热轧与平面应变压缩试验的有限元对比分析

通过Gleeble-1500热模拟试验机测得的铝合金压缩的实验数据,根据弹塑性热力耦合大变形有限元理论,建立了本构方程,获得了热轧过程中的数值仿真模型;分析了轧制过程中应力及应变率场的变化规律及平均应变率、摩擦系数等参数对轧制变形的影响,并与平面应变压缩条件下试件的变形情况进行了对比.计算结果表明:轧后轧件中心节点处的应变与平面应变压缩试验中的应变比较接近,但应变率的变化情况却存在较大的差别.提出的变压缩速度的平面应变压缩方法,能够更准确地模拟材料热轧过程中的变形和软化机制.     

高弹态未硫化橡胶在不同变形模式下的循环加卸载力学行为

对高弹态未硫化橡胶实施了不同变形模式(单轴拉伸和压缩)的循环力学实验,以考察其黏超弹性响应以及应变率对力学行为的影响。结果表明,材料的拉伸、压缩性能与加载速率和变形历史均有着显著的相关性。对于循环拉伸变形,应力-应变曲线的非线性特征较为明显,材料表现出类似于屈服的性质,且随着循环次数和变形量增加,迟滞损耗的积累趋势逐渐减弱;而对于循环压缩变形,应力-应变关系接近线弹性,且随着循环次数和变形量增加,材料刚度和迟滞损耗积累的趋势都逐渐增强。针对上述实验结果,在经典的Bergstrom-Boyce模型中引入损伤变量,对未硫化橡胶的应变率相关性和Mullins效应进行表征。理论计算结果表明,这种非线性黏超弹模型能够较好地描述未硫化橡胶在不同载荷条件下的变形响应与应力软化特征。     

微薄梁三点弯曲的尺度效应研究

应用高灵敏度的力传感器以及时间序列电子散斑干涉法,同时测出了不同厚度纯镍薄片三点 弯曲试件的抗力与变形,得到薄梁中心点处的载荷与挠度曲线. 应用Fleck和Hutchinson 的偶应力理论,结合平面应变弯曲模型,建立了薄梁处于弹性状态和弹塑性状态的 控制方程, 应用Runge-Kutta法进行数值求解,并将计算得到的载荷-挠度曲线以及无量纲化弯矩-表面 应变曲线和实验结果进行了比较. 在理论计算过程中,没有拟合任何材料参数,所有的材料 参数均来自实验测量的结果,材料特征尺度也是根据Stolken和Evans的工作给出 的. 结果表明: 应用偶应力理论预测的结果和实验结果符合良好,而经典理论的预测结果与 实验不相符合.     

改进的Jones—Nelson物理非线性材料模型与橡胶复合材料层板分析

Ｊｏｎｅｓ－Ｎｅｌｓｏｎ模型是复合材料物理非线性应力应变关系的一种描述方法，它通过建立材料刚度与应变能密度的关系以及考虑纤维增强复合材料的物理非线性问题，非线性的材料矩阵只为应变能的子数，使得材料模型可以方便地应用地复杂应力状态下，通过 在大变形的应变应力变量和变泊松比概念，对Ｊｏｎｅｓ－Ｎｅｌｓｏｎ模型进行了改进，解决了材料特性的扩展问题和收敛问题，同时考虑了大变形中纤维铺设角度的重新取向，使其     

有机玻璃非线性应力松弛行为表征与本构模型

通过实验测定了有机玻璃(PMMA)在不同初始应变率和初始应变下的应力松弛曲线。分析了三种应变率(0.001s~(-1)、0.01s~(-1)和0.1s~(-1))下不同初始应变(5%、7%、10%和15%)的实验结果。结果表明:初始加载应变率会影响有机玻璃高分子链段的运动速度,进而对应力松弛行为产生影响;不同初始应变对应力松弛的影响,则体现为不同变形程度对PMMA高分子链运动速度的影响。将PMMA的应力松弛视为两个阶段即应力快速松弛段和应力缓慢松弛段,基于对这两个阶段的分析,提出了一个非线性应力松弛本构模型。对实验得到的应力松弛曲线采用上面提出的本构模型进行数值拟合,吻合结果良好。此外,还对模型中参数的影响因素进行了分析讨论。结果表明该本构模型可以用于描述PMMA的非线性应力松弛行为。     

用于高强度材料的SHPB实验添加垫块法

提出了用于高强度材料的改进的SHPB实验方法添加垫块法,运用数值模拟方法,利用有限元程序LS-DYNA3D分析了添加垫块实验方法的合理性和可行性。根据一维应力波理论,给出了数据处理的修正方法。作为应用实例,采用改进的实验方法对高强度的Al2O3陶瓷材料的动态力学性能进行了研究,得到了比常规方法较高的应变率及应力应变范围的动态应力应变曲线,表明Al2O3陶瓷为应变率相关的非线性弹脆性材料。结果表明,添加垫块实验方法可有效地防止实验中压杆端面的变形,提高试件的应力应变及应变率水平。添加垫块实验方法为在SHPB装置上实现高强度材料的动态实验提供了一种方便实用的途径。     

铝板轧制过程中的理论与实验研究

本文采用三维弹塑性有限元法对铝板轧制过程进行了计算机模拟,文中同时考虑材料、几何双重非线性,利用 Update-Lagrangian 叠加原理,对铝板轧制从非稳态到稳态进行了模拟计算.首次应用密栅云纹法研究了铝板横向变形.理论计算结果同实测结果有较好的吻合.文中还给出了一些实验难于处理的金属流动信息和应力分布信息.     

高强度钢板热成形本构理论与实验分析

热成形(热冲压)过程中硼钢的热、力、相变耦合关系是研究热成形理论的基础, 同时也是决定热成形工艺及数值模拟准确性的关键因素. 对热成形硼钢进行高温拉伸及淬火实验: 硼钢板材试样在奥氏体化(950℃)后保温一定时间, 然后在连续冷却的同时施加拉伸力, 记录此过程中力、位移、膨胀量及温度的变化. 通过对不同冷却速率及不同拉伸力情况下上述物理量的变化规律及微观组织性能的分析, 研究硼钢相变过程中的热、力、相变耦合关系. 建立了硼钢相变过程中的热、力、相变耦合模型. 通过引入混合定律对热成形过程中的多相材料热力学参数和力学性能进行等效分析; 对热成形应变组成及其形成机理进行了分析, 引入了相变体积应力及相变塑性应力等新概念. 硼钢高温流动应力采用修改的Norton-Hoff形式, 并通过实验确定了流动应力的材料常数. 在此基础上将热、力、相变耦合关系引入热成形本构方程中, 分别建立了高强度钢板热成形的全量形式及增量形式本构方程. 对U形零部件热成形过程进行了数值模拟, 并与实验结果进行比较, 结果证明建立的本构理论的有效性.      

ARALL 材料拉伸力学性能的试验研究

利用自行研制的旋转盘式间接杆杆型冲击拉伸试验装置对ＡＲＡＬＬ材料以及施加预应力的ＡＲＡＬＬ材料进行了３个应变率（２００、５００、１３００ｓ－１）的冲击拉伸试验，得到了两种材料在不同应变率下的完整的应力应变曲线。结果表明ＡＲＡＬＬ材料在高速加载条件下的变形可以分为弹性变形、塑性变形和材料失稳后的残余变形三个部分，每个部分都有不同的变形机理。结果还表明，随着应变率的增加两种材料的屈服应力、失稳应力以及失稳应变均相应增加，表现出明显的应变率强化和动态韧性现象。最后根据材料在不同应变率下的试验结果，建立了ＡＲＡＬＬ材料计及应变率影响的三段线性本构模型。     

可压缩粘弹性材料Ⅱ型动态扩展裂纹尖端场

为了研究粘性效应作用下的动态扩展裂纹尖端渐近场,建立了可压缩粘弹性材料II型动态扩展裂纹的力学模型,推导了可压缩材料Ⅱ型动态扩展裂纹的本构方程.在稳态蠕变阶段,弹性变形和粘性变形同时在裂纹尖端场中占主导地位,应力和应变具有相同的奇异量级r-1/(n-1).通过渐近分析求得了裂纹尖端应力、应变和位移分离变量形式的渐近解,并采用打靶法求得了裂纹尖端应力、应变和位移的数值结果,给出了应力、应变和位移随各种参数的变化曲线.数值计算表明,弹性变形部分的可压缩性对Ⅱ型裂尖应力场影响甚微,而对应变场和位移场影响较大.裂尖场主要受材料的蠕变指数n和马赫数M的控制.当泊松比ν =0.5时,可以退化为不可压缩粘弹性材料Ⅱ型动态扩展裂纹.     

Superplastic characteristics of high purity yttria-stabilized zirconia polycrystals

The mechanical characteristics of superplastic yttria-stabilized zirconia polycrystals have been analyzed as a function of stress, temperature and grain size. The evolution of the stress exponent n with stress found in high purity materials is similar to that observed in superplastic metals. True creep parameters can be ascribed to the deformation mechanism at high stresses. By contrast, the creep parameters exhibit a continuous evolution with stress, temperature and grain size at low stresses. The threshold stress formalism used in conventional and high strain rate superplastic metals accounts for the mechanical characteristics observed in fine-grained zirconia polycrystals.     

薄膜多层滚压的表面粗糙度晶体学分析

提出了晶体学模型来研究多层薄膜滚压的表面粗糙度。研究了单层滚压模型,提出滚压的三维模型和二维模型（平面应变）,讨论了晶粒大小和应力状态的影响。把单个单层滚压和多层滚压模型、三维模型和平面应变模型进行了比较,发现对于所涉及的二维和三维模型来说,单层滚压和多层滚压模型并无明显的差别。因此在这些情况下,可以用单个单层滚压模型来代替多层滚压模型从而节省CPU时间。     

结构强度在线分析的光电复合法

本文将电测方法与光弹性方法结合在一起,导出了应用读数应变εd就可以获取准静态情况下结构的载荷P,危险截面位置和最大应力峰值σmax(X0Y0Z0)的理论计算方法,从而为结构的在线强度评价提供一种实验分析方法,文中导出的εd-t,P-t,k-t及σ－t诸曲线谱的相关性理论,不仅揭示了εd与未知载荷P,无量纲应力系数K及应力σ之间的关系,更重要的是这种分析思想与计算机技术结合在一起,可以达到设备强度自动报敬警的作用,作为一个实例,作者应用本理论成功地解决了某钢厂热轧线上一个粗轧机机架在线的应力强度评价难题。     

Path dependence and multiaxial behavior of a polycrystalline NiTi alloy within the pseudoelastic and pseudoplastic temperature regimes

Several multiaxial experiments on polycrystalline NiTi have been conducted within a wide temperature range. In this vein, the pseudoelastic as well as the pseudoplastic behavior are investigated within the distinct temperature regimes. Isothermal and temperature varying thermomechanical loading paths are applied by means of an active temperature control in order to characterize the path dependence of pseudoelasticity and the multiaxial one-way effect of the alloy. The main focus is on the determination of the dependence of the loading sequence, the related non-linearity of the material and the combined material interaction, e.g., referring to reorientation processes for complex loading paths with respect to pseudoelasticity and the one-way effect. Isothermal tension/compression/torsion experiments are performed on an austenitic microstructure spanning all four quadrants of the axial/torsional strain subspace. In this regard, it is deduced in the course of this contribution that the apparently qualitatively different material behavior for different strain paths in the pseudoelastic temperature regime might be explained by the axial/torsional and tension/compression asymmetry. Furthermore, some multidimensional axial/torsional stress controlled experiments are realized with loading on a martensitic and unloading being implemented both on martensitic and austenitic microstructures. Here, the peculiarity of the one-way effect referring to apparently different transformation temperatures is ascribed to the loading history of the specimen material and to differently oriented martensite variants. In order to elucidate these effects, potential explanations for the pseudoelastic path dependence and the non-linearity in the material behavior with reference to the multiaxial one-way effect are presented.     

Stress relaxation,creep and set recovery of elastomers

The stress relaxation, creep and recovery behaviour of a cross-linked unfilled natural rubber has been investigated at moderate stresses in tension. The aim being to extend the idea, initially developed by Alan Gent in his seminal 1962 paper on the relaxation behaviour of rubber, in order to understand and examine the time dependent mechanisms that are present in elastomers under strain. A method based upon the Boltzmann superposition principle was used to compare the creep compliance with a measurement of its recovery after release from a range of constant loads held for different times. The creep behaviour was seen to exhibit the usual linear dependence on the logarithm of time. The recovery data was also seen to reduce onto a single recovery curve for any given applied tensile stress for a range of loading times using the Boltzmann superposition principle. The differences between the relative rates of the creep and the recovery behaviour can in part be attributed to the non-linearity in the stress–strain behaviour exhibited in tension of the elastomer.     

Temperature development based on technological analysis: Fast rolling as an example

Summary The mean rise in temperature with plastic strain due to the work of plastic deformation, taking account of the temperature-dependence of yield stress, is investigated when conduction effects are neglected and results for one particular example are presented. An adaptation of the well-known Siebel-Karman analysis for sheet rolling is examined, and results for the particular case of rolling a medium carbon steel are given in detail. The temperature-dependent property of the yield stress in cold-rolling at moderate reductions is shown not to have a large effect on factors such as roll-torque, though the actual temperature increase generated is a significant one. Examples are given of how the temperature of the material increases in its passage through the roll gap.     

The prediction of time-dependent non-linear stresses in viscoelastic materials: I. Selection of constitutive equation and strain measure

The theories for the prediction of time-dependent, non-linear stresses in viscoelastic materials such as polymers are reviewed, and it is noted that the commonly observed stress non-linearity may be ascribed either, as is usually done, to memory-function non-linearity or, alternatively, to strain-measure non-linearity. To investigate the latter alternative whilst retaining a general memory-function non-linearity, a single-integral constitutive equation of the Bird—Carreau type is employed but with an arbitrary strain measure I in place of the normally employed Finger tensor F. This model includes as special cases a large proportion of the constitutive equations previously employed for predictive purposes and in particular with a linear memory function it is shown to be indistinguishable, with the normally conducted shear experiments, from the successful BKZ model.In the new model the shear component I₁₂ of the strain measure can be found from experimental results obtained in the startup of steady shear flow, without specification or restriction of memory-function non-linearity. The form of I₁₂ found from experiment is quite non-linear in shear a for ¦a¦> 2, and hence differs from the F tensor for which F₁₂ = a. The same form for I₁₂ found for a variety of polymer solutions and a polymer melt and consequently a simple function describing I₁₂ is proposed as a new, material-independent, strain measure.     

Initial thermo-mechanical post-buckling of beams with temperature-dependent physical properties

The objective of this work is to analyze the elastic buckling and initial post-buckling behavior of slender beams subjected to uniform heating. The beams are assumed to be double-hinged with fixed ends, preventing thermal expansion. Consequently, destabilizing compressive forces arise that may lead to beam buckling. When the temperature is further increased, the beam experiences finite displacements, with the result that the analysis is geometrically non-linear. The modulus of elasticity and the thermal induced strain, key material properties for this problem, are temperature-dependent. Thus, the coefficients of the governing equations are not constant. This suggests the physical non-linearity of the mathematical model. Hence, the analysis is geometrically and physically non-linear. The analysis is sensitive to the beam initial temperature, as the thermal strain is a function of the initial and final temperatures. The material is considered to be linear elastic, and consequently viscoelastic and plastic effects are not taken into account. Furthermore, the beam cross-section properties are assumed to be constant, which is consistent with the small strain formulation. A perturbation method is applied to the governing non-linear differential equations so that the initial post-buckling behavior may be analytically determined when temperatures above the critical temperature are applied to the beam. To illustrate the application of the formulation we present a case study for the aluminum 7075-T6 alloy, a material commonly used in aerospace and naval industries. Nonetheless, it is expected similar behavior for other metallic materials. The curves that define the variation of the modulus of elasticity, the thermal strain and the yield stress with temperature are considered in our analysis. The change in length, reaction forces at the supports and geometric configurations are obtained as a function of temperature and the beam slenderness ratio. The critical buckling loads and temperatures and the initial post-buckling analysis are also calculated in the context of the temperature-independent physical properties. Our results emphasize the importance of modeling the material's non-linearity if accuracy is required. However, from a practical application point of view results are acceptable if temperature-independent physical properties are employed, especially for large slenderness ratios.     

Nonlinear vibration analysis of micro-plates based on strain gradient elasticity theory

In this study, non-linear free vibration of micro-plates based on strain gradient elasticity theory is investigated. A general form of Mindlin’s first-strain gradient elasticity theory is employed to obtain a general Kirchhoff micro-plate formulation. The von Karman strain tensor is used to capture the geometric non-linearity. The governing equations of motion and boundary conditions are obtained in a variational framework. The Homotopy analysis method is employed to obtain an accurate analytical expression for the non-linear natural frequency of vibration. For some specific values of the gradient-based material parameters, the general plate formulation can be reduced to those based on some special forms of strain gradient elasticity theory. Accordingly, three different micro-plate formulations are introduced, which are based on three special strain gradient elasticity theories. It is found that both geometric non-linearity and size effect increase the natural frequency of vibration. In a micro-plate having a thickness comparable with the material length scale parameter, the strain gradient effect on increasing the non-linear natural frequency is higher than that of the geometric non-linearity. By increasing the plate thickness, the strain gradient effect decreases or even diminishes. In this case, geometric non-linearity plays the main role on increasing the natural frequency of vibration. In addition, it is shown that for micro-plates with some specific thickness to length scale parameter ratios, both geometric non-linearity and size effect have significant role on increasing the frequency of non-linear vibration.