identified.

Data for:Online nondestructive testing for fine steel wire rope in electromagnetic interference environment

Abstract: The diameter of fine steel wire rope (FSWR) is generally a few millimeters. Its magnetic flux leakage (MFL) signal is weak, and the number of magnetic sensors installed for defect detection is limited because of the small diameter. In FSWR production workshops, different kinds of machinery work together, deteriorating the power quality and making the spatial electromagnetic environment complex; the weak MFL is thus interfered with further. It is difficult to carry out online nondestructive testing (NDT) of FSWR in the process of manufacturing. In this paper we present a novel MFL method for FSWR NDT in a strong electromagnetic interference environment. We use a three-dimensional finite element method (FEM) to analyze the MFL signals. A simplified magnetic circuit is presented to excite the FSWR; the circuit comprises two half-sized radial magnetizing ring NdFeB magnets, and because there is no need for a magnetic yoke, the device is simple and light. A single Hall sensor is used to measure the flux leakage field. A stable performance power system is designed for the NDT power supply, which is not only resistant to voltage sags, but also has very low output noise. To enhance the signal-to-noise ratio (SNR) of the MFL defects signal, a signal conditioning and processing circuit are designed to enhance the detectability of signals in MFL data. The novel and small FSWR NDT system realizes on-line testing in an environment of strong electromagnetic interference, and for the experiment with a 1.5-mm-diameter wire rope twisted by 19 wires, the minimum damage of a pit on half of a wire can be identified.

A hybrid nondestructive testing method of pulsed eddy current testing and electromagnetic acoustic transducer techniques for simultaneous surface and volumetric defects inspection

Abstract: In this paper, a hybrid nondestructive testing (NDT) method combining with the pulsed eddy current testing (PECT) method and the electromagnetic acoustic transducer (EMAT) method has been proposed and validated through numerical simulations and experiments. First, a numerical code is developed for the simulation of hybrid EMAT/PECT signals based on the developed EMAT and PECT code. Second, based on the numerical simulation, the influences of the eddy current induced by the excitation coil and the eddy current due to the velocity effect of the ultrasonic wave are compared and analyzed. In addition, the features of the EMAT and the PECT signals are analyzed respectively. Third, several signal separation and extraction methods are proposed on the basis of the spectral analysis and filtering strategies for extracting EMAT and PECT signals from the mixed signals of the hybrid EMAT/PECT method and their validity are evaluated through experiments. Finally, hybrid EMAT/PECT experiments are conducted and three types of defects (surface defects, bottom thinning defects, composite defects) in an aluminum plate and a SUS304 plate are successfully detected at the same time using the proposed hybrid NDT method and the signal feature parameters. Based on the numerical and experimental results, the proposed hybrid PECT/EMAT NDT method is demonstrated both high detectability and high efficiency for detecting surface and volumetric defects at the same time.

Noncontact Laser Ultrasonic Inspection of Ceramic Matrix Composites (CMCs)

Abstract: Ceramic matrix composites (CMCs) are poised to revolutionize jet engine technology by enabling operation temperatures well beyond those possible with current superalloys, while reducing active cooling requirements and engine weight. Manufacturing of parts formed by silicon-carbide (SiC) fibers in a SiC matrix is now well advanced, with the first non-structural static components entering service in 2017 with the CFM Leap® engine that uses SiC/SiC turbine shrouds. In order to expand the scope of application of CMCs to rotating parts, such as turbine blades, much work is being conducted to understand and characterize the modes of failure of these materials at temperatures beyond ∼ 1100 ° C . In this context, the ability of nondestructively monitoring the formation and progression of damage in CMCs specimens during high-temperature mechanical testing is critical. However, the elevated temperature precludes the possibility of using sensors placed in direct contact with the specimen and therefore severely restricts the range of available NDE techniques. This paper provides the first experimental assessment of the feasibility of noncontact laser ultrasonic inspection of SiC/SiC flat coupons. An Nd: Yag laser is used to excite ultrasonic waves on one side of the specimen while a Michelson interferometer detects the signals emerging on the other side at the epicenter position. The lasers are mounted on synchronized linear stages to form C-scans as in conventional immersion ultrasonics while ablation damage to the surface of the specimen is prevented by operating the lasers at low power density. Despite the complex microstructure of the SiC/SiC material it is found that the measured waveforms are remarkably similar to those observed when conducting the same tests in aluminum specimens. Moreover, it is shown that it is possible to image interlaminar defects caused by impacts, and monitor crack opening under tensile load. Finally, very good signal stability is observed when temperature is increased from 25 to 1250 °C which confirms the feasibility of laser monitoring at high temperature and is consistent with the good thermal stability of ceramic materials.

Robust quantitative depth estimation on CFRP samples using active thermography inspection and numerical simulation updating

Abstract: A quantitative evaluation of delaminations in composite laminates encounters multiple difficulties due to the anisotropic behaviour of the laminate. Extensive calibrations for each structure are required and each depth needs certain manual modifications for optimal performance of the estimation routines. In this manuscript, a robust technique is developed using a numerical model to estimate the thermal diffusivity through the anisotropic material which improves defect depth estimation between each layer. Three different calibration depths are necessary to compute the diffusivity through a certain stacking sequence of a multi-layered composite laminate. The results are compared with the state-of-the-arts experimental evaluation techniques and with a regular numerical model. It is seen that especially for deeper defects, the optimised numerical model delivers more accurate results due to the considered anisotropic diffusivity.

Improve the signal to noise ratio and installation convenience of the inductive coil for wire rope nondestructive testing

Abstract: Inductive coil is used as the nondestructive testing sensor of a wire rope or a pipe, because of its low cost and high durability. However, the winding structure is complex and difficult to design during the field test. Hundreds or even thousands of turns are needed to improve the signal to noise ratio (SNR) and the data processing is cumbersome. In this paper, based on the theoretical analysis and 3D transient magnetic field simulation, a kind of iron core is presented as coil winding skeleton for the wire rope nondestructive testing. Additional iron core plays a role of magnetism concentration, where the magnetic flux leakage (MFL) path is changed and the MFL of the defect is converged to the core. Therefore, the SNR of the coil which is wound on the iron core is improved, and the coil winding skeleton is simplified with the iron core structure optimization. Meanwhile, the influence of the coil cross-section area on the test result analysis is eliminated, and the influence of the lift-off distance between coil and wire rope on the detection result is also reduced. Finally, it is proved by experiment that the SNR of coil with the iron core proposed in this paper is increased almost six times, which makes it easier for defect analysis.

Ultrasonic imaging of nonlinear scatterers buried in a medium

Abstract: In this paper, an ultrasonic technique for imaging nonlinear scatterers, such as cracks, buried in a medium is proposed. The method called amplitude modulation consists of a sequence of three acquisitions for each line of the image. The first acquisition is obtained by transmitting with all elements of the phased array. Next the second and third acquisitions are obtained by transmitting with odd elements only and even elements only, respectively. An image revealing nonlinear scattering from the medium is reconstructed line by line by subtracting the responses measured with second and third acquisitions (odd elements and even elements) from the response obtained with all elements transmitting. The method was implemented on three different conventional multi-channel electronic platforms equipped with different ultrasonic probes (center frequency 3–5 MHz, 64 or 128 elements). A crack (6 mm-deep x 24 mm-long) in a stainless steel block was investigated. With all probes and multi-channel electronic platforms, higher detection specificity of the crack was obtained with amplitude modulation compared with conventional ultrasound imaging. Image contrast (ratio between crack response amplitude over background amplitude) was increased by 5 dB with amplitude modulation compared with conventional ultrasound imaging.