A STUDY OF B-P UNIFIED VISCOPLASTIC CONSTITUTIVE MODEL

By combining the Bodner-Partom constitutive model and equivalent stress function, finite element methods and program on analyzing non-elastic deformation and stress for thermal viscoplastic material are studied in this paper, and it's the first time that this material model is used in a kind of engineering software-MARC. Thermal viscoplastic behavior of high temperature alloy GH536 specimen with gap is analyzed by this program. The research results show it is feasible to analyze thermal viscoplastic behavior of specimen or structure by applying B-P model.     

Thermo-mechanical modeling of orthogonal machining process by finite element analysis

A plane strain finite element method is used with a new material constitutive equation for 1020 steel to simulate orthogonal machining with continuous chip formation. Deformation of the workpiece material is treated as elastic–viscoplastic with isotropic strain hardening, and the numerical solution accounts for coupling between plastic deformation and the temperature field, including treatment of temperature-dependent material properties. To avoid numerical errors associated with large deformation of elements, automatic remeshing is used, with at least 15 rezonings required to achieve a satisfactory solution. Effects of the uncertainty in the constitutive model on the distributions of strain, stress and temperature around the shear zone are presented, and the model is validated by comparing average values of the predicted stress, strain, strain rate and temperature at the shear zone with experimental results. Parametric effects associated with cutting speed and initial work temperature are considered in the simulations.     

SS304不锈钢在室温单轴循环加载下的时相关棘轮行为

对SS304不锈钢在室温单轴循环加载下的时相关棘轮行为和时相关应变循环特性进行了实验研究．讨论了材料在不同加载速率、不同保持时间和不同加载波形下的循环软／硬化行为和棘轮行为特征．结果表明：即使在室温下,SS304不锈钢也表现出明显的时相关效应;材料的循环变形行为,特别是棘轮行为明显地依赖于加载速率和保持时间以及加载波形的形状．研究得到了一些有助于时相关循环变形行为本构描述的结论．     

Experimental investigation of plastic deformation of stainless steel for cyclic loading at low temperature

Cyclic plastic deformation of stainless steel SUS 304 is experimentally investigated at low temperature of liquid nitrogen (77 K) under various cyclic loading conditions Thin walled tubular specimens are subjected to cyclic loading under constant strain ranges. At low temperature, the material shows remarkable hardening by cyclic loading comparing with cyclic loading at room temperature. The hysteresis curves of stress-strain relations by cyclic loading are saturated by increasing the cycle numbers. The saturation tendency depends on loading direction. The saturated stress values are related with cumulative plastic strain of cyclic loading. The prestraining is given at 77 K by axial and torsional loadings, and subsequent cyclic loading under constant strain range is conducted at 77 K. The cyclic stress-strain curves are saturated by increasing cyclic numbers. At small cyclic numbers, cyclic plastic deformation depends on the prestrain direction. The directional effect of pre-strain on cyclic loading becomes small with increasing number of cycles.     

Effect of High Strain-Rate Deformation and Aging Temperature on the Evolution of Structure,Microhardness, and Wear Resistance of Low-Alloyed Cu–Cr–Zr Alloy

The effect of the preliminary high strain-rate deformation, performed via the method of dynamic channel-angular pressing (DCAP), and subsequent annealings on the tribological properties of a dispersionhardened Cu–0.092 wt % Cr–0.086 wt % Zr alloy has been investigated. It has been shown that the surfacelayer material of the alloy with a submicrocrystalline (SMC) structure obtained by the DCAP method can be strengthened using severe plastic deformation by sliding friction at the expense of creating a nanocrystalline structure with crystallites of 15–60 nm in size. It has been shown that the SMC structure obtained by the high strain-rate DCAP deformation decreases the wear rate of the samples upon sliding friction by a factor of 1.4 compared to the initial coarse-grained state. The maximum values of the microhardness and minimum values of the coefficient of friction and shear strength have been obtained in the samples preliminarily subjected to DCAP and aging at 400°С. The attained level of microhardness is 3350 MPa, which exceeds the microhardness of the alloy in the initial coarse-grained state by five times.     

Determination of material constants and prediction of fracture in sintering of ceramic products

Methods of determining material constants required for finite element simulation of sintering of ceramic green bodies are proposed. The flow stress and shrinkage strain-rate in sintering are measured from a simple compression test with a constant load. The critical difference in volumetric strain in the criterion for sintering fracture is determined from a three-point bending test with a constant load. Using the measured material constants, the densification behaviour in compaction and sintering processes of circular tiles composed of a coloured upper layer and uncoloured substrate with a groove is simulated by the viscoplastic finite element method. The occurrence of sintering fracture is predicted from the difference of volumetric strain calculated by the finite element simulation. The accuracy of the calculated results is improved by using the flow stress with strain-rate and temperature sensitivities and the history of shrinkage strain-rate.     

Extension of Oxley's predictive machining theory for Johnson and Cook flow stress model

This paper presents an extension of Oxley's predictive analytical model for forces, temperatures and stresses at primary (shear zone) and secondary (tool-chip interface zone) deformation zone for Johnson and Cook flow stress model. The effect of strain in addition to strain-rate and temperature at tool–chip interface, which is ignored by many researchers, is considered in the present analysis. The extension is made inline with Oxley's predictive machining theory by introducing the term n_eq for Johnson and Cook material flow stress model. The term n_eq becomes strain hardening exponent (n) for power law flow stress model used by Oxley and can be found for other material models too. Johnson and Cook flow stress model that considers the effect of strain, strain-rate, and temperature on material property is widely used nowadays in finite element method simulation and analytical modeling due to its simple form and easy to use. The extension of Oxley's theory is verified for orthogonal cutting test data from the available literature for 0.38% carbon steel [Oxley, P.L.B., 1989. The Mechanics of Machining: An Analytical Approach to Assessing Machinability. Ellis Horwood Ltd., England] and AISI 1045 steel [Ivester, R.W., Kennedy, M., Davies, M., Stevenson, R., Thiele, J., Furness, R., Athavale, S., 2000. Assessment of machining models: progress report. Machining Science and Technology 4, 511–538] and found in good agreement.     

Themo-elasto-viscoplastic modelling of friction stir welding

Abstract

A coupled two-dimensional Eulerian thermo-elasto-viscoplastic model has been developed for modelling the friction stir welding process. First, a coupled thermo-viscoplastic analysis is performed to determine the temperature distribution in the full domain and the incompressible material flow around the spinning tool. Next, an elasto-viscoplastic analysis is performed outside the viscoplastic region to compute the residual stress. Both frictional heat and plastic deformation heat generation are considered in the model. Furthermore, this is the only known model computing residual stress accounting for plasticity caused by both thermal expansion and mechanical deformation due to material spinning. The computed residual stress is verified by comparing to experimentally measured data.     

差温无模预成形锻造的有限元模拟研究

利用热力耦合有限元方法 ,对AISI10 4 5钢圆柱体坯料在环形加热源局部加热下的差温无模预成形锻造过程进行了模拟研究。模拟结果表明 ,在坯料中获得和控制大梯度分布的不均匀温度场是实现无模预成形锻造的关键。复杂零件成形必须合理地组合加热源和冷却源 ,以便产生所需的温度梯度场。在快速变形条件下 ,变形过程中的持续加热对金属的流动影响甚微 ,变形前的温度分布基本上决定了金属的流动 ,但在慢速变形条件下 ,变形过程中的持续加热对金属的流动产生一定的影响 ,其影响程度视零件复杂程度而异。从该文研究结果可以预见 ,差温无模锻造可以发展成为一种无模锻造制坯新工艺     

Microstructural evolution and mechanical properties of niobium processed by equal channel angular extrusion up to 24 passes

We have systematically investigated the microstructural evolution of niobium (Nb) subjected to severe plastic deformation via equal channel angular extrusion (ECAE) up to 24 passes. The starting Nb billet material consists of a centimeter-scale grain size with a columnar structure. We have found that the grain size reduction of the Nb is almost saturated at ∼300 nm after eight passes of ECAE. However, the population of high-angle grain boundaries continues to increase with further ECAE, and no saturation appears to have been reached at 24 passes. We have evaluated the mechanical properties of the samples with different number of ECAE passes over a wide range of strain rates, from quasi-static to high strain rates. We have used strain-rate jump tests to examine the strain-rate sensitivity (SRS) of the processed samples and found that the SRS of the ECAE-processed Nb is ∼0.012, which is a factor of three smaller than that of the coarse-grained counterpart. The activation volume derived for plastic deformation indicates that the double-kink formation of screw dislocations is still the predominant deformation mechanism in the ECAE-processed Nb. Quasi-static true stress-strain curves exhibit elastic-nearly perfectly plastic behavior. The quasi-static yield strength is also nearly saturated after eight passes of ECAE. High-strain-rate compressive true stress-strain curves show uniform flow softening. However, the dynamic peak stress keeps rising with an increased number of ECAE passes, suggesting a strong grain boundary contribution to dynamic strengthening. Scanning electron microscopy of post-loaded surfaces displays a morphology of diffuse shear bands accompanying highly compressed grains. In our report, we demonstrate that grain boundaries of severely deformed metals play different roles at low, quasi-static vs. high-strain rates of mechanical loading. The difference is primarily determined by the strength of grain boundaries acting as dislocation barriers at different loading rates. This discovery is significant for the understanding of the effect of the microstructure as a function of the applied loading rate.     

喷丸强化过程及冲击效应的数值模拟

运用大型有限元软件LS-DYNA建立了喷丸强化处理过程的三维有限元模型, 研究了弹丸冲击作用下, 铝合金材料Al 2024-T3动态响应过程中的应力波结构、应变率效应及应力波衰减效应等动态参量; 研究了弹丸搭接率对于残余应力场的影响, 建立了高覆盖率多丸粒强化模型; 研究了冲击顺序、材料应变率及初始残余应力对强化效应的影响; 研究了喷丸强化后的表面微观变形特征. 分析结果表明, 弹丸高速冲击引起的弹塑性双波会在材料内部形成高应变率效应; 弹丸搭接率ζ对于强化效应有明显影响, ζ=1/2是近似的临界值; 不同冲击顺序对于强化效果的影响较小, 材料应变率对于强化效果有显著影响; 初始残余应力对于喷丸强化最终形成的残余应力场的影响取决于弹丸冲击速度; 经过喷丸强化处理, 零件表面形成微米级凹坑, 并且随着冲击次数和喷丸覆盖率的提高, 凹坑深度逐渐增加.     

TWIP钢的动态力学性能及变形机制研究

利用Zwick/Roell Z100万能材料试验机和Hopkinson拉杆对TWIP钢进行了准静态及动态力学性能的研究。基于力学实验结果,修正了Johnson-Cook动态本构模型中应变硬化项以及应变强化项。采用X射线衍射(XRD)、扫描电镜(SEM)、电子背散射衍射(EBSD)技术对TWIP钢拉伸变形前后的组织进行了观察与分析。结果表明：TWIP钢在准静态加载下表现为负应变率敏感性,动态加载时表现为正应变率敏感性。拉伸过程中,孪生诱发塑性是TWIP钢的主要变形机制,同时滑移也起到重要作用;动态加载下TWIP钢中形变孪晶的起始应变和孪晶体积分数均小于准静态加载过程;形变孪晶的生成以及孪晶相互作用导致的晶粒细化,使TWIP钢兼具高强度、高塑性及高动态吸能性能,在抗冲击、抗爆领域具有广泛的应用前景。     

Distribution of the carbide phase in the viscoplastic layer during the rotational friction-welding of various steels

The paper presents the viscoplastic phenomena observed during the rotational friction-welding process of high-speed steel M2 (AISI SAE) with carbon steel 1060 (AISI SAE). The structure, phase composition and distribution of the carbide phase in the friction plane and adjoining zones are examined by using optical microscopy (OM) with a system for quantitative analysis, and scanning electron microscopy (SEM) in addition to EDS analysis. The experimentally-checked mathematical relationship is suggested for the calculation of the radial movements of the carbide phase, immediately next to the friction plane. On the basis of qualitative and quantitative evaluation of the microstructural and rheological appearances, a model of carbide phase distribution is also established in the viscoplastic and viscous layers in the area of the friction plane.     

A study of axial compression of metallic hemispherical domes

The present paper deals with the experimental and computational analysis of the deformation behaviour of the metallic spherical shells subjected to axial compression. Axial compression of aluminium spherical shells of R/t values ranging from 26 to 45 was performed between two parallel platens. In experiments all the spherical shells were found to collapse in an axisymmetric mode. A finite element computational model of the development of the axisymmetric mode of collapse is also presented. In the proposed finite element model the material of the deforming dome has been idealized as rigid viscoplastic. Experimental and computed results are compared to validate the computational model. Effects of different process parameters on the deformation behaviour of the shells are presented and discussed.     

Effects of cyclic loading on temperature evolution of ULTIMET superalloy: experiment and theoretical modeling

High-speed, high-resolution infrared t hermography, as a non-contact, full-field, and nondestructive technique, was used to study the temperature variations of a cobalt-based ULTIMET alloy subjected to cyclic fatigue. During each fatigue cycle, the temperature oscillations, which were due to the thermal-elastic-plastic effects, were observed and related to stress-strain analyses. The change of temperature during fatigue was utilized to reveal the accumulation of fatigue damage . A constitutive model was developed for predicting the thermal and mechanical responses of ULTIMET alloy subjected to cyclic deformation. The model was constru cted in light of internal-state variables, which were developed to characterize the inelastic strain of the material during cyclic loading. The predicted stress -strain and temperature responses were found to be in good agreement with the e xperimental results.     

FE analysis on Deformation and Temperature Nonuniformity in Forming of AISI-5140 Triple Valve by Multi-Way Loading

In triple valve forming process by multi-way loading severely nonuniform deformation and temperature distributions are prone to occur, which may lead to poor forming quality and macro-micro defects. A 3D coupled thermo-mechanical rigid-viscoplastic finite element (FE) model for multi-way loading forming of AISI-5140 steel equal diameter triple valve was developed based on DEFORM-3D. Through comprehensive simulation and analysis, the influences of main process parameters on the forming process and nonuniformity of deformation and temperature were studied. The results showed that: (1) the degree of deformation nonuniformity decreased with the increase of the punch loading speed, initial temperature of billet, or the decrease of the friction factor; (2) the average temperature of forming body increased as the punch loading speed, initial temperature of billet and the friction increased, while the degree of temperature nonuniformity decreased with the increase of punch loading speed or decrease of initial billet temperature.     

超塑成形理论与实践——王仲仁教授在超塑性研究领域的贡献

回顾了王仲仁教授在超塑性研究领域的一系列重要研究成果。Sn-Pb共晶超塑性材料薄壁管复合加载实验表明,在复杂应力状态下超塑材料遵守Mises屈服准则,并与Tresca屈服准则也接近,给出了超塑材料在应变速率强化条件下的屈服轨迹;研究了超塑变形过程中晶粒和孔洞的长大规律及其对变形中流动应力的影响,导出了包含晶粒长大和孔洞长大影响的超塑性本构关系;提出了测定超塑材料摩擦系数的理论校准曲线和应变速率敏感性指数的变截面拉伸试验法;开发了带有动凸模的微机控制的1000kN超塑成形机,研制了当时国内最大的微机控制的5000kN超塑成形机;在模具型腔超塑成形研究方面,成功挤压了130型汽车连杆锻模,是迄今为止尺寸最大的超塑成形模具钢型腔。     

乙醇助燃剂燃烧环境中Q235碳钢的氧化行为研究

研究了常用的碳素结构钢在乙醇助燃剂火场环境气氛中的高温氧化行为,以期为初步判断典型火场温度、燃烧时间以及是否存在助燃剂提供依据。对Q235碳钢分别在乙醇燃烧气氛中进行5,10,20和30 min的氧化实验,并实测温度曲线,分析了其氧化行为和氧化层中单质碳的形成特点。结果表明,Q235碳钢在乙醇燃烧环境中形成网状氧化层,随时间的增加而发生起皱和剥落,在表面氧化层未检测到FeO物相;在氧化层和基体界面处沉积有游离状的碳单质,并且随着氧化时间的增加,碳含量增加。本实验温度条件下未形成FeO物相,氧化物的相组成特点可为火场的燃烧温度判别提供参考;乙醇热解产生的单质碳多沉积于表面能高的区域,如基体和氧化层界面处,也可为火灾现场是否使用乙醇助燃剂提供判断依据。     

考虑不连续屈服的钛合金高温变形本构模型

以发生不连续屈服的钛合金高温变形流动曲线特性为基础,讨论各阶段的变形机制。利用位错增殖动态理论和统一粘塑性理论,构建反映变形温度和应变速率影响且能描述不连续屈服、下屈服点后存在轻微应变硬化、动态再结晶等变形特性的高温粘塑性本构模型。将所建模型应用于新型亚稳β钛合金Ti2448发生明显不连续屈服的高温变形,并用改进的遗传算法确定模型中的相关材料参数。预测值与实验值误差在5%以内,表明这种基于内变量构建的本构模型不仅物理意义清晰,能够有效描述发生不连续屈服的钛合金高温变形,而且具有较强的外推能力,能为其它钛合金的本构模型构建提供参考。