Near-surface Defects Research Articles

Identifying Native Point Defects in the Topological Insulator Bi2Te3.

We successfully identified native point defects that occur in Bi2Te3 crystals by combining high-resolution bias-dependent scanning tunneling microscopy and density functional theory based calculations. As-grown Bi2Te3 crystals contain vacancies, antisites, and interstitial defects that may result in bulk conductivity and therefore may change the insulating bulk character. Here, we demonstrate the interplay between the growth conditions and the density of different types of native near-surface defects. In particular, scanning tunneling spectroscopy reveals the dependence on not only the local atomic environment but also on the growth kinetics and the resulting sample doping from n-type toward intrinsic crystals with the Fermi level positioned inside the energy gap. Our results establish a bias-dependent STM signature of the Bi2Te3 native defects and shed light on the link between the native defects and the electronic properties of Bi2Te3, which is relevant for the synthesis of topological insulator materials and the related functional properties.

Non-destructive evaluation of pipes by microwave techniques and artificial neural networks

Near-field imaging based on an electromagnetic sensor has been widely used for nondestructive detection. An approach to detect the near-surface defects in pipeline coatings and dielectric pipelines is proposed. Based on the characteristics of resonant frequency shifts, a novel method using artificial neural network (ANN) is established to quantitatively evaluate circular-section shape defects in pipes, such as air bubbles in pipeline coating layers or qualitative characterize non-circular section-shape defects. The proposed method has three important modules: a new resonator for data acquisition, a signal-processing algorithm for data preprocessing, and an ANN for quantitative imaging. In the designed sensor, we extend the tip of the sensing ring and introduce an appending in the ring gap for high sensitivity. Simulations show that the sensor can detect a defect with a radius as small as 0.7 mm. The raw resonant frequency shifts obtained by the sensor scanning at an angle interval around the specimen first are preprocessed by curve fitting, sampling, and adaptive data interpolation or truncation. Then, using an ANN, the relationships among resonant frequency shifts, external radius of the specimen, and defect size are modeled for imaging of circular-section shape defects. Preliminary simulations and measurements illustrate the efficacy of the method. Consequently, a contactless, high-resolution, near-field imaging measurement based on sensor scanning for inspecting pipe structures is obtained.

EBSD pattern simulations for an interaction volume containing lattice defects

A quantitative understanding of the effect of the spatial distribution and density of lattice defects on the electron backscatter diffraction patterns requires careful consideration of the electron-matter interaction volume and the traction free boundary condition on the deformation field for near-surface defects. In this work, we couple a depth-specific dynamical electron scattering simulation with an approximate crystal deformation model to generate a single diffraction pattern from an interaction volume containing lattice defects. Two case studies are considered, namely a single edge dislocation and a low angle grain boundary. Their displacement fields, derived from the three-dimensional Yoffe-Shaibani-Hazzeldine’s dislocation field model, are fed into the simulation and the resulting diffraction patterns are cross-validated using the HR-EBSD technique. In addition, diffraction contrast associated with defect deformation field is investigated with the virtual beam technique. Pattern diffuseness is quantitatively analyzed in the frequency domain as a function of dislocation density.

Detection of wood defects using low acoustic impedance-based PZT transducers

Early detection of common defects in wooden structures is an important factor to avoid costly maintenance and improve the life expectancy of structures. A novel qualitative technique has been developed and evaluated for detecting near-surface abnormalities and defects in wooden structures. The proposed setup consisted of a piezoelectric ceramic lead zirconate titanate transducer and an impedance evaluation board. This setup is cost-effective and can be easily employed over various wooden structures. The proposed setup was evaluated on a polymeric material with a debonding. The main experimental validations were conducted on wood specimens with unfilled carved cavities and also putty-filled holes. In addition, the setup was tested for the poor gluing in a wooden specimen. The measured impedance for each location was recorded and normalized to the reference baseline. The proposed setup could detect the cavities and putty-filled regions successfully. It also could detect the approximate geometry of defects. The glued and unglued regions were detected as well.

A Model-Based Study of Transmit-Receive Longitudinal Arrays for Inspection of Subsurface Defects

Abstract Dual matrix transmit-receive longitudinal (TRL) arrays have been shown to provide an improved signal to noise ratio in the near field zone which makes them the most suitable array configuration for the inspection of near-surface defects. This study aims to compare the performance of different configurations for transmit-receive longitudinal matrix arrays. For this purpose, four matrix configurations of 2 × 32, 4 × 16, 4 × 32, and 8 × 16 elements are investigated using EXTENDE CIVA modeling package. The array operating frequencies investigated are either 5 MHz or 10 MHz. The effect of different natural focal depths, arrays separation distances, dynamic electronic depth focusing, and electronic beam skewing for these TRL arrays are considered in models prepared in CIVA. The inspection of a series of flat bottom holes extended up to a few millimeters under the surface using the selected TRL configurations is also investigated in the study. It is found that the performance of focusing for near-surface areas is more efficient using the 4 × 16 and 8 × 16 elements configurations as compared with the others, and the signal amplitudes of the defects located deeper in the target material are almost independent of the configuration.

Detection of near-surface defects in rails combining Green's function retrieval of ultrasonic diffuse fields and sign coherence factor imaging

A method combining Green's function retrieval theory and sign coherence factor (SCF) imaging is presented to detect near-surface defects in rails. The defects are close to the ultrasonic phased array and near-surface acoustic information of defects is obscured by the non-linear effects of the initial wave signal in directly acquired responses. To overcome this problem, cross-correlations of the diffuse field signals captured by the array transducer are performed to reconstruct the Green's function. SCF imaging is used to further improve the spatial resolution and signal-to-noise ratio (SNR) of near-surface defects in rails. Experiments are conducted on two rails containing two and four defects, respectively. The results show that these defects can be clearly identified when using the reconstructed Green's function. However, the images of near-surface defects are masked and cannot be distinguished when using directly captured signals and total focus imaging. The proposed method reduces the background noise and allows for effective imaging of near-surface defects in rails.

누설 자속 탐상을 위한 고성능 소형 자기 영상 장치 개발

Magnetic flux leakage inspection is one of the most effective methods of nondestructive testing because it allows both surface crack and near-surface defects on magnetized ferromagnetic materials to be detected. Therefore, there is an increasing need for a high-performance device that measures the magnetic field distribution to achieve successful inspection results. It consists of magnetic sensors, a magnetizer, and supporting hardware including a preamplifier, a data processing module, and a communication module. This paper proposes a new high-performance compact magnetovision device for magnetic flux leakage inspection. It contains 32 linearly-arranged Hall sensors, each with a spatial resolution of 1 mm, and a small-sized magnetizer with a permanent magnet. In addition, a preamplifier, a fast data processing module with several function blocks, and a communication module are implemented as a form of SoC (System on a Chip) to minimize the size of the device. With these features, this device can be conveniently adopted into various applications. The performance of the proposed device was validated through experiments.

Practical evaluation of velocity effects on the magnetic flux leakage technique for storage tank inspection

Magnetic flux leakage (MFL) is a technique commonly used to inspect storage tank floors. This paper describes a practical evaluation of the effect of scanning velocity on defect detection in mild steel plates with thicknesses of 6 mm, 12 mm and 16 mm using a fixed permanent magnetic yoke. Each plate includes four semi-spherical defects ranging from 20% to 80% through-wall thickness. It was found that scanning velocity has a direct effect on defect characterisation due to the distorted magnetic field resulting from induced eddy currents that affect the MFL signal amplitude. This occurs when the inspection velocity is increased and a reduction in the MFL signal amplitudes is observed for far-surface defects. The opposite applies for the top surface, where an increase is seen for near-surface MFL amplitudes when there is insufficient flux saturating the inspection material due to the concentration of induced flux near the top surface. These findings suggest that procedures should be altered to minimise these effects based on inspection requirements. For thicker plates and when far-surface defects are of interest, inspection speeds should be reduced. If only near-surface defects are being considered then increased speeds can be used, provided that the sensor range is sufficient to cope with the increased signal amplitudes so that signal clipping does not become an issue.

Microstructure and cyclic deformation behavior of a 3D-printed Ti–6Al–4V alloy

The main purpose of this investigation was to identify microstructure and cyclic deformation behavior of a 3D-printed Ti–6Al–4V alloy via selective electron beam melting (SEBM). Abundant orientated α-lamellae were present in the prior columnar β grains along the building direction. Four distinct high-angle α/α grain boundaries were detected as stipulated by the Burgers orientation relationship in the β→α phase transformation at a relatively slow cooling rate during SEBM, including the unique low-energy prismatic/basal (PB) grain boundaries with a misorientation angle of 90° in the 3D-printed Ti–6Al–4V alloy. The alloy exhibited superior strength and adequate ductility, along with a certain extent of strain-rate sensitivity. Cyclic stabilization largely sustained, while cyclic softening occurred at higher strain amplitudes. Although fatigue life could be described by Smith, Watson and Topper (SWT) model, it would be better predicted via a total strain energy-based model. Crack initiation occurred from near-surface defects of entrapped gas pores or residual unmelted zones, and crack propagation was characterized by typical fatigue striations. The results obtained would help promote the safe and reliable applications of 3D-printed titanium components.

Lifetime assessment of the process-dependent material properties of additive manufactured AlSi10Mg under low-cycle fatigue loading

Systematic low-cycle fatigue (LCF) experiments are carried out on additive manufactured AlSi10Mg specimens for several material conditions with varying layer thickness, heat treatment, building direction and surface quality. The deformation behaviour depends significantly on the heat treatment. It is outlined that the process control and heat treatment can produce fatigue properties comparable with the cast material, whereby an as-built specimen surface leads to a lifetime reduction in all cases. The experiments are accompanied with detailed metallo- and fractographic investigations. For all tested LCF specimens, the defect type and the failure origin defect size are characterized in terms of the √area parameter by using scanning electron microscopy. The failure of the specimen is mostly caused by lack of fusion surface or near-surface defects, whereby the defect size is determined by the SLM process parameters, such as building direction, surface quality and layer thickness. On the basis of the experimental data and the observed defects, a mechanism-based, deterministic lifetime model is developed and adapted to the specific damage mechanisms of the additive manufactured AlSi10Mg alloy.

Contour design to improve topographical and microstructural characteristics of Alloy 718 manufactured by electron beam-powder bed fusion technique

Additive manufacturing (AM) processes are being frequently used in industry as they allow the manufacture of complex parts with reduced lead times. Electron beam-powder bed fusion (EB-PBF) as an AM technology is known for its near-net-shape production capacity with low residual stress. However, the surface quality and geometrical accuracy of the manufactured parts are major obstacles for the wider industrial adoption of this technology, especially when enhanced mechanical performance is taken into consideration. Identifying the origins of surface features such as satellite particles and sharp valleys on the parts manufactured by EB-PBF is important for a better understanding of the process and its capability. Moreover, understanding the influence of the contour melting strategy, by altering process parameters, on the surface roughness of the parts and the number of near-surface defects is highly critical. In this study, processing parameters of the EB-PBF technique such as scanning speed, beam current, focus offset, and number of contours (one or two) with the linear melting strategy were investigated. A sample manufactured using Arcam-recommended process parameters (three contours with the spot melting strategy) was used as a reference. For the samples with one contour, the scanning speed had the greatest effect on the arithmetical mean height (Sa), and for the samples with two contours, the beam current and focus offset had the greatest effect. For the samples with two contours, a lower focus offset and lower scan speed (at a higher beam current) resulted in a lower Sa; however, increasing the scan speed for the samples with one contour decreased Sa. In general, the samples with two contours provided a lower Sa (∼22 %) but with slightly higher porosity (∼8 %) compared to the samples with one contour. Fewer defects were detected with a lower scanning speed and higher beam current. The number of defects and the Sa value for the samples with two contours manufactured using the linear melting strategy were ∼85 % and 16 %, respectively, lower than those of the reference samples manufactured using the spot melting strategy.

Influence of surface conditions and specimen orientation on high cycle fatigue properties of Inconel 718 prepared by laser powder bed fusion

High-cycle fatigue testing of nickel-based superalloy Inconel 718 made by laser powder-bed fusion was performed on round uniform-gauge or hourglass specimens for various specimen orientations, stress ratios and surface conditions (as-built and machined). Only surface conditions, specifically near-surface process defects, influenced fatigue properties. There was a systematic bias of the azimuthal position of crack initiation sites. Crack initiation in specimens with as-built surfaces occurred at 50 µm diameter near-surface features. These dimensions were used in conjunction with fatigue limits derived from tests suspended at 107 cycles to calculate threshold ΔK values of 2–6 MPa √m due to as-built surface conditions.

Effect of miniaturization and surface roughness on the mechanical properties of the electron beam melted superalloy Inconel®718

In this work, the Ni-based super alloy Inconel 718 manufactured via electron beam melting is investigated. Typical microstructure of Inconel 718, which was processed by electron beam melting, consists of columnar oriented dendritic structure with strong texture along building direction in hatch region, whereas microstructure differs in contour region, which is supposed to influence mechanical properties in the as-built state. As highly complex geometries are possible to manufacture with additive manufacturing techniques, the influence of miniaturization and surface roughness on microstructural and mechanical properties has to be understood in detail. Therefore, samples were processed with different initial sizes and subsequently tested in as-built and polished condition. Before performing mechanical tests, process-induced microstructure was determined by scanning electron microscope as well as distribution of defects and geometrical deviations by microfocused computed tomography. To characterize the mechanical properties, different testing methods, both tensile and fatigue tests, were carried out. Present investigations show almost similar microstructures in large-scale and small-scale Inconel 718 volumes. However, small-scale volumes show higher number of defects in the form of surface and near-surface defects. Furthermore, as-built specimens show geometrical deviations when compared to initial CAD diameter, which makes the implementation of an average equivalent diameter mandatory. It can be demonstrated that small-scale as-built volumes have reduced mechanical properties, whereby ultimate tensile strength is reduced by 60% and fatigue strength is reduced by 75%, showing that increased defect density and as-built surface roughness have a higher impact on fatigue properties and are the dominating reason for early failure in the as-built state due to multiple crack initiation.

Near-Surface Mass Defect in Models of Locally Heterogeneous Solid Mechanics

Abstract This article deals with the model of the locally heterogeneous elastic body. The model accounts for long-range interaction and describes near-surface non-homogeneity and related size effects. The key systems of model equations are presented. From the viewpoint of the representative volume element, the boundary condition for density and the limits of applicability of the model are discussed. The difference of mass density in the near-surface body region from the reference value (near-surface mass defect) causes a non-zero stressed state. It is indicated on the strong dependence of the surface value of density from the curvature of the surface of thin fibres. The effect of the near-surface mass defect on the stressed state and the size effect of surface stresses have been investigated on an example of a hollow cylinder. Size effect of its strength has been studied as well.

Developing Data Driven Models to Study Electrocatalytic CO2 Reduction on Ceria

As the rates of carbon dioxide emissions have continually increased on a global scale, mainly due to high demand of fossil fuel burning for transportation and generating electricity, an inevitable interest in managing and recycling the CO2 byproducts has emerged. New solutions are focusing on sustainable reduction of CO2 to CO, which can be conveniently converted to useful hydrocarbon liquid fuel products, such as methane, using existing technologies and infrastructure, as well as renewably generated electricity. Of particular interest in this field is the development and improvement of effective CO2 reduction catalysts, which ceria-based materials have been shown to be suitable for intermediate temperature solid oxide fuel cells. However, understanding the physical mechanisms by which ceria-based catalysts reduce CO2 has posed a challenging problem. In this work, recently-developed continuum approaches for the near-surface chemistry of high-temperature mixed conductors and data driven model-building routines have been combined to gain a quantitative understanding of the physical phenomenon of high-temperature CO2 reduction on ceria catalysts. The main insights pertain to the surface and near-surface defect structure of these materials, where the presence of oxygen vacancies is linked to the enhanced catalytic activity. The experimental data is in-operando scanning surface potential probe measurements performed on specially-designed patterned and thin-film test cells at temperature and under polarization. Preliminary results of data-driven model building will be presented and discussed.

Wavenumber Imaging of Near-Surface Defects in Rails using Green's Function Reconstruction of Ultrasonic Diffuse Fields.

Wavenumber imaging with Green’s function reconstruction of ultrasonic diffuse fields is used to realize fast imaging of near-surface defects in rails. Ultrasonic phased array has been widely used in industries because of its high sensitivity and strong flexibility. However, the directly measured signal is always complicated by noise caused by physical limitations of the acquisition system. To overcome this problem, the cross-correlations of the diffuse field signals captured by the probe are performed to reconstruct the Green’s function. These reconstructed signals can restore the early time information from the noise. Experiments were conducted on rails with near-surface defects. The results confirm the effectiveness of the cross-correlation method to reconstruct the Green’s function for the detection of near-surface defects. Different kinds of ultrasonic phased array probes were applied to collect experimental data on the surface of the rails. The Green’s function recovery is related to the number of phased array elements and the excitation frequency. In addition, the duration and starting time of the time-windowed diffuse signals were explored in order to achieve high-quality defect images.

Phase Coherence Imaging for Near-Surface Defects in Rails Using Cross-Correlation of Ultrasonic Diffuse Fields

In this paper, phase coherence imaging is proposed to improve spatial resolution and signal-to-noise ratio (SNR) of near-surface defects in rails using cross-correlation of ultrasonic diffuse fields. The direct signals acquired by the phased array are often obscured by nonlinear effects. Thus, the output image processed by conventional post-processing algorithms, like total focus method (TFM), has a blind zone close to the array. To overcome this problem, the diffuse fields, which contain spatial phase correlations, are applied to recover Green’s function. In addition, with the purpose of improving image quality, the Green’s function is further weighted by a special coherent factor, sign coherence factor (SCF), for grating and side lobes suppression. Experiments are conducted on two rails and data acquisition is completed by a commercial 32-element phased array. The quantitative performance comparison of TFM and SCF images is implemented in terms of the array performance indicator (API) and SNR. The results show that the API of SCF is significantly lower than that of TFM. As for SNR, SCF achieved a better SNR than that of TFM. The study in this paper provides an experimental reference for detecting near-surface defects in the rails.

Influence of structure and properties of surface layer on fatigue durability of hardened steels strengthened by combined electromechanical treatment

Using the example of hardened carbon steels (steel 45, U8), the effect of combination of various surface hardening technologies is considered (using electromechanical processing, surface plastic deformation, non-abrasive ultrasonic finishing and their combination) on changes in structural state and surface microhardness, cyclic durability of hardened specimens and fatigue failure mechanisms. The studies were carried out by the methods of optical and scanning electron microscopy and by microhardness and fatigue tests. It is shown that for the investigated steels in quenched state, a high-speed pulsed thermo-deformation effect during electromechanical processing is accompanied by an increase in the surface microhardness (by more than 50 %) and decrease in the fatigue limit (by 20 – 30 %). Such a change in properties is associated with formation in the surface layer of substantially non-equilibrium, inhomogeneous in chemical composition, ultradispersed phases with increased hardness. At the same time, in the near-surface metal volumes tempering processes of the hardened structure proceed with the formation of softening zones and tensile residual stresses, accompanied by a decrease in the microhardness in these zones and the fatigue limit of the specimens. Such effects reduce some of the materials performance characteristics during surface hardening. The ways to improve the properties of such products due to additional technological operations require further studies. Combined surface hardening (based on electromechanical processing, surface plastic deformation and non-abrasive ultrasonic finishing) of carbon steels allows, due to variations in the intensity of temperature and deformation effects, to purposefully change the structural-phase composition and stress-strain state of the surface and near-surface metal layers. As a result, it becomes possible to form a balanced complex of strength and fatigue characteristics of the samples, depending on the preliminary heat treatment of steel. The operations of surface plastic deformation and non-abrasive ultrasonic finishing after electromechanical hardening, due to intensive plastic deformation provide smoothing of the surface and healing of near-surface defects and allow correction of stress-strain state of the processed metal. It provides an increase in microhardness in the tempering zone by 20 – 25 % and the fatigue limit of the samples by 25 – 30 %.

Passive Thermography for Detection of Damaging Events during Quasi-Static Tensile Testing

In order to determine the material properties of anisotropic continuous fiber-reinforced polymers, such as stiffness and strength, quasi-static tensile tests are carried out, in which the test specimen is subjected to a controlled load until failure. In opposite to ductile metals, exact localization of the failure on the test specimen or observation of the initial or final failure in time is not possible because of the brittle failure mechanism. The sample often breaks down into many small fragments. In order to get a deeper understanding of the failure behavior and to optimize the material and the design of the specimen, a characterization of the damage progress under load by passive thermography was implemented. This is a suitable method for the detection of near-surface defects and provides very promising results, especially in combination with sophisticated evaluation methods of active thermography. An important influencing factor is the analysis of the amount of energy released during a micro-damaging event. In this paper, we show an approach to increase the contrast of single damaging events and a possibility to visualize the damaging progress, especially for near-surface defects. The measurements were realized with continuously fiber-reinforced materials.

Numerical Simulation and Experimental Study of Fluid-Solid Coupling-Based Air-Coupled Ultrasonic Detection of Stomata Defect of Lithium-Ion Battery.

Aiming at the characteristics of the periodic stacking structure of a lithium-ion battery core and the corresponding relationship between the air-coupled ultrasonic transmission initial wave and the wave propagation mode in each layer medium of a lithium-ion battery, the homogenized finite element model of a lithium-ion battery was developed based on the theory of pressure acoustics and solid mechanics. This model provided a reliable method and basis for solving the visualization of ultrasonic propagation in a lithium-ion battery and the analysis of ultrasonic time-frequency domain characteristics. The finite element simulation analysis and experimental verification of a lithium-ion battery with a near-surface stomata defect, near-bottom stomata defect and middle-layer stomata defect were performed. The results showed that the air-coupled ultrasonic transmission signal can effectively characterize the stomata defect inside a lithium-ion battery. The energy of an air-coupled ultrasonic transmission signal is concentrated between 350–450 kHz, and the acoustic diffraction effect has an important influence on the effect of the ultrasonic and stomata defect. Based on the amplitude response characteristics of the air-coupled ultrasonic transmission wave in the stomata defect area, a C-scan of the lithium-ion battery was performed. The C-scan result verified that air-coupled ultrasonic testing technology can accurately and effectively detect the pre-embedded stomata defect and natural stomata defect in a lithium-ion battery, which is able to promote and expand the application of the technology in the field of electric energy security.