40Cr钢应力磁化过程中的磁化反转特征

金属磁记忆检测技术是一种可早期检测铁磁构件应力集中程度的新方法,但进一步定量评价和广泛应用的瓶颈问题是复杂的应力磁化反转特征.对40Cr钢圆棒试件在不同最大拉力下进行反复加载-卸载拉伸试验,测定试件表面某确定点处漏磁场与拉应力的关系.试验结果表明,当试件处于弹性变形阶段时,漏磁场强度与拉应力的变化规律为线性关系;当试件受力超过屈服强度时,漏磁场强度与拉应力的变化规律变为折线,表现为先减小后增大再减小的应力磁化反转现象.随着最大拉力的增大,应力磁化反转极值点位置向较高拉应力方向移动,漏磁场强度最大变化量△Bmax也逐渐增大.     

含小孔铁磁平板单向拉伸的漏磁场和应力分布

 研究了含小孔铁磁平板单向拉伸时应力集中状况与漏磁场分布的关系。测量了在不同拉应力作用条件下漏磁场分布的规律。对漏磁场的分布关系、漏磁场坐标变化率的分布规律,与理论上应力、应力坐标变化率的分布关系进行了比较和分析,发现存在很有应用价值和理论价值的内在规律,可为铁磁试件应力集中程度的磁性方法检测提供借鉴作用。     

铁磁试件应力磁化过程中的磁化反转效应

在反复加载-卸载条件下,测量了35号冷轧钢试件在不同的最大拉应力作用下试件表面某确定点漏磁场随拉应力的对应关系,并进行了讨论和分析。建立了两种应力磁效应的数学模型,即应力磁化反转效应的应力等效磁场模型和应力磁化反转效应的磁导率模型。理论模型都能很好地反映应力磁化耦合的反转现象,和试验数据基本一致。理论模型的建立进一步证明了金属磁记忆定量检测的可能性。     

不同材料在不同应力状态下的磁记忆信号特征

磁记忆是力磁效应的一种表现,目前关于力磁效应的基本规律及机理研究还没有统一的定论。为了进一步对磁记忆效应的机理及不同材料不同应力状态下磁记忆信号特征进行试验性的研究,对不同铁磁材料(Q235,20号钢,45号钢)圆棒试件进行静载拉伸试验,分别测量某固定点处在线加载和在线卸载2种不同应力状态下的磁记忆信号。试验结果表明:3种不同材料在线加载时磁记忆信号各表现出了不同的力磁效应;在线卸载时测得的磁记忆信号更能有利地分析构件的应力阶段;弹性阶段时磁荷梯度GF变化幅度不大,接近屈服阶段时,磁荷梯度变化幅度变大,可以利用磁荷梯度对低碳钢材料应力定量评价。试验结果为磁记忆效应机理研究提供了一定的依据,也为金属磁记忆检测的定量检测提供了潜在可能性。     

不同应力状态下带钢的破鳞机理

利用自主设计的拉矫机对热轧带钢进行不同应力状态的破鳞试验,采用金相显微镜、扫描电镜和能谱分析对破鳞前后氧化皮的表面和界面进行分析,研究了带钢不同应力状态下的破鳞机理.试验结果表明,随着拉应力和压应力的增大,氧化皮全剥落率不断增大,并逐渐趋于稳定,且压应力的破鳞效果强于拉应力;拉应力作用下氧化皮以分层剥落为主,外层Fe3O4发生剥落,内层FeO仅在缺陷处发生少量剥落;而压应力作用下氧化皮以全剥落为主,临界弯曲应力随剥落量的增大而增大.     

横向拉应力对晶粒取向Si－Fe的磁畴结构及磁声发射强度的影响

观察了晶粒取向３％Ｓｉ－Ｆｅ在横向磁化场及横向拉应力作用下，磁畴结构和磁声发射强度的变化。在横向拉应力下，横向条状畴形成并增长。应力增大，横向条状畴碎化形成点状畴结构。磁畴与磁声发射强度变化相对应，揭示了磁场发射强度受横向拉应力影响的实质。     

磁化状态对粘结NdFeB磁体磁稳定性影响分析

粘结NdFeB磁体的磁化状态不仅直接决定磁体对外提供的磁场大小,也对磁体的磁稳定性、材料用量有重要影响。通过对不同磁化状态反向退磁一定时间后磁通密度变化率的分析可知,饱和磁化后反向退磁的磁体具有更优的磁稳定性。反向退磁幅度越大,磁体在气隙中磁通密度受外界退磁场导致的损失率越小,磁性能越稳定。反向退磁幅度大于9%时,磁体具有相对较好的磁稳定性;同时,材料用量越多的磁体磁稳定性越好。     

块石胶结充填体组成材料的力学特性

目前,国内外对于块石胶结充填体组成材料力学特性的研究较少,而材料的力学特性在一定程度上影响充填效果的好坏.基于目前的研究成果,借助室内单轴抗压、抗拉试验,得到块石胶结充填体各组成材料的应力-应变关系曲线、载荷-时间关系曲线.结果表明:3种组成材料强度差异性大;骨料试件的强度远远大于尾砂充填体试件与骨料-尾砂试件;骨料试件与骨料-尾砂试件峰后应力跌落较快,表现为脆性特征;尾砂充填体试件在到达峰值应力后下滑迟缓,说明其具有一定残余强度;尾砂充填体试件与骨料试件在单轴荷载的条件下具有初始压密、线弹性变形、塑性变形和失稳破坏4个阶段.     

热轧钢板矫直工艺的有限元分析

采用有限元软件ANSYS对热轧钢板矫直工艺进行模拟,研究了钢板同一初始状态下采用两种不同矫直工艺参数模拟后的结果,以及在同一矫直工艺参数下对钢板施加不同初应力载荷的模拟结果.探讨了钢板矫直过程中残余应力的变化情况以及矫直工艺参数与钢板残余应力的关系.模拟结果表明:在矫直过程中钢板表面都要经过拉一压-啦一压-拉的反复弯曲过程;对于纵向残余应力分布不均的通过模拟矫直能够达到明显的改善,并且在上矫直辊增加凸度值对消除横断面上的内应力分布不均是非常有效的.     

环形薄壁类粉末冶金零件压坯残余应力分析

针对环形薄壁类粉末冶金零件压坯的残余应力进行分析,采用修正的Drucker-Prager Cap弹塑性本构模型对金属粉末压坯压制、卸载和脱模过程进行数值模拟,同时考虑不同零件压坯几何参数(高径比和厚径比)以及压制工艺条件(摩擦条件和脱模角度)对压坯内残余应力的影响.研究结果表明,压坯脱模后的残余应力分布规律是压坯侧表面出现明显的压应力层,内部为拉应力区域,且越靠近压坯中心值越小.随着高径比和厚径比的增加,其残余应力逐渐减小;随着摩擦系数的增加,压坯表面的轴向残余压应力不断增大且压坯内部的拉应力也不断加大;适当增加脱模角度有利于压坯内应力释放而减小残余应力.通过正交模拟试验及方差分析可知,厚径比和脱模角度对金属粉末成形脱模后压坯残余应力影响更为显著.     

Direct Experimental Observations on Concurrent Microstructure and Magnetic Property Developments in Non-Grain Oriented Electrical Steel

Non-grain oriented electrical steel, with minor in-grain orientation gradients, was subjected to interrupted tensile deformations and concurrent microtexture, magnetic property and residual stress measurements. After the upper yield point, clear signatures of mechanical stress relief were observed. Changes in orientation gradients led to annihilation of low-angle (1 to 3 deg) boundaries. Prior deformation compressive residual stresses became tensile and magnetic properties improved. Beyond an optimum true strain of 0.01, this boundary annihilation ceased, compressive stresses were generated, and magnetic properties degraded.     

硬质合金金刚石涂层的残余应力分析

采用有限元方法分析硬质合金金刚石涂层的热残余应力,采用轴对称二维几何模型和自由边界条件,考虑了涂层厚度及不同基体对应力场的影响。分析表明,在多晶金刚石涂层中存在压应力,在基体中出现了拉应力,这种应力结构分布对材料性能的影响是有利的。在明锐界面出现了大的剪切应力(415MPa),应力奇异场出现在明锐界面靠近自由边界处。涂层压应力随基体中钴含量的增加而增加,随涂层厚度的增加而减小。因此可以借助这两个因素来适当控制和调节材料中的应力场以得到性能优良的产品。     

Yield behavior of a mild steel after prestraining and aging under reversed stress

Tubular specimens of mild steel are subjected to tensile prestraining and then aged under compressive stresses along the Bauschinger curve, at various temperatures below 175 °C. The compressive (reversed) stress imposed on the specimen during aging, called here the aging stress, is employed up to a maximum level of 90 pct of the prestress. Reloading tests in tension and in compression are carried out at room temperature, and the critical aging stress at which the yield stresses in the two reloading directions become equal is determined as a function of aging temperature. The critical aging stress decreases with increasing aging temperature and becomes constant at aging temperatures above 100 °C. Under such critical aging conditions, the aging index, i.e., the ratio of yield stress to prestress, rises with increasing aging temperature and approaches a saturation value of 0.86 at 150 °C. It is shown that stress aging is an effective means of reducing the Bauschinger effect while preserving the work hardening induced by the prestrain.     

Characteristics of the martensitic transformation and the induced two-way shape memory effect after training by compressive pseudoelastic cycling in Cu-Zn-Al single crystals

Training effects on the characteristics of the transformation when using pseudoelastic compressive cycling are studied from σ-ε curves and calorimetric measurements. Several characteristics produced by tensile cycling and observed in thermally induced martensitic transformations also appear in this case. For a fixed number of training cycles, the efficiency of the two-way shape memory effect (TWSME) obtained after compressive tests is higher than that measured when tensile experiments are done. In a certain range, the efficiency of the resulting TWSME after compressive training increases with the magnitude of the applied stress. Values of the stress (threshold stress) which trained samples with an induced TWSME are able to overcome when going to martensite are presented for two sets of samples. These opposing stresses are related to different degrees of the TWSME inhibition. These values increase with the number of compressive cycles and show a saturation behavior.     

Cold-Cracking Assessment in AA7050 Billets during Direct-Chill Casting by Thermomechanical Simulation of Residual Thermal Stresses and Application of Fracture Mechanics

Thermally induced strains and stresses developed during direct-chill (DC) semicontinuous casting of high strength aluminum alloys can result in formation of micro-cracks in different locations of the billet. Rapid propagation of such micro-cracks in tensile thermal stress fields can lead to catastrophic failure of ingots in the solid state called cold cracking. Numerical models can simulate the thermomechanical behavior of an ingot during casting and after solidification and reveal the critical cooling conditions that result in catastrophic failure, provided that the constitutive parameters of the material represent genuine as-cast properties. Application of fracture mechanics, on the other hand, can help to derive the critical crack length leading to failure. In the present research work, the state of residual thermal stresses was determined in an AA7050 billet during DC casting by means of ALSIM5. Simulation results showed that in the steady-state conditions, large compressive stresses form near the surface of the billet in the circumferential direction, whereas in the center, the stresses are tensile in all directions. Magnitudes of von Mises effective stresses, the largest component of principal stresses and the fracture mechanics concepts, were then applied to investigate the crack susceptibility of the billet.     

Residual stress-affected diffusion during plasma nitriding of tool steels

Plasma nitriding of tool materials is common practice to improve the wear resistance and lifetime of tools. Machining-induced compressive residual stresses in shallow layers of some tenths of microns are observed accompanied by other characteristic properties of machined surfaces in these high-strength materials. After plasma nitriding of M2 high-speed steel, previously induced compressive residual stresses remain stable and the depth of diffusion layers decreases with increasing compressive residual stresses. This article reports investigations of plasma nitrided samples with different levels of residual stresses induced prior to the nitriding process. For comparison, experiments with bending load stresses during plasma nitriding have also been carried out. The plasma nitriding treatment was performed at constant temperature of 500 °C with a gas mixture of 5 vol pct N₂ in hydrogen. Nitriding time was varied from 30 to 120 minutes. All samples were characterized before and after plasma nitriding concerning microstructure, roughness, microhardness, chemical composition, and residual stress states. Experimental results are compared with analytical calculations on (residual) stress effects in diffusion and show a clear effect of residual and load stresses in the diffusion of nitrogen in a high-strength M2 tool steel.     

Effect of Shot Blasting and Shot Peening Parameters on Residual Stresses Induced in Connecting Rod

This paper investigates the effect of shot blasting and shot peening parameters on residual stresses induced in connecting rod. Compressive residual stresses are induced using shot peening to increase fatigue life of connecting rod. Shot peening is also responsible for increase in surface roughness. Surface roughness is detrimental for fatigue life of the connecting rod. This necessitates shot blasting to reduce surface roughness. Shot peening and shot blasting processes are analysed to find optimum process parameters which will induce required value of compressive residual stress on the surface of connecting rod. Compressive residual stresses induced in the connecting rod specimen have been experimentally measured using X-ray stress analyser. The experimental results have been analysed using grey relational analysis to find optimum values of process parameters for target value of compressive residual stress and surface roughness. The experimental investigation and the analysis of it have resulted in achieving the desired value of compressive residual stress, which is 10.5% higher over the existing connecting rod. Surface roughness also decreases to 3.84 Ra which is 8.5% lesser than specified value to achieve better fatigue life.     

Residual stress measurements by x-ray diffraction in the ferritic-austenitic interface weldment

A mathematical model of calculating residual stresses at the weld interface of ferritic-austenitic steels has been developed. The Kurdjumov-Sachs orientation relationship (110)_bcc[111]// (11 l)_fcc[011] was determined at the interface of the transition joint. The resultant bcc-fcc lattice misfit gives rise to significant residual stresses. The performed X-ray analysis establishes macro- and microstress profiles extending up to 22 mm and 55 μm, respectively. The profiles indicate the development of compressive and tensile stresses on either side of the weld interface. With respect to this, tensile stresses in increasing sequence were computed from the parent metal toward the interface and compressive stresses were determined from the interface inward toward the weld bead in decreasing sequence. A remarkable stress discrepancy between the two profiles was observed, with the macrostresses falling in the range of +185 to −245 MPa and the microstress level ranging between +340 and −420 MPa. While the developed interfacial residual stresses are due to the difference in the bcc-fcc lattice parameters, the discrepancy observed in the determined stress level has its origin in the varying percentage of the two phases involved within a narrow mixing zone at the weld interface.     

Thermal residual stresses in functionally graded and layered 6061 Al/SiC materials

The thermal residual stresses that develop in spray atomized and codeposited functionally graded and layered 6061 Al/SiC metal-matrix composites (MMCs) during cooling from the codeposition temperature to ambient temperature were studied using thermo-elastoplastic finite element analysis. In an effort to investigate the effect of layered and graded structures on the residual stress distribution, the composites with homogeneous distribution of SiC particulates were also analyzed. The effect of SiC volume fraction in the SiC-rich layers and the effect of SiC-rich layer thickness on the residual stresses were investigated. Based on the present study, it was found that the residual stress distribution is very distinct for the aluminum and the SiC-rich layers in the layered materials. As the volume fraction of SiC increases in the SiC-rich layer, the magnitude of residual stresses also increases. The radial stress was found to be tensile in the aluminum layers and compressive in the SiC-rich layers. It was also found that, as the thickness of the SiC-rich layer increases, the magnitude of radial stress in the aluminum layers increases, and that in the SiC-rich layers decreases. In the graded material, the lower region of each layer exhibits tensile radial stress, and the upper region of each layer shows compressive radial stress in order to maintain continuity between layers during cooldown. In general, the layered and the graded materials have greater residual stresses and more complicated stress distribution, as compared with those in the composite materials with homogeneous distribution of SiC particulates.