Surface-breaking Defects Research Articles

Focused Rayleigh wave EMAT for characterisation of surface-breaking defects

Developments towards higher resolution and the ability to detect small defects are bringing a step-change in non-destructive testing. This paper presents a new method for increasing resolution, using a focused electromagnetic acoustic transducer (EMAT) optimised to generate Rayleigh waves at 2MHz. This high frequency allows detection of mm-depth defects, and the focusing allows sizing of much shorter defects than is possible when using standard EMATs. The focusing behaviour and the aperture angle effect are analysed using laser vibrometry and finite element modelling, showing that a reduced aperture shifts the focal point from the designed value and increases the focal depth. The dual-EMAT has excellent signal to noise ratio (up to 30dB) and has been used in single shot mode to image a variety of surface-breaking defects, including detecting and positioning a pair of real defects in an aluminium billet sample, and a machined defect of 2mm length, 0.2mm width, and 1.5mm depth, giving an upper limit on the defect length of 2.1±0.5mm. The results can be used to design an EMAT with optimised focal behaviour for defect detection.

Defect tolerance of friction stir welds in DH36 steel

Abstract Friction stir welding of steel is in the early stages of development. The aim to commercialise this process creates a trade-off between welding time, cost and quality of the joint produced. Therefore, it becomes critical to analyse the lower quality bound of steel friction stir welds in conventional square edge butt welding configuration. Work has been undertaken to evaluate the microstructure and fatigue performance of 6 mm thick DH36 steel plates friction stir welded with sub-optimal process conditions, resulting in the development of embedded and surface breaking flaws. The defective weldments were characterised to understand the nature of the flaws and a programme of mechanical testing was undertaken (including fatigue assessment) to determine the relationship between the flaw geometry, location and weld quality. A number of characteristic flaws were identified and seen to interact with the samples' fatigue fracture mechanisms. Samples with wormholes at the weld root produced the lowest fatigue performance. Fracture from incomplete fusion paths at the retreating side of the welds' top surface was seen to correspond to the highest recorded fatigue lives. The work provides an insight into the complex nature of characteristic flaws in steel friction stir welds and their interaction with fatigue behaviour.

Systematic investigation of the fatigue performance of a friction stir welded low alloy steel

A comprehensive fatigue performance assessment of friction stir welded DH36 steel has been undertaken to address the relevant knowledge gap for this process on low alloy steel. A detailed set of experimental procedures specific to friction stir welding has been put forward, and the consequent study extensively examined the weld microstructure and hardness in support of the tensile and fatigue testing. The effect of varying welding parameters was also investigated. Microstructural observations have been correlated to the weldments’ fatigue behaviour. The typical fatigue performance of friction stir welded steel plates has been established, exhibiting fatigue lives well above the weld detail class of the International Institute of Welding even for tests at 90% of yield strength, irrespective of minor instances of surface breaking flaws which have been identified. An understanding of the manner in which these flaws impact on the fatigue performance has been established, concluding that surface breaking irregularities such as these produced by the tool shoulder’s features on the weld top surface can be the dominant factor for crack initiation under fatigue loading.

Laser ultrasonic characterisation of branched surface-breaking defects

Surface-breaking defects often have a geometry which is more complicated than the ‘normal slot’ used in many calibration tests, and this geometry will affect the reflection and transmission of ultrasonic surface waves incident on the defect. We present here measurements on defects with varied branched geometries, designed to simulate stress corrosion cracking (SCC) type defects, characterising the geometry using laser-based ultrasonic generation and detection of Rayleigh waves. We show the behaviour of the near-field enhancement and the far-field reflection as a function of branch position and length, and signal analysis which can be used to gain further information for characterising the defect geometry. The experimental results and finite element method (FEM) models presented in this paper highlight the potential of this technique to test components prone to developing SCC, in order to identify and characterise surface-breaking defects.

Eddy currents testing probe with magneto-resistive sensors and differential measurement

Magneto-resistive (MR) sensors have been applied for eddy currents testing (ECT) usually in the inspection of buried defects (at low frequency operation) but they can also be used for the detection of surface breaking defects using higher frequencies. Although the MR sensors bandwidth is high (up to hundreds of MHz), the operating frequency of eddy current probes using these MR sensors is usually much more limited. The presence of inductive coupling in the sensors interconnections results in an additional voltage contribution to the measured signal whose frequency is equal to the primary magnetic field and whose amplitude therefore increases with frequency. Depending on the probe design, this undesired voltage contribution can surpass the sensor response when the selected operating frequency is moderately high. In this paper, a MR sensors based EC probe designed for the detection and characterization of surface breaking defects is presented. The probe and the measurement setup were designed to evaluate two compatible techniques targeting the reduction of the inductive coupling on the measured signals and thus enabling higher frequency operation. One measurement technique relies on using two sensors in a differential measurement while the other technique employs heterodyning principles to generate a low frequency component to recover the magnetic field detected by the sensors. The results using the employed techniques improved by more than 20dB the signal to noise ratio at which defects can be detected and by around 70 times the relative variation of the measured signal when operating at 100kHz. An application result in friction stir welding samples demonstrates the ability to detect crack defects with depth around 400μm.

A DIFFERENTIAL DPSM BASED MODELING APPLIED TO EDDY CURRENT IMAGING PROBLEMS

This paper deals with an innovative implementation of a semi-analytical modeling method, called the Distributed Points Source Method (DPSM), in the case of an eddy current problem. The DPSM has already shown great potentialities for the versatile and computationally efficient modeling of complex electrostatic, electromagnetic or ultrasonic problems. In this paper, we report a new implementation of the DPSM, called differential DPSM, which shows interesting prospects for the modeling of complex eddy current problems such as met in the non-destructive imaging of metallic parts. In this paper, the used eddy current imaging device is firstly presented. It is composed of an eddy current (EC) inducer and a magneto optical set-up used to translate the magnetic field distribution appearing at the surface of the imaged part, into a recordable optical image. In this study, the device is implemented for the time-harmonics (900 Hz) imaging of a two-layer aluminum based assembly, featuring surface-breaking and buried defects. Then, the basics of the time-harmonics DPSM modeling are recalled, and the differential approach is presented. It is implemented for the modeling of the interactions of the eddy current imaging device with the considered flawed assembly in the same operating conditions as the experimental implementation. The comparison between experimental and computed data obtained for millimetric surface and buried defects is presented in the form of complex magnetic cartographies and Lissajous plots. The obtained results show good agreement and open the way to the modeling of complex EC problems. Furthermore, the low computational complexity of the differential DPSM modelings makes it promising to consider for the solving of EC inverse problems.

Scanning laser source Lamb wave enhancements for defect characterisation

Surface-breaking defects in thin storage structures can cause costly component failure if left undetected. Here, a method for detecting and characterising surface breaking defects using near-field Lamb wave enhancements is presented for measurements in which a laser generation source passes over the defect. A dual laser scanning system is used to generate and detect Lamb waves in aluminium plates containing v-shaped laser micro-machined slots of different depths. Time–frequency analysis techniques are used to identify and track the magnitudes of individual wave-modes across a scan at different frequencies, and large enhancements in signal magnitude at certain frequencies are observed for the fundamental A0 and S0 wave modes when the laser source passes over the defect lip. The mechanisms responsible for this enhancement are identified and examined, and a characterisation process for identifying the position and severity of the defect is presented.

Active Whisking-Based Remotely Deployable NDE Sensor

The fundamental sensitivity characterization of a novel whisking sensor for applications in nondestructive evaluation is presented. The whisking sensors, originally developed for proximity detection applications in autonomous robotics are evaluated for measurements of surface roughness and surface form change. These surface parameters are the representatives of the typical changes associated with corrosion and surface breaking defects in real structures. The authors demonstrate that the whisking sensor can be used to accurately quantify surface roughness in the range 14-53 μm with excellent correlation to a standard reference. Furthermore, it is shown that that the sensor can detect 14 mm diameter flat bottomed holes with depths ranging from 0.4 to 1.0 mm. In contrast to conventional ultrasonic and eddy current techniques, the sensor is insensitive to surface liftoff, producing an error of only 1.2% for liftoffs of several mm. This liftoff insensitivity is a highly desirable characteristic for real-world deployment of the sensors, and the authors describe how the sensor can be incorporated into autonomous inspection robots.

Railroad inspection based on ACFM employing a non-uniform B-spline approach

The stresses sustained by rails have increased in recent years due to the use of higher train speeds and heavier axle loads. For this reason surface and near-surface defects generate by Rolling Contact Fatigue (RCF) have become particularly significant as they can cause unexpected structural failure of the rail, resulting in severe derailments. The accident that took place in Hatfield, UK (2000), is an example of a derailment caused by the structural failure of a rail section due to RCF. Early detection of RCF rail defects is therefore of paramount importance to the rail industry. The performance of existing ultrasonic and magnetic flux leakage techniques in detecting rail surface-breaking defects, such as head checks and gauge corner cracking, is inadequate during high-speed inspection, while eddy current sensors suffer from lift-off effects. The results obtained through rail inspection experiments under simulated conditions using Alternating Current Field Measurement (ACFM) probes, suggest that this technique can be applied for the accurate and reliable detection of surface-breaking defects at high inspection speeds. This paper presents the B-Spline approach used for the accurate filtering the noise of the raw ACFM signal obtained during high speed tests to improve the reliability of the measurements. A non-uniform B-spline approximation is employed to calculate the exact positions and the dimensions of the defects. This method generates a smooth approximation similar to the ACFM dataset points related to the rail surface-breaking defect.

An investigation of effects of welding residual stresses on creep crack growth for a low alloy butt weld

R5 is an assessment procedure for the high temperature response of structures and R5 Volume 7 specifies the creep crack growth assessment for low alloy welds. In power plants, almost all welds operating in creeping regime have been post weld heat treated (PWHT). However, low levels of welding residual stresses may still exist. It is known that the welding residual stresses in weldments can be difficult to estimate, as their magnitude and distribution depend on materials, geometry, weld procedure and heat treatment.For high temperature low alloy steel, CrMoV, butt welds, R5 Volume 7 suggests that a bending welding residual stress of 60 MPa should be assumed for a circumferential defect following a post weld heat treatment. For conservatism, the relaxation of this low level of the welding residual stress in a service component operating at high temperatures for a long period time has been discounted in R5. As the actual welding residual stresses can be difficult to estimate, it is necessary to carry out a detailed investigation for the effects of the welding residual stresses on the creep crack growth and a number of associated crack growth parameters.In this paper, for a typical low alloy steel butt weld in ½Cr½Mo¼V (‘½CMV’) pipework with 2¼Cr1Mo weld metal under a given set of internal pressure, temperature and system moment, a series of creep crack growth analyses have been conducted using the R5 Volume 7 assessment procedure, assuming the welding residual stresses of 10 MPa–100 MPa and three different crack initiation times. A postulated service-initiated internal circumferential defect (semi-elliptical surface-breaking surface defect) of 2.3 mm deep by 15 mm long has been used.The investigation results are presented by a series of graphs which show the effects of the welding residual stresses and different crack initiation times on the creep crack growth for the weld studied. After a detailed discussion, conclusions can be drawn.

Routes for GMR-Sensor Design in Non-Destructive Testing

GMR sensors are widely used in many industrial segments such as information technology, automotive, automation and production, and safety applications. Each area requires an adaption of the sensor arrangement in terms of size adaption and alignment with respect to the field source involved. This paper deals with an analysis of geometric sensor parameters and the arrangement of GMR sensors providing a design roadmap for non-destructive testing (NDT) applications. For this purpose we use an analytical model simulating the magnetic flux leakage (MFL) distribution of surface breaking defects and investigate the flux leakage signal as a function of various sensor parameters. Our calculations show both the influence of sensor length and height and that when detecting the magnetic flux leakage of μm sized defects a gradiometer base line of 250 μm leads to a signal strength loss of less than 10% in comparison with a magnetometer response. To validate the simulation results we finally performed measurements with a GMR magnetometer sensor on a test plate with artificial μm-range cracks. The differences between simulation and measurement are below 6%. We report on the routes for a GMR gradiometer design as a basis for the fabrication of NDT-adapted sensor arrays. The results are also helpful for the use of GMR in other application when it comes to measure positions, lengths, angles or electrical currents.

Experimental validation of an eddy current probe dedicated to the multi-frequency imaging of bore holes

This paper deals with the experimental characterization of an array probe dedicated to the eddy current imaging of sub-millimetric surface breaking defects appearing in bore holes of metallic parts. The probe is constituted of a large inducer generating an uniformly oriented EC flow within the inspected material, and a sensing array probe featuring bobbin coils to sense the radial component of the magnetic field resulting from the eddy currents/defects interactions within the wall of the bore hole. The probe was designed with accurate defect characterization in view, i.e. to provide multi-frequency and high spatial resolution images with a reduced acquisition time, so as to enhance the informative content of the acquired eddy current data. An experimental set-up was build in order to validate the imaging performances of such a probe. A prototype featuring a large inducer and a single sensing coil which can be accurately positioned in the sensing area, has been developed in order to evaluate the sensing performances as well as to study the influence of the sensing array configuration on the imaging performances. The experimental results demonstrate a good sensing ability of the designed probe in the 10–800kHz frequency range, with peak-signal-to-noise ratios higher than 36dB at 10kHz (and 62dB at 800kHz) for defects featuring dimensions as small as 0.4mm×0.2mm×0.2mm. Furthermore, a staggered row arrangement of the sensing array was proposed so as to significantly reduce the error due to the sampling step resulting from the pickup coils geometry (from 35% to less than 9% in the worst case). The experimental evaluation of the probe provides promising prospects for the accurate characterization of defects, by means of advanced multifrequency signal processing algorithms.

Lamb wave near field enhancements for surface breaking defects in plates

Near field surface wave ultrasonic enhancements have previously been used to detect surface breaking defects in thick samples using Rayleigh waves. Here, we present analogous surface wave enhancements for Lamb waves propagating in plates. By tracking frequency intensities in selected regions of time-frequency representations, we observe frequency enhancement in the near field, due to constructive interference of the incident wave mode with those reflected and mode converted at the defect. This is explained using two test models; a square based notch and an opening crack, which are used to predict the contribution to the out-of-plane displacement from the reflected and mode converted waves. This method has the potential to provide a reliable method for the near field identification and characterisation of surface breaking defects in plates.

The vibration dipole: A time reversed acoustics scheme for the experimental localisation of surface breaking cracks

A combination of time reversed acoustics and nonlinear elastic wave spectroscopy techniques is introduced to localize surface breaking defects in a non-destructive manner. Reciprocal time reversal is applied at two neighbouring positions in order to create a vibration dipole with high amplitudes. At surface breaking cracks, nonlinear elastic effects are triggered by the shear forces due to induced friction of the crack interfaces. By mapping the nonlinearity generated by the vibration dipole over the sample surface, the position of a surface breaking crack can be visualized. The technique is tested on an industrial steel sample containing a closed crack.

MLP for Defect Detection from Power Plants Flow Pipelines Equipped by Principal Component Analysis

Abstract This research effort proposes an intelligent control approach for Defect detection of flow pipelines in power plants by applying Multilayer Perceptron (MLP) for classification by which equipped with Principal Component Analysis (PCA). This fusion has been applied to have an intelligent defect detection algorithm of power plants flow pipelines. Among various methods of Non Destructive Testing (NDT), Magnetic Flux Leakage (MFL) technique is the most useful method due to its efficiency and low cost. For this reason models were developed to determine more accurate surface-breaking defects along the applied field when using the magnetic flux leakage technique. The theoretical model fits the experimental MFL results from simulated defects. For MFL sensors, the normal magnetic leakage field is subsequently used for evaluation of defects .Three different defects are analytically performed for this research. These are named Data type1 up to 3. In our previous works, we applied linear discriminate analysis (LDA) and observed that the results were more accurate in some cases but this algorithm is simpler and so fast rather than previous one, also mentioned method in this paper is so useful and could be simply simulate.

Enhancement of ultrasonic surface waves at wedge tips and angled defects

The behaviour of sound waves interacting with wedges has attracted interest from researchers in geophysics and non-destructive testing. We consider here the near-field behaviour of Rayleigh waves incident on wedges and surface-breaking defects which propagate at an angle to the surface, such as rolling contact fatigue on rails. It has been shown that, for a detection point on the edge of the crack tip, a very large signal enhancement is observed for shallow angles. We explain this behaviour through considering the effect of the defect geometry, with changes in the frequency·thickness product leading to mode-conversion of the incident Rayleigh wave.

Thermographic detection of surface breaking defects using a scanning laser source

Surface breaking defects are detected using a scanning laser source to deposit heat into a sample surface. Any lateral flow of heat is disturbed by such a defect, with a change in thermal spot shape being detected by an infrared camera. An apparent increase in temperature as the laser passes over a defect is due to a localised change in emissivity, allowing defects to be distinguished from surface markings. Defects have been detected in both stainless steel and aluminium.

Modelling and experimental measurements of idealised and light-moderate RCF cracks in rails using an ACFM sensor

Alternating current field measurement (ACFM) sensors can be used to detect surface breaking defects in metal components. In rails rolling contact fatigue (RCF) cracks form due to the wheel–rail contact stresses. These cracks are surface breaking and can have complex shapes. A COMSOL model has been developed for a commercial ACFM system and RCF cracks in rails. In this paper model results are compared to experimental measurements using an ACFM pencil probe for calibration defects machined into a rail and real RCF defects (light and moderate categories; <20 mm surface length) in a rail removed from service. X-ray tomography has been used to determine the size and morphology of the real RCF cracks for input into the model. It has been shown that the model can be used to determine the change in normalised Bx signal due to the presence of calibration defects machined into a new rail. The model has also been used to compare the experimental data for the real RCF cracks to the reconstructed model cracks and a semi-elliptical approximation to their shape.

Non-contact ultrasonic detection of angled surface defects

Non-destructive testing is an important technique, and improvements are constantly needed. Surface defects in metals are not necessarily confined to orientations normal to the sample surface; however, much of the previous work investigating the interaction of ultrasonic surface waves with surface-breaking defects has assumed cracks inclined at 90° to the surface. This paper explores the interaction of Rayleigh waves with cracks which have a wide range of angles and depths relative to the surface, using a non-contact laser generation and detection system. Additional insight is acquired using a 3D model generated using finite element method software. A clear variation of the reflection and transmission coefficients with both crack angle and length is found, in both the out-of-plane and in-plane components. The 3D model is further used to understand the contributions of different wavemodes to B-Scans produced when scanning a sample, to enable understanding of the reflection and transmission behaviour, and help identify angled defects. Knowledge of these effects is essential to correctly gauge the severity of surface cracking.

An experimental study of EMAT ultrasonic surface waves modes in railhead

A lab-based laser-EMAT (Electromagnetic acoustic transducers) system has been developed to observe the ultrasonic surface wave propagation and interaction with surface breaking defects on the sample rail head surface. The frequency- wavenumber dispersion curves of the dominant surface wave modes were determined using 2D-FFT method based on the measurements of the out-of-plane displacement of the surface wave. A complete picture of the ultrasonic surface wave on the sample surface over time was reconstructed with exceptionally high spatial and temporal resolution. Despite the curvature of the rail head, the ultrasonic surface guided wave propagating down the rail is found to have similar properties to Rayleigh waves by direct comparison to those observed on flat samples using the same technique. Rail breaks caused by rolling contact fatigue (RCF) cracks are of growing concern to the modern high speed railway industry (1). The fatigue crack initiates on or very close to the rail rolling surface, which is not related to any material defect. Its occurrence is increasing on high speed passenger lines, mixed and heavy haul railways and can lead to expensive rail grinding in the attempt to remove it, premature removal of the rails and complete rail failure. The rolling contact fatigue damage on rails can be divided in headchecks, squats and spalling. They are caused by a combination of high normal and tangential stresses between rail and wheel, which cause sever shearing of the rolling surface layer of rail head and either fatigue or exhaustion of ductility of material. The microscopic cracks produced propagations through the heavily deformed surface layer of railhead at a shallow angle to the rail running face (about 10 ◦ ∼15 ◦ ) until it reaches a depth where the steel retains its original isotropic properties (2,3). At this stage the crack is a few millimeters deep into the rail head, and usually the crack may simply lead to spalling of material from the rail surface. However, for reasons still not very clearly understood, isolated oblique crack can down into the rail, and if not to detect by ultrasonic testing car, it may cause the rail to break. RCF damages are much more severe from the point of view of the structure integrity for high speed railway track.