Residual stress evaluation in dissimilar welded joints using finite element simulation and the L CR ultrasonic wave

Subsurface stresses in welded structures increase the likelihood of fatigue cracks and environmental induced material degradation. The ability to evaluate stresses at the surface as well as in the interior of welded structural members would substantially increase the accuracy of structure life estimation. The longitudinal critically refracted (L _CR) wave is a bulk longitudinal mode that travels within an effective depth underneath the surface. It may be used to detect in-plane subsurface stresses in the structures. This paper presents a three dimensional thermo-mechanical analysis to evaluate welding residual stresses in dissimilar plate-plate joint of AISI stainless steel 304 and Carbon Steel A106-B type. After finite element simulation, the residual stresses were evaluated by L _CR ultrasonic waves. Finally the results of two methods were compared and verified by hole-drilling method. This paper introduces a combination of ??Finite Element Welding Simulation?? and ??Ultrasonic Stress Measurement using the L _CR Wave?? which is called as ??FEL _CR??. The capabilities of FEL _CR??. The capabilities of FEL _CR in residual stress measurement are confirmed here. And also this paper evaluates residual stresses of dissimilar welded joints by LCR ultrasonic waves. It has been shown that predicted residual stress from three dimensional FE analyses is in reasonable agreement with measured residual stress from LCR method and also the results of both are verified with hole-drilling experimental measurements.     

Regularities of the change in the coercive force under biaxial asymmetric deformation of steel 3

The change in the coercive force under biaxial asymmetric (tension and compression in mutually perpendicular directions) cyclic deformation of cross-shaped steel 3 specimens in the elastic region of deformations was studied. Specimens were deformed beforehand under biaxial asymmetric loading to various degrees of plastic deformation. It was demonstrated that the elastic-deformation dependences of the coercive force measured along the tension and compression directions are qualitatively similar to those under uniaxial tension or compression. It was also shown that, under cyclic elastic loading, these dependences are reversible for well-annealed steel and have a hysteresis that expands with increasing degree of plastic deformation for plastically deformed steel. The possible causes of the hysteresis in the dependence of the coercive force on the elastic cyclic deformations under biaxial loading are discussed. It was supposed that the hysteresis of the coercive force was caused by the appearance of free (not bound in carbide phases) carbon atoms playing the role of interstitial impurity atoms for the α-iron lattice in plastically deformed carbon steels. The possibility of estimating the stressed-strained state of steel under biaxial loading using a magnetic method was discussed.     

The influence of biaxial elastic deformation on the coercive force and local remanent magnetization of construction steels

The influence of biaxial elastic tension-compression and tension-tension deformation on the coercive force and remanent magnetization of Cτ3 steel and 09Γ2C pipe steel cross-shaped specimens is studied. It is shown that coercive force grows monotonically with increasing load in the direction of compression under elastic tension-compression deformation and almost does not change under elastic tension-tension deformation. The local remanent magnetization decreases for all the considered types of biaxial elastic deformation. The possibility of using these methods for the evaluation of the stressed state of complexly deformed carbon steel products is discussed.     

低频振动塑性成形粘弹塑性模型的体积效应分析

采用Kirchner对应变时间历程的基本假设,针对振动拉伸建立一个一维粘弹塑性模型;利用MATLAB中的符号计算,推导粘弹塑性本构方程的显式表达式.通过确立粘弹塑性边界并对本构方程进行数值求解,可以确定金属在振动加工过程中,其应力应变在粘弹性与粘塑性之间的变化情况.通过计算瞬时应变的大小与屈服限建立粘弹性变形和粘塑性变形的判断准则.在考虑粘弹塑性本构关系中的后继屈服情况、应变历程、应变率历程及弹性应变等因素后,可以确定单轴振动拉伸时材料变形的动态应力和平均应力.根据所给定的振型参数和材料力学性能参数,结合特定的振动拉伸实例,分别得出金属在准静态拉伸和振动拉伸时的动态应力-时间、动态应力-应变和平均应力-应变率的变化趋势等,实现基于粘弹塑性本构关系的低频振动塑性成形的体积效应机理分析.     

Thermal stresses due to time exponentially decaying laser pulse: elasto-plastic wave propagations

In the present study, thermal stress developed in the substrate material, which is subjected to a laser heating pulse is formulated. The closed form solutions for the temperature and stress fields due to time exponentially decaying laser pulse are presented. The Laplace transformation method is employed when deriving the governing equations. The elastic and plastic propagation of the stress waves are considered and the depth of the plastic zone is predicted. In order to account for the recoil pressure generated during the evaporation process, stress boundary at the free surface of the workpiece is considered. It is found that the magnitude of stress wave, due to stress boundary at the surface, well exceeds the elastic limit of the substrate material. Once the magnitude of the recoil pressure reduces considerably, elastic wave is generated. This occurs after t^*=0.032. Since the elastic wave propagates faster than the plastic wave, both waves meet at some depth below the surface. This, in turn, defines the depth of the plastic zone. In the present case, the depth of elastic zone extends to about x^*=9.2 below the surface. The magnitude of the stress wave generated due to temperature gradient is less than the yield strength of the substrate material; in which case, its magnitude decreases with increasing depth from the surface.     

Time reversal method for guided wave inspection in pipes

The application of the time reversal method in pipe-like structures based on finite element method (FEM) is investigated. A steel pipe model measuring 70 mm x 3.5 mm is used to analyze the reflection coefficient of the L(0,2) mode with the time reversal process. Simulation results show that the time reversal array method is beneficial to the improvement of the signalto-noise ratio of a guided wave inspection system. As the intercepting window is widened, more energy is included in re-emitted signals, which leads to a large reflection coefficient of the L(0,2) mode. In parallel, a circumferential locating method based on the time reversal method is described. The time reversal process used for guided wave inspection leads to the temporal and spatial focusing. When the time reversal signals are re-emitted, the angular profile obtained at the axial location of the defect can be used to determine the circumferential location of the defect. Except for a pipe with one defect, the circumferential locating method has been verified on another pipe model with two defects. Meanwhile, the elements number of the time reversal array has been discussed for enhancing the discrimination of the defect circumferential location.     

Assessment of local deformation using EBSD: Quantification of accuracy of measurement and definition of local gradient

Electron backscatter diffraction (EBSD) in conjunction with scanning electron microscopy was used to assess the magnitude of microstructural scale deformation (local deformation) for deformed Type 316 stainless steel. Local misorientation, which is an averaged misorientation between neighboring measurement points, is often used for assessment of local deformation. However, the local misorientation is unsuitable for a quantitative evaluation because it depends not only on the local deformation but also on various factors such as measurement accuracy and the distance between points used in the misorientation calculation (step size). In this study, first, the measurement accuracy was quantified by a parameter called background noise. The factors which affect the measurement accuracy were then discussed from measurements under various conditions. Secondly, in order to reduce the influence of measurement conditions and exclude the dependency of step size in the local deformation assessment, a parameter called the local gradient G_L was proposed. The local gradient clearly showed the spatial distribution of local deformation regardless of the measurement accuracy, and it had hardly any effect from grid pattern and step size. Finally, the local gradient was correlated with the plastic strain from which it was revealed that the strain gradient near a notch root could be estimated by the EBSD measurement.     

A FEM study on the mechanical responses of pseudoelastic TiNi alloys to a particle normal loads

In this study, the models of four materials including three sorts of pseudoelastic TiNi alloys and a stainless steel (as a contradistinction) enduring a particle's normal loads were individually simulated based on bilinear strain hardening law by means of finite element method. Owing to the pseudoelasticity, TiNi alloys proved to have high elastic strain limit and low pseudo-Yong's modulus, with which the special mechanical response was created under normal loads. The results shown that pseudoelastic TiNi alloys occurred plastic deformation more difficult than the stainless steel, and the critical load of plastic deformation increased with the increasing elastic strain limit and the decreasing pseudo-Young's modulus. Plastic regions of three TiNi alloys with the elastic strain limit 0.02, 0.04 and 0.06 were 0.60, 0.32, 0.047 times of that of the stainless steel, respectively. When the pseudoelastic TiNi alloys endured a particle's normal loads, the phenomena of decreasing contact stress, von Mises stress and increasing the elastic strain were also observed in this FEM study. In terms of above results, the wear mechanism of pseudoelastic TiNi alloys was discussed finally.     

Diagnostics of Creep of Heat-Resistant Steels on the Basis of Measurements of the Ultrasonic Wave Velocity in Nondestructive Testing of Energy Equipment: I. Probes and Tools for Measuring the Velocity of Ultrasound

The designs of probes for measuring the time of propagation of bulk and surface ultrasonic waves and examples of their use for diagnostics of the process of metal creep are described. The results of measurements of the surface wave velocity in the elastic and plastic domains are given. It is shown that in the elastic domain (up to the yield limit of the metal) change in the surface wave velocity is not greater than 0.02%. In the plastic domain, a decrease in the surface wave velocity is observed and the magnitude of the decrease depends linearly on the loading rate.     

Substrate dependence of the scratch resistance of CrN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> coatings on steel

Hard CrN_x coatings were sputter deposited on hot work tool steel (HWTS) and high speed steel (HSS) in an industrial PVD reactor. Coatings were deposited under various nitrogen flows. The thickness, density, hardness, elastic modulus, composition, and stress were determined for the coatings. The specimens were subjected to scratch testing. Two different failure mechanisms were investigated: chipping and complete coating removal. For all specimens, the coating-to-substrate adhesion was that good that adhesion did not limit the scratch resistance. Therefore, the minimum loads at which a given type of failure was initiated were not a measure for the coating-to-substrate adhesion. The scratch resistance was better for coatings on HSS than for coatings on HWTS. This is due to the higher hardness of the HSS. Substrate independent measures for the scratch resistance of the coating were obtained by considering critical track widths instead of critical loads. The hardening of the coating--substrate systems due to the coating was investigated. The uncoated substrates exhibited track width independent scratch hardness. For the coated specimens the scratch hardness increased with increasing track width until chipping of the coating occurred. Complete coating removal coincided with a decrease in hardness. Although the elastic properties, hardness, and thickness of all coatings were more or less equal, CrN_1.0 coatings outperformed CrN_0.6 coatings in scratch tests both on HSS and on HWTS.     

Coercive-force hysteresis of carbon steels during elastic cyclic tensile deformation

Dependences of the coercive force and mechanical stresses on cyclic tensile in the region of elastic deformation of steel samples with various carbon contents (Armco iron, 3, 8) are investigated. It is shown that the dependence of the coercive force on cyclic elastic tensile deformation is reversible for well-annealed samples. The dependence for plastically deformed steels has a hysteresis loop that is significantly determined by the carbon content and increases with an increasing degree of plastic deformation. It is assumed that the hysteresis of the coercive force as a function of elastic cyclic tensile deformations in plastically deformed carbon steels is caused generally by the appearance “free” carbon atoms, i.e., those not bound in carbide phases, which act as interstitial impurity atoms in the lattice of α-iron.     

Microstructure and Tribological Properties of a HfB<Subscript>2</Subscript>-Containing Ni-Based Composite Coating Produced on a Pure Ti Substrate by Laser Cladding

A HfB₂-containing Ni-based composite coating was fabricated on Ti substrates by laser cladding, and its microstructure and tribological properties were evaluated during sliding against an AISI-52100 steel ball at different normal loads and sliding speeds. The morphologies of the worn surfaces were analyzed by scanning electron microscopy (SEM) and three-dimensional non-contact surface mapping. The results show that wear resistance of the pure Ti substrate and NiCrBSi coating greatly increased after laser cladding of the HfB₂-containing composite coating due to the formation of hard phases in the composite coating. The pure Ti substrate sliding against the AISI-52100 counterpart ball at room temperature displayed predominantly adhesive wear, abrasive wear, and severe plastic deformation, while the HfB₂-containing composite coating showed only mild abrasive wear and adhesive wear under the same conditions.     

Micro-Raman depth analysis of residual stress in machined germanium

Raman spectroscopy has proved to be a useful nondestructive technique for measuring residual stresses in semicondutors. The Raman microprobe is used to investigate the effects of machine parameters on residual stresses in single point diamond turned germanium (Ge). A profiling technique that provides a method of obtaining the residual stress information as a function of depth with depth resolutions of 10.0 nm is discussed. This method is used to analyze the asymmetrically broadened and shifted spectral features in the machined samples. Residual stresses are sampled across ductile feed cuts in (100) Ge wafers, which were single point diamond turned using various feed rates (12.5, 25 nm/rev), rake angles (0°, −10°, −30°), and clearance angles (6°, 16°). In general a region of plastically deformed material that shows slight compressive stresses exist near the surface of the diamond turned sample. The compressive surface stress increases to a maximum at a depth of ≈ 50 nm beneath the surface at which point the stress rapidly changes sign. The rapid sign change is indicative of the transition from plastic to elastic deformation. Deeper probe regions exhibit increasing tensile stresses, which reach a maximum and then relax to zero at greater depths in the sample. A related study of the stress field occurring around Vicker's hardness indents provides a link between theoretical and experimental stress profiles and demonstrates the accuracy of the micro-Raman profiling technique.     

Inelastic nonlinear behavior of steel trusses cooled down from a heating stage

This paper is to study the inelastic nonlinear behavior of steel trusses cooled down from a heating stage, a problem that was rarely investigated before. Bilinear elastoplastic model is adopted for steel. Softening of elastic modulus and yield stress at elevated temperatures is considered by the reduction factors of Eurocode3. The plastic strain resulting from strain reversal is included in the stress-strain curves. Instantaneous modulus is proposed for simulating the unloading (loading) behavior caused by cooling (heating). Both the thermal and plastic strains are considered in the force-displacement relation for truss members. The temperature-deflection curves obtained for a simple truss serve as the benchmark problem for checking the validity of other approaches in treating the heating and cooling stages. For general applications, the elastoplastic finite element derived was adopted to solve the cooling responses of preheated steel trusses by the Newton-Raphson method. The permanent deformation and residue stress computed for each member are useful for evaluating the reusability and safety of steel trusses after a fire attack.     

Plastic buckling of a cylindrical shell in pure bending

An approximate analysis is presented of the bifurcation of an elastic-plastic circular cylindrical shell subjected to pure bending. The method of analysis is an extension into the plastic range of an earlier analysis[8]; a relatively simple solution is obtained by making various approximating assumptions, and by a judicious choice of the eigenmode. The results show that the critical outer fibre stress is up to about 35% greater than the critical stress of an axially compressed cylinder, using J₂ flow theory, in contrast to the elastic or elastic-plastic (deformation theory) cases, where these two stresses are practically equal.     

Energy dependence of the relative light output of YAlO<Subscript>3</Subscript>:Ce,Y<Subscript>2</Subscript>SiO<Subscript>5</Subscript>:Ce,and YPO<Subscript>4</Subscript>:Ce scintillators

The nonlinear dependence of the relative light output on the energy deposited in single-crystal scintillation materials YAlO₃:Ce (YAP:Ce), Y₂SiO₅:Ce (YSO:Ce), and YPO₄:Ce (YPO:Ce) has been studied. The investigations have been conducted under quasi-monochromatic X-ray excitation in the energy range of 9.5–100 keV. In addition to the standard technique for measuring the nonproportional scintillator response based on the dependence of the full-energy peak position on the energy of incident radiation, a method is proposed for measuring the light output by X-ray fluorescence peaks. Using this method for YAP:Ce, it is possible to investigate the nonlinear dependence of the light output on the photon energy in the energy range of 2–40 keV. Along with this method, the K-dip spectroscopy method has been proposed and tested by measuring the dependence of the relative light output on the electron energy in the range of 0.1–80.0 keV. The processes resulting in the loss of the scintillation material efficiency at a high ionization density are considered.     

An electron microprobe and cathodoluminescence study of chemical reactions between tool and workpiece when turning steel with alumina-based ceramics

Crater wear of alumina-based ceramic cutting tools when machining steel is predominantly dependent upon superficial plastic deformation. Such tool surface deformation may be greatly affected by chemical reactions with workpiece material. Crater surfaces of worn alumina-based ceramic tools in Coromant grade CC 620 (a pure ceramic, containing Al₂O₃ and ZrO₂) have been analysed by electron microprobe and cathodoluminescence after turning steel SS 2541 (similar to AISI 4337). It was found that the deformed surface layer had increased concentrations of iron and magnesium. Both these elements were probably present as spinel phases FeO. Al₂O₃ and MgO. nAl₂O₃ in solid solution. The spinel phases have higher yield strengths and probably also higher ductility than alumina itself, which may explain why wear rates are reduced when such compounds are found at the alumina tool-chip interface.     

磁控溅射微纳米梯度镀层对45钢及40Cr钢弹性变形性能的影响

利用磁控溅射离子镀技术在屈强比不同的45钢及40Cr钢基体上制备Cr、类石墨碳(Graphite-like-carbon,GLC)及CrTiAlN微纳米梯度镀层,通过考察镀层/金属复合体系弹性模量和屈服强度的变化来研究镀层对基体弹性变形行为的影响。研究结果表明:磁控溅射离子镀层可以改善金属基体的弹性变形性能,但镀层对不同金属基体力学参量的影响不同;屈强比低的45钢,镀层可同时提高弹性模量和屈服强度,在v=0.1mm/min的低速拉伸时,45钢试样的屈服强度可提高20%左右;屈强比高的40Cr钢,镀层对其屈服强度基本无影响,但对弹性模量提高的程度更显著,当拉伸速度为0.7mm/min时,40Cr钢试样的弹性模量可提高60%,而45钢仅为22%;镀层/基体复合体系的弹性模量随拉伸速度的增加表现为先增大后减小的特点,其变化规律不同于金属基体;镀层的高硬度和良好的膜基匹配能力有助于阻碍工件表面及亚表面微塑性形变区的扩展,从而提高试样的力学性能。     

Data recording system of a dispersion interferometer based on a CO<Subscript>2</Subscript> laser

The data recording system of a multichannel double-pass dispersion interferometer based on a CO₂ laser is described. This system has been designed to record the linear density of plasmas in a real-time mode with a time discreteness of 4 μs and resolution 〈N _e L〉 ～ 0.34 × 10¹³ cm^?2 (N _e is the electron component of the plasma density, and L is the plasma size in the wave propagation direction) in the range of linear density variations of up to 10¹⁷ cm^?2. The system is built from unified recording modules that use fast ADCs to record the shape of photodetector and modulator signals and FPGA-based digital units of dataflow processing to form results of measurements. The single-channel recording module of the dispersion interferometer has been tested under actual experimental conditions of the GDL gas-dynamic trap and the TEXTOR tokamak (Julich, Germany).     

Effect of Plastic deformation and its localization zones on magnetic characteristics of steel 45

Attachable transducers were used to measure the demagnetizing current, which is proportional to the coercive force, and Barkhausen noise parameters for steel 45 specimens depending on the degree of their plastic deformation in measuring the magnitudes along and perpendicular to the tension direction. The demagnetizing current measured along the preliminary tension direction was shown to increase monotonically; the rate of its changes is maximal at the initial stage of plastic deformation. The demagnetizing current measured perpendicular to the applied tension direction also increases monotonically but within a narrower range. Near local inhomogeneities formed during the deformation, substantial changes in the coercive force and Barkhausen noise take place. The results obtained are explained on the basis of the current concept of the anisotropy of internal stresses in preliminarily plastically deformed materials.