双向应力状态下铝硅合金ZL105塑性行为的实验研究

本文对典型的拉、压性能不同的金属材料铝硅合金ZL105的塑性行为,屈服、强化、体积变化和流动特性,在双向应力状态下进行了全面和系统的实验研究,深入地探讨了影响材料塑性行为的主要因素,得出了新的结果和重要的结论。实验结果表明,平均应力直接影响材料的塑性行为,平均压应力使材料的韧性增强,而平均拉应力使材料的脆性增强。平均压应力不利于塑性变形的发生,相应平均压应力增大的应力状态,材料的屈服应力提高。与流行的塑性体积膨胀的观点不同,在平均应力是压应力的应力状态下,塑性体积收缩,因此,经典的正交准则不再适宜描述该材料的流动性质。材料的强化性质明显地依赖于应力状态。     

软化Mohr-Coulomb材料强度参数的测定方法

为了得到试件的粘聚力和内摩擦角随轴向塑性压应变变化的曲线提出本方法。试件的弹塑性本构关系遵循相关联的Mohr-Coulomb强度准则;对常规三轴试验,试件受力进入塑性状态后,处在棱椎状屈服面的棱上,加载过程遵循Koiter流动法则。按经典塑性力学理论,推导得到轴向塑性压应变与轴向应力与轴向应变的关系;在常规三轴试验机上获得不同围压下试件的全程应力-应变曲线,进而可得到各自围压下轴向塑性压应变随加载过程的变化曲线;把来自不同围压下对应同一轴向塑性压应变的应力分别代入屈服面方程,即可求得对应的粘聚力和内摩擦角。结果表明,Mohr-Coulomb材料的两个强度参数的变化由轴向塑性压应变确定。轴向塑性压应变可以作为塑性变形的状态参数,它和试件的受力过程可以唯一确定试件的变形过程。     

软化材料厚壁筒的解析解及其稳定性分析

将弹塑性材料的应力应变全过程曲线简化为三线性模型(弹性-线性软化-残余理想塑性),并假设材料服从Tresca屈服准则和关联流动法则,推导出受内压厚壁筒的解析解.在这个解析解的基础上,讨论了厚壁筒的平衡稳定性问题,内压达到临界载荷时,厚壁筒丧失稳定性,其临界载荷就是软化塑性材料厚壁筒的承载能力.     

he analytical solutions of thick-walled cylinder of softening material and its stability

将弹塑性材料的应力应变全过程曲线简化为三线性模型(弹性-线性软化-残余理想塑性), 并 假设材料服从Tresca屈服准则和关联流动法则,推导出受内压厚壁筒的解析解. 在这个解析解 的基础上,讨论了厚壁筒的平衡稳定性问题,内压达到临界载荷时,厚壁筒丧失稳定性,其 临界载荷就是软化塑性材料厚壁筒的承载能力.     

不同拉压特性的厚壁圆筒极限内压统一解

厚壁圆筒在实际工程领域中应用广泛,若能精确计算出极限内压,对预防事故发生,降低风险有重要意义.工程中存在许多材料,其拉压强度和拉压模量均存在差异,这些差异对极限内压的大小有显著影响.以往研究表明,仅考虑拉压强度与拉压模量的一个方面,计算结果与实际情况存在一定的误差.本文基于双剪统一强度理论,综合考虑中间主应力效应及材料拉压强度和拉压模量的不同,推导了内压作用下厚壁圆筒的弹、塑性状态的应力分布及弹性极限内压、塑性极限内压与安定极限内压的统一解,通过与其他文献对比分析验证了本文计算结果的正确性,分析了半径比、统一强度理论参数、拉压强度比与拉压模量系数对弹性极限内压、塑性极限内压及安定极限内压的影响.结果表明:统一解均随半径比和统一强度理论参数的增大而增大,随拉压强度比的增大而减小,弹性极限内压随材料拉压模量系数的增大而减小,当壁厚增加到一定值后,安定极限内压随材料拉压模量系数的增大而减小;材料的拉压模量不同、拉压强度差异对厚壁圆筒的安定性影响显著,考虑中间主应力效应可使材料的潜能得到更充分发挥,极限内压随半径比的变化规律可为选择合理壁厚提供参考,该结论可为厚壁圆筒的工程应用提供理论依据.     

拉压性能不同材料厚壁圆筒与厚壁球壳的极限压力

本文就拉压屈服极限不同的理想弹塑性材料厚壁圆筒及厚壁球壳在内压力作用下进行了应力分析，得到了依赖于压拉比的弹性极限载荷与塑性极限载荷．由分析结果可见，拉压性能不同材料的弹性极限载荷与塑性极限载荷均有所提高，并且随着壁厚的增加提高量也有显著增加．     

拉压性能不同材料厚壁圆筒与厚壁球壳的极限压力

本文就拉压屈服极限不同的理想弹塑性材料厚壁圆筒及厚壁球壳在内压力作用下进行了应力分析，得到了依赖于压拉比的弹性极限载荷与塑性极限载荷。由分析结果可见，拉压性能不同材料的弹性极限载荷与塑性极限载荷均有所提高，并且随着壁厚的增加提高量也有显著增加。     

一种散体材料SHPB被动围压试验体应力修正方法

本文利用有限元仿真给出了一种修正方法，并用数值仿真和试验验证了该方法的可靠性。研究表明:散体材料SHPB被动围压试验中，试样厚度远小于厚壁圆筒长度时，端部效应会导致厚壁圆筒不均匀凸出变形，计算材料的体应力-应变关系不能将厚壁圆筒应力状态简化为平面应力问题；厚壁圆筒处于弹性状态下，通过厚壁圆筒理论计算出的径向力与真实径向力存在一定比例关系，在一定范围内，折算系数与试样实时厚度呈二次函数关系。     

考虑拉压强度差效应的厚壁圆筒承载能力分析

应用双剪统一强度理论，考虑材料的拉压异性和同性，推导了在内压力和轴力联合作用下的厚壁圆筒的塑性极限载荷表达式．在该表达式中，当反映中间主应力效应的系数取不同的值时，就能得到按Tresca屈服准则、线性逼近的Mises屈服准则和双剪应力屈服准则的计算结果，并且绘制了在相应准则下的极限应力线图．从而可知：在三维应力状态下，应用该理论，可以获得极限载荷分析的精确解；极限载荷线图与三种屈服准则的屈服曲线是相吻合的；计算的结果可以用于拉压异性和同性的材料，为工程应用提供了理论依据．     

拉压性能不同材料厚壁圆筒和厚壁球壳的极限压力分析

本文用广义双剪应力强度理论对拉压性能不同的材料制成的厚壁圆筒和厚壁球壳进行了弹塑性应力分析，得出与拉压比有关的弹性极限内压力、塑性极限内压力、弹塑性区的应力以及弹塑性内压力与弹塑性半径之间的关系式．     

孪晶对多晶纯钛塑性变形影响的实验研究

本文介绍了不同温度下多晶纯钛试件的单调拉伸、压缩试验和不同应变的压缩加卸载试验及其结果,同时介绍了拉断试样以及不同压缩应变下卸载试样的金相观察结果。通过直接比较不同温度下多晶纯钛的单调拉伸、压缩试验结果以及相应的金相分析,得到了由孪晶单独引起的流动应力的增加与应变之间的定量关系。本文还根据Hall-Petch关系和孪晶体积分数的演化方程,建立了孪晶引起的流动应力的增加与应变之间的唯象解析关系。试验结果表明此唯象关系较好地描述了孪晶对多晶纯钛塑性变形的影响。     

Correlation between swift effects and tension–compression asymmetry in various polycrystalline materials

The Swift phenomenon, which refers to the occurrence of permanent axial deformation during monotonic free-end torsion, has been known for a very long time. While plastic anisotropy is considered to be its main cause, there is no explanation as to why in certain materials irreversible elongation occurs while in others permanent shortening is observed.In this paper, a correlation between Swift effects and the stress–strain behavior in uniaxial tension and compression is established. It is based on an elastic–plastic model that accounts for the combined influence of anisotropy and tension–compression asymmetry. It is shown that, if for a given orientation the uniaxial yield stress in tension is larger than that in compression, the specimen will shorten when twisted about that direction; however, if the yield stress in uniaxial compression is larger than that in uniaxial tension, axial elongation will occur. Furthermore, it is shown that on the basis of a few simple mechanical tests it is possible to predict the particularities of the plastic response in torsion for both isotropic and initially anisotropic materials. Unlike other previous interpretations of the Swift effects, which were mainly based on crystal plasticity and/or texture evolution, it is explained the occurrence of Swift effects at small to moderate plastic strains. In particular, the very good quantitative agreement between model and data for a strongly anisotropic AZ31–Mg alloy confirm the correlation established in this work between tension–compression asymmetry and Swift effects. Furthermore, it is explained why the sign of the axial plastic strains that develop depends on the twisting direction.     

Microscopic modelling of volume expansion and pressure dependence of plastic yielding in crystalline solids

Summary The present paper deals with a nonlinear finite element procedure for the macroscopic rate-independent analysis of micromechanics of crystalline solids undergoing finite elastic-plastic deformations including plastic volume expanding effects. Particular attention is focused on deviations from the Schmid law of the critical resolved shear stress to model different stress-deflection behavior in uniaxial tension and compression tests. Numerical simulations of elastic-plastic boundary-value problems should demonstrate the efficiency of the algorithm and discuss the influence of different model parameters on stress-carrying and deformation behavior observed experimentally. Accepted for publication 3 June 1997     

Axial compression and energy absorption characteristics of high-strength thin-walled cylinders under impact load

Non-linear finite element software LS-DYNA is used to analyze the axial compression behavior and energy absorption of a high-strength thin-walled member under an impact load. To elucidate the effect of dynamic impact on the strain rate, the Cowper-Symonds equation is applied to analyze the plastic state of stress and the onset of dynamic yielding under different strain rates, such that the modeled deformation behavior of the member is consistent with the actual situation. Results for the thin-walled members made of mild steel and dual phase steel are compared. Assuming two different materials with equal sectional areas, an analysis confirms that the energy absorption of high-strength steel thin-walled component is better than the mild steel thin-walled component. Hence, thin-walled tubes made of high-strength steel are investigated using a series of analysis. The relationships between displacement and load, average load and energy absorption properties are obtained.     

Three-dimensional micromechanical modeling of voided polymeric materials

A three-dimensional micromechanical unit cell model for particle-filled materials is presented. The cell model is based on a Voronoi tessellation of particles arranged on a body-centered cubic (BCC) array. The three-dimensionality of the present cell model enables the study of several deformation modes, including uniaxial, plane strain and simple shear deformations, as well as arbitrary principal stress states.The unit cell model is applied to studies on the micromechanical and macromechanical behavior of rubber-toughened polycarbonate. Different load cases are examined, including plane strain deformation, simple shear deformation and principal stress states. For a constant macroscopic strain rate, the different load cases show that the macroscopic flow strength of the blend decreases with an increase in void volume fraction, as expected. The main mechanism for plastic deformation is broad shear banding across inter-particle ligaments. The distributed nature of plastic straining acts to reduce the amount of macroscopic strain softening in the blend as the initial void volume fraction is increased. In the case of plane strain deformation, the plastic flow is observed to initiate across inter-particle ligaments in the direction of constraint. This particular mode of deformation could not have been captured using a two-dimensional, plane strain idealization of cylindrical voids in a matrix.The potential for localized crazing and/or cavitation in the matrix is addressed. It is observed that the introduction of voids acts to relieve hydrostatic stress in the matrix material, compared to the homopolymer. It is also seen that the predicted peak hydrostatic stress in the matrix is higher under plane strain deformation than under triaxial tension (with equal lateral stresses), for the same macroscopic stress triaxiality.The effect of void volume fraction on the macroscopic uniaxial tension behavior of the different blends is examined using a Considère construction for dilatant materials. The natural draw ratio was predicted to decrease with an increase in void volume fraction.     

Effect of surface/interface stress on the plastic deformation of nanoporous materials and nanocomposites

Surface and interface play an important role on the overall mechanical behaviors of nanostructured materials. We investigate the effect of surface/interface stress on the macroscopic plastic behaviors of nanoporous materials and nanocomposites, where both the surface/interface residual stress and surface/interface elasticity are taken into account. A new second-order moment nonlinear micromechanics theory is developed and then reduced to macroscopically isotropic materials. It is found that the effect of surface/interface residual stress is much more prominent than that of the surface/interface elasticity, causing strong size effect as well as asymmetric plastic deformation for tension and compression. The variation of yield strength is more prominent with smaller pore/inclusion size or higher pore/inclusion volume fraction. For a representative nanoporous aluminum, the surface effect becomes significant when the pore radius is smaller than about 50 nm. When hard inclusions are embedded in a ductile metal matrix, the interface effect and resulting size effect are much smaller than that of nanoporous materials. The results may be useful for evaluating the mechanical integrity of nanostructured materials.     

Measurement of curved-surface deformation in cylindrical coordinates

Stereo vision is used to measure the strain field of a round tension test specimen in a cylindrical coordinate system. Initially, the displacement fields of the specimen are measured relative to a world coordinate system erected by the stereo vision. Through coordinate transformations, the measured displacement fields expressed in world coordinates are then converted to the displacement fields expressed in cylindrical coordinates. By differentiating the axial and circumferential displacements in the axial and circumferential directions, the axial, circumferential and shear strains are determined. Results indicate that the measured mean value of the axial strains is in good agreement with the measurements of the extensometer and the strain gage. The Poisson's ratio obtained by the circumferential and axial strains is close to .33 in the elastic state. The mean error of the computed shear strain is approximately .03 percent in the smaller elastic deformation and .08 percent in the larger plastic deformation.     

拉压异性线性等向强化材料厚壁球壳极限分析

运用Mohr屈服准则对承受内压的拉压屈服强度不同的线性等向强化材料的厚壁球壳进行了极限载荷分析，得到了依赖于拉压比和强化模量的厚壁球壳极限载荷解析式和依赖于拉压比和强化模量的厚壁球壳极限载荷分析式。结果表明，材料拉压屈服强度的不同和强化特性对厚壁球壳极限载荷均有一定的影响。     

An experimental study of inhomogeneous cyclic plastic deformation of 1045 steel under multiaxial cyclic loading

An experimental study was conducted on the inhomogeneous cyclic plastic deformation of 1045 steel under multiaxial cyclic loading. Thin-walled tubular specimens were used and small strain gages were bonded on the specimen surface to characterize the local deformation. The controlled loading paths included cyclic tension–compression, cyclic torsion, proportional axial-torsion, 90°-out-of-phase axial-torsion, and fully reversed torsion with a constant axial stress. The maximum stress in each experiment was lower than the lower yield stress of the material. It was found that the cyclic plastic deformation within the gage section of the specimen under multiaxial stress state followed the three-stage process that was observed from uniaxial loading, namely, incubation, propagation, and saturation. The plastic deformation was significantly inhomogeneous during the propagation stage, and the inhomogeneity continued through the saturation stage. The duration of each stage and the saturated strains were dependent on the cyclic stress amplitude and the loading path. Multiaxial stress state reduced the incubation stage. With identical equivalent stress magnitude, the nonproportional loading path resulted in the shortest incubation and propagation stages, and the saturated equivalent plastic strain magnitude was the smallest. Although the deformation over the gage section was inhomogeneous, the plastic deformation in a given local area was found to be practically isotropic.