Effect of mechanical stresses on the initial susceptibility and hysteresis characteristics of an ensemble of nanoparticles

Effect of uniaxial mechanical stresses on the initial susceptibility, saturation remanence, and coercive force of materials in which single-domain nanoparticles are carriers of magnetic properties has been studied theoretically. It has been shown that at small elastic stresses the above characteristics vary linearly; with increasing stresses, the growth of tensile stresses leads to a nonmonotonic variation of these characteristics, and the growth of compressive stresses, to a continuous decrease in the susceptibility and remanence and to an increase in the coercive force. With increasing plastic deformations, the position of the maximum in the stress dependence of the saturation remanence (or initial susceptibility) and the position of the minimum in the coercive force dependence are shifted to the region of greater values.     

应力对20号冷轧钢试件磁性特征的影响

测量了20号冷轧钢在不同拉应力作用后,试件端点表面磁感应强度随拉应力的变化关系曲线,并对端点表面剩余磁感应强度和矫顽力随拉应力的关系进行了分析。当外作用拉应力小于试件的拉伸屈服点应力时,残余应力对端点表面剩余磁感应强度和矫顽力的影响很小;当外拉应力大于试件的屈服点应力时,端点表面剩余磁感应强度和矫顽力将急剧增加发生跃变。以此可以利用端点表面剩余磁感应强度和矫顽力在屈服点发生突变的性质对材料所受的应力状况进行探测,能够对试件负载是否已超出屈服点作出有效判断,为磁性检测方法的研究提供了新的思路。     

On the role of residual internal stresses and dislocations on Barkhausen noise in plastically deformed steel

The dependence of Barkhausen noise on elastic and plastic deformations, achieved in tension and in compression, has been investigated both in Armco iron and a low carbon steel. These materials exhibit quite different behaviours, especially with regard to the effect of plastic deformation: a tensile plastic deformation (>1%) induces a marked increase in Barkhausen noise for Armco iron while it induces a steep decrease in the low carbon steel. The comparison between the tensile and compressive behaviours, as well as between the elastic and plastic regimes of deformation enables us to attribute these effects to two underlying mechanisms, i.e. effect of residual internal stresses through magneto-elastic coupling and dislocation–domain wall interaction. In Armco iron, the latter mechanism seems to have the strongest influence on the Barkhausen noise, while in the low carbon steel the influence of residual internal stresses prevails.     

Effects of laser irradiation on iron loss reduction for Fe–3%Si grain-oriented silicon steel

The effects of laser irradiation on iron loss reduction for Fe–3%Si grain-oriented silicon steel sheet were investigated. The local tensile residual stress states near the laser irradiated cavity lines were observed by using the new X-ray stress measurement method for a single crystal. Although the higher laser power induced the larger tensile residual stresses, the minimum iron loss was obtained at the medium tensile residual stress conditions of about 100–200 MPa. The increase of Vickers hardness was observed with increasing laser power, which was the mark of the plastic deformations induced by the laser irradiation. The tensile residual stress reduces eddy current loss and the plastic deformation increases hysteresis loss of the material. The total iron loss is determined by the balance of these two effects of laser irradiation.     

Effect of plastic deformations on the dependence of the remanence of steels on elastic tensile stresses

Maximum values of the remanence and the positions of the maxima, which correspond to the average value of body stresses (internal stresses of the first kind), have been found based on the measurements of the dependence of the remanent magnetization of ferromagnetic steels on the elastic tensile stresses. The internal stresses were varied over wide limits with the aid of different amounts of a preliminary plastic tensile deformation. The results obtained make it possible to use magnetic methods of measurements to determine residual stresses and control their changes during exploitation, which offers new possibilities for nondestructive testing of the stress-strain state of articles and constructions.     

基于磁滞无损技术的起重机钢梁疲劳寿命分析

针对起重机的钢梁结构的疲劳问题,选取起重机所用钢材制得3组试样进行测试,每组试样的中间标距部分标记了3个位置,随后对试样进行拉伸试验,拉伸前后分别进行矫顽力测试,对拉伸试验过程进行仿真工作,将得到的仿真应力云图与矫顽力云图进行了对比。测试及仿真结果表明：随着外加拉应力的增大,材料的矫顽力(Hc)逐渐增大,矫顽力呈现阶段性的变化规律,这可能与残余应力在材料内部产生位错的钉扎效应有关,矫顽力云图表明试样内部的晶体结构受到不同程度的破坏,矫顽力也随之增加。进一步对矫顽力与应变数据进行理论分析,得到矫顽力 应变拟合曲线与拟合方程。     

基于矫顽力的铁磁性材料应力检测技术

针对铁磁性材料应力检测问题,分析了铁磁性材料矫顽力和材料所受应力的关系,提出了通过检测铁磁性材料的矫顽力实现检测应力的方法。采用电磁检测原理测量铁磁性材料的矫顽力,选取3个受到不同拉应力的铁磁性圆环,在圆环对称位置分别缠绕激励线圈和检测线圈,采用正弦交流信号作为激励,测量圆环的磁滞回线,计算3个圆环的矫顽力值。结果表明,圆环受力后磁滞回线的形状发生改变,矫顽力随着材料所受应力的增大而增大。     

W18Cr4V高速钢激光相变强化层的残余应力

利用Ｘ射线衍射法对Ｗ１８Ｃｒ４Ｖ高速钢激光相变强化层的残余应力进行了测定和分析。试验结果表明，相变强化层表面的残余应力为压应力，内部为拉应力，近似按正弦函数分布。其大小和分布状态随工艺参数的变化而改变。当改变工艺参数使表面熔化时，表面为拉应力。     

Residual stress characterization in structural materials by destructive and nondestructive techniques

Transmutation of nuclear waste is currently being considered to transform long-lived isotopes to species with relatively short half-lives and reduced radioactivity through capture and decay of minor actinides and fission products. This process is intended for geologic disposal of spent nuclear fuels for shorter durations in the proposed Yucca Mountain repository. The molten lead-bismuth-eutectic will be used as a target and coolant during transmutation, which will be contained in a subsystem vessel made from materials such as austenitic (304L) and martensitic (EP-823 and HT-9) stainless steels. The structural materials used in this vessel will be subjected to welding operations and plastic deformation during fabrication. The resultant residual stresses cannot be totally eliminated even by stress-relief operations. Destructive and nondestructive techniques have been used to evaluate residual stresses in the welded and cold-worked specimens. Results indicate that tensile residual stresses were generated at the fusion line of the welded specimens made from either austenitic or martensitic stainless steel, with reduced stresses away from this region. The magnitude of residual stress in the cold-worked specimens was enhanced at intermediate cold-reduction levels, showing tensile residual stresses in the austenitic material while exhibiting compressive stresses in the martensitic alloys. Comparative analyses of the resultant data obtained by different techniques revealed consistent stress patterns.     

焊缝与螺栓混合连接梁柱钢节点的残余应力有限元分析

考虑温度变化对钢材物理性能和力学性能的影响,以热-弹塑性理论为基础,建立梁柱节点顶底角钢焊缝、螺栓混合连接的三维仿真模型.利用有限元分析软件ANSYS模拟钢结构梁柱节点在焊接和冷却阶段的温度场,并求解得到残余应力的分布.结果表明,焊缝区域主要以拉应力为主,残余应力在焊缝和螺栓等不同部位的分布不同.最大残余应力出现在顶角钢与梁翼缘的焊缝处,最小残余应力出现在柱与角钢连接处,分别是设计强度值的220.8%和11.45%,与《钢结构设计规范》中的规定基本相符.     

The role of residual stress in neutral pH stress corrosion cracking of pipeline steels – Part II: Crack dormancy

This investigation provides a quantitative analysis of the effect of Type I residual stresses on the occurrence of pitting and stress corrosion cracking (SCC) formation in pipeline steel exposed to neutral pH aqueous environments. It has been shown that SCC generated in neutral pH environments can be readily blunted due to plastic deformation (room temperature creep) and/or extensive anodic dissolution. As a result, a high positive tensile residual stress gradient is necessary for continued growth of SCC in pipeline steels exposed to this neutral pH environment. The tensile residual stress represents a large mechanical driving force for crack nucleation and short crack growth. Active cracks may become dormant as the near-surface residual stress gradient changes, due to self-equilibration, from highly tensile to a lower tensile state or to a compressive state. The change in residual stress level can occur within 1 mm of the surface, resulting in a large proportion of dormant SCC.     

Correlation between the stress-strain state parameters and magnetic characteristics of carbon steels

A method for reconstruction of the stress-strain diagrams of steel objects from their magnetic characteristics determined directly during tension has been suggested. The coercive force, residual induction in the major hysteresis loops, and the maximum magnetic permeability are recommended as parameters for nondestructive testing of stresses and strains.     

Changes in the state of heat-resistant steel induced by repeated hot deformation

This work deals with the problems of structural regeneration by thermal restoration treatment (TRT). These include the lack of a structural sign showing that TRT is possible, a consensus on TRT modes, the data on the necessary relaxation depth of residual stresses, or criteria of structural restoration. Performing a TRT without solving these problems may deteriorate the properties of steel or even accelerate its destruction. With this in view, the purpose of this work is to determine experimentally how the residual stress state changes under thermal and mechanical loads in order to specify the signs of the restoration of structure and structural stability. The object of this research is unused 12Cr1MoV steel that has been aged naturally for 13 years. Using X-ray dosimetry with X-ray spectral analysis, we study the distribution of internal residual stresses of the first kind during the repeated hot deformation. After repeated thermal deformation, the sample under study transforms from a viscoelastic Maxwell material into a Kelvin-Voigt material, which facilitates structural stabilization. A sign of this is the relaxation limit increase, prevention of continuous decay of an α-solid solution of iron and restoration of the lattice parameter.     

Specific Features of the Nucleation and Growth of Fatigue Cracks in Steel under Cyclic Dynamic Compression

The processes of the fracture of 40Kh and U8 steels under cyclic dynamic compression are studied. It has been found that the main cause for the fracture of the cyclically compressed specimens is the propagation of cracks due to the effect of residual tensile stresses, which arise near the tips of the cracks at the stage of the unloading of the specimens. The growth rate of a crack has the maximum value at the initial stage of its propagation in the vicinity of the stress concentrator. As the crack propagates deep into the specimen, its growth rate decreases and depends only slightly on the real cross section of the specimen. The model of the process of the fatigue fracture of the steels under dynamic loading by a cyclically varied compressive force is proposed. It has been found that the high fatigue endurance is provided by tempering at 200°C for the 40Kh steel and at 300°C for the U8 steel.     

超声滚压加工对X80管线钢焊接残余应力的影响

建立超声表面滚压加工（ultrasonic surface rolling process,USRP）的三维有限元模型,开发了模拟焊接的移动双椭球热源子程序,利用有限元软件ABAQUS模拟了X80管线钢焊缝不同方向的焊接残余应力,在此基础上叠加USRP的超声振动与静载荷的综合作用,模拟了表面塑形变形、应力和应变,耦合后分析了USRP前后残余应力的变化规律.结果表明,经过USRP处理,X80管线钢表面焊缝区由三向残余拉应力变为三向残余压应力,随着USRP次数的增加,残余压应力数值不断增大,残余应力σ_x,σ_y,σ_z变化规律基本相同.     

Yield locus evolution and constitutive parameter identification using plane strain tension and tensile tests

An inverse calibration strategy to determine the constitutive parameters of phenomenological advanced yield criteria in a convenient and economical manner is presented. The studies on the shape of the yield loci for various steel types revealed that their work contours exhibit almost no evolution in the vicinity of the equibiaxial tension after roughly 4% equivalent strain. In other words, the ratio between balanced biaxial and uniaxial stresses of the yield locus reaches a saturation value after undergoing some deformation. Accordingly, the balanced biaxial stresses can be associated with the tensile flow stresses in the rolling direction by means of a constant factor, which can be generalized for different steel families. Based on this, an alternative inverse-analysis strategy using the tensile and plane strain tension tests will be proposed and validated within this work. Cup drawing tests have been applied to assess the accuracy of the optimized yield loci for different strain paths.     

Influence of fabrication stresses on fatigue life of friction stir welded aluminium profiles

Influence of clamping and welding procedure on fatigue life of a friction stir (FS) welded aluminium profile has been analysed. Two series of aluminium profiles (series A and B) have been fatigue tested. The different clamping conditions in series A and B gave rise to different failure positions that could not be explained neither by the effective notch stress method, nor by the critical distance methods. The clamping in series A introduced plastic deformation at the weld notch that gave rise to tensile residual stresses. The failure position could be explained from the value of the stress intensity factor provided that the residual stresses were considered.     

Investigation of the Effect of Residual Stress Gradient on the Wear Behavior of PVD Thin Films

The control of residual stresses has been seldom investigated in multilayer coatings dedicated to improvement of wear behavior. Here, we report the preparation and characterization of superposed structures composed of Cr, CrN and CrAlN layers. Nano-multilayers CrN/CrAlN and Cr/CrN/CrAlN were deposited by Physical Vapor Deposition (PVD) onto Si (100) and AISI4140 steel substrates. The Cr, CrN and CrAlN monolayers were developed with an innovative approach in PVD coatings technologies corresponding to deposition with different residual stresses levels. Composition and wear tracks morphologies of the coatings were characterized by scanning electron microscopy, high-resolution transmission electron microscopy, atomic force microscopy, x-ray photoelectron spectroscopy, energy-dispersive x-ray spectroscopy, x-ray diffraction and 3D-surface analyzer. The mechanical properties (hardness, residual stresses and wear) were investigated by nanoindentation, interferometry and micro-tribometry (fretting-wear tests). Observations suggest that multilayer coatings are composed mostly of nanocrystalline. The residual stresses level in the films has practically affected all the physicochemical and mechanical properties as well as the wear behavior. Consequently, it is demonstrated that the coating containing moderate stresses has a better wear behavior compared to the coating developed with higher residual stresses. The friction contact between coated samples and alumina balls shows also a large variety of wear mechanisms. In particular, the abrasive wear of the coatings was a combination of plastic deformation, fine microcracking and microspallation. The application of these multilayers will be wood machining of green wood.     

Non-destructive measurement of residual stresses in carbon steel weld joints

Abstract

Residual stress measurements have been carried out on 8 and 12 mm thickness carbon steel single V weld joints by X -ray diffraction and ultrasonic techniques. The maximum tensile and maximum compressive stresses on the surface of the 12 mm carbon steel weld joint are higher than those on the surface of the 8 mm carbon steel weld joint. The results also indicate that the variation in the surface residual stress across the weld is different from that for the through thickness residual stresses. The effect of constraint, which depends on the thickness of the weld joint, is found to influence the surface and through thickness residual stresses. The effect of stress relief annealing heat treatment on residual stresses was also studied.     

Modelling the effects of tool-edge radius on residual stresses when orthogonal cutting AISI 316L

Tool-edge geometry has significant effects on the cutting process, as it affects cutting forces, stresses, temperatures, deformation zone, and surface integrity. An Arbitrary-Lagrangian–Eulerian (A.L.E.) finite element model is presented here to simulate the effects of cutting-edge radius on residual stresses (R.S.) when orthogonal dry cutting austenitic stainless steel AISI 316L with continuous chip formation. Four radii were simulated starting with a sharp edge, with a finite radius, and up to a value equal to the uncut chip thickness. Residual stress profiles started with surface tensile stresses then turned to be compressive at about 140 μm from the surface; the same trend was found experimentally. Larger edge radius induced higher R.S. in both the tensile and compressive regions, while it had almost no effect on the thickness of tensile layer and pushed the maximum compressive stresses deeper into the workpiece. A stagnation zone was clearly observed when using non-sharp tools and its size increased with edge radius. The distance between the stagnation-zone tip and the machined surface increased with edge radius, which explained the increase in material plastic deformation, and compressive R.S. when using larger edge radius. Workpiece temperatures increased with edge radius; this is attributed to the increase in friction heat generation as the contact area between the tool edge and workpiece increases. Consequently, higher tensile R.S. were induced in the near-surface layer. The low thermal conductivity of AISI 316L restricted the effect of friction heat to the near-surface layer; therefore, the thickness of tensile layer was not affected.