延性材料全阶段单轴本构关系获取方法

材料拉伸直至破坏的单轴本构关系对于材料与结构的大变形分析和裂纹问题中的变形行为研究有重要意义。该文提出一种将实验与数值分析相结合获取材料拉伸直至颈缩破断全程本构关系的新方法——TF法(tensile test-finite element method)。通过构造初始微小缺陷实现拉伸试样的颈缩有限元模拟;应用自制对中夹具,结合DIC(digital image correlation)方法和应变传感器来测量材料拉伸全程的标距位移、试样颈缩根部直径和颈缩区轮廓线;应用逐步逼近原理并借助开发的ANSYS APDL命令流程序实现材料全程单轴真实本构关系求解的迭代计算。应用TF方法得到的全程材料本构关系对SS316L和T225NG合金拉伸试样的颈缩行为进行有限元模拟分析,结果表明:颈缩区两端的载荷-位移曲线、最小截面直径-位移曲线和颈缩区轮廓线的数值分析结果与实验结果吻合良好;对SS316L小曲率半径漏斗试样的载荷-位移曲线的模拟结果也与实验结果吻合良好。还给出SS316L和T225NG合金两种材料的Ramberg-Osgood全程单轴本构模型参数与等效破断应力应变,讨论两种材料单轴拉伸试样破断时颈缩根部横截面的应力分布。     

应力三轴度和应变率对6063铝合金力学性能的影响及材料表征

对6063铝合金试样在不同应力三轴度和不同应变率下进行拉伸试验,得到了该合金在这两种情况下的力学性能.研究结果表明:随着应力三轴度的减小,材料的等效弹性模量、等效屈服应力减小,但等效断裂应变增大;随着应变率逐渐增大,材料的屈服强度和断裂强度略有增大;断裂应变明显减小;抗拉强度基本不变.Johnson-cook本构模型及其断裂应变模型可以用来描述6063铝合金在不同三轴应力度和不同应变率下的本构及失效关系.通过材料表征,得出了Johnson-cook本构模型及其断裂应变模型中各个参数,为有限元(ABAQUS)模拟提供帮助.     

10Ni5CrMoV钢循环塑性变形的力学本构关系

针对屈服强度为785 MPa级别的调质态10Ni5Cr Mo V钢开展了系列应变幅的循环加载试验,并分析了循环应力应变特性,结果表明:循环塑性变形条件下表观弹性模量随应变幅的增大呈线性降低特点;循环塑性应变显著降低屈服强度,且屈服强度随应变幅的增大呈线性降低特点;塑性变形段的真应力和真应变塑性分量的对数呈良好的线性关系。基于上述结果提出了描述循环应力应变关系的单对数强化弹塑性本构关系模型σ=σp+χ·lgεp,并建立了10Ni5Cr Mo V钢循环应力应变本构方程。     

超高性能混凝土梁正截面受弯承载力理论研究

基于64组超高性能混凝土（ultra high performance concrete, UHPC）抗压性能试验数据,分别建立了峰值压应变ε₀、立方体抗压强度f_cu与轴心抗压强度f_c之间的关系以及弹性模量E_c与立方体抗压强度f_cu的关系;基于复合材料力学,建立了受拉区UHPC等效拉应力;基于平截面假定,建立了UHPC梁正截面受弯承载力计算公式,推导了受压区等效矩形应力图形参数、计算公式,并结合UHPC受压本构确定等效矩形应力图形参数。通过28根试验梁的相关数据,验证UHPC梁正截面受弯承载力计算公式及等效矩形应力图形参数取值的可行性。研究结果表明,等效矩形应力图形参数取值较为合理,梁正截面受弯承载力计算值与试验值吻合良好。     

2024-T42铝合金低中应变率力学性能及本构关系

为研究2024-T42铝合金的低中应变率力学性能和本构关系,采用电子万能实验机和高速液压伺服材料实验机,进行常温下2024-T42铝合金准静态、低中应变率拉伸实验,得到材料在不同应变率下的应力-应变曲线。考虑颈缩对真实应力-真实应变的影响,采用仿真反演方法对颈缩后的真实应力-真实应变曲线进行修正,并基于Johnson-Cook本构模型进行拟合。结果表明:2024-T42铝合金在低中应变率范围的率敏感性较弱;但具有较强的应变硬化效应;基于仿真分析的反演修正方法能较好重构材料颈缩点后的真实应力-真实应变曲线;并通过铝管压溃实验和仿真分析,验证了反演修正方法的合理性和所获本构模型参数的准确性。     

拉伸试样颈缩过程中I1的分布函数

采用拉伸试样颈缩区应力方程的Bridgman解推导了颈缩区最小截面上的应力第一不变量I1的计算公式,并对EH50钢试样拉伸过程中颈缩区不同位置的应力分布进行了分析.结果表明,圆拉试样拉伸至不同颈缩程度时,应力第一不变量I1值从最小截面中心处到外边缘随r/a变化呈幂函数分布,颈缩区截面中心处I1随应变量呈线性增加,相关性方程:I1=517+1153ε.     

弹塑性本构关系的一种新提法

本文基于应力空间和应变空间的概念,提出了一种能够概括目前广泛应用的Reuss和Hill本构方程的新的弹塑性本构方程。在新的本构方程中,塑性应变增量和应力之间的关系是与塑性常数矩阵相联系的,根据对塑性常数矩阵本征值的讨论,可以将一般的各向异性塑性体划分成全异性和半异性塑性体。最后本文还给出了适合于一般塑性体的广义等效应力和广义等效应变的定义。     

0Cr18Ni24Mo6N奥氏体焊缝金属循环塑性变形的力学本构关系

针对0Cr18Ni24Mo6N奥氏体焊缝金属开展了系列应变幅的循环加载试验,并分析了循环应力应变特性。结果表明:循环塑性变形条件下表观弹性模量随应变幅的增大呈线性降低特点;循环塑性应变显著降低屈服强度,且屈服强度随应变幅的增大呈线性降低特点;塑性变形段的真应力和真应变塑性分量满足单对数强化弹塑性本构关系,并建立了0Cr18Ni24Mo6N奥氏体焊缝金属循环应力应变本构方程。     

OCr18Ni24Mo6N奥氏体焊缝金属循环塑性变形的力学本构关系

针对OCr18Ni24Mo6N奥氏体焊缝金属开展了系列应变幅的循环加载试验,并分析了循环应力应变特性.结果表明:循环塑性变形条件下表观弹性模量随应变幅的增大呈线性降低特点;循环塑性应变显著降低屈服强度,且屈服强度随应变幅的增大呈线性降低特点;塑性变形段的真应力和真应变塑性分量满足单对数强化弹塑性本构关系,并建立了OCr18Ni24 Mo6N奥氏体焊缝金属循环应力应变本构方程.     

基于等效全程单轴本构关系的应力三轴度分析

该文利用有限元辅助测试(Finite element Aided Testing,FAT)方法测得了四种工程材料的等效全程单轴本构关系,基于此全程本构关系对拉伸过程中漏斗试样的应力三轴度演化进行了有限元分析,并同基于Bridgman应力修正后的单轴本构关系得到的漏斗试样漏斗根部截面中心处应力三轴度计算公式结果进行了比较。得到了四种材料的等效全程单轴本构关系、破断应变、破断应力和漏斗根部截面上应力三轴度分布随截面中心Von-Mises等效应变的变化规律。同时基于三轴度理论还讨论了漏斗型试样的拉伸破断机理以及断口的形貌。     

材料模型对1Cr18Ni9Ti管材拉伸有限元仿真的影响

为研究材料模型对有限元模拟1Cr18Ni9Ti管拉伸的影响,将管材单向拉伸试验获取的真实应力应变曲线分别拟合成线性硬化和指数硬化材料模型,并用于有限元模拟。经对比分析认为,采用真实应力应变模型的分析结果与实验结果吻合良好,并能正确显示出颈缩发生时刻和颈缩形状;采用指数硬化模型的有限元模拟结果接近真实应力应变模型,颈缩区应力应变分布略显分散;采用线性硬化模型的有限元模拟结果未能显示实际管拉伸后期的局部颈缩形状。     

Some necessary conditions for a maximal solution to a family of elliptic equations

We established some necessary conditions that a class of non-local variational problems has a maximal element over a preassigned set, _p, of functions. The exact nature of the set _p is not specified; this depends—in part—on the physical process being modeled. However we do impose some very general conditions on a maximizing element so that our computations may be carried out.

The class of problems we consider occurs in various engineering problems. In particular we apply our results to a class of problems similar to that encountered in the determination of optimum two-dimensional bearing profiles for Hydrodynamic Lubrication Theory.     

316LN不锈钢高温拉伸颈缩区塑变行为

为准确测量颈缩发生后的应力-应变行为,本文综合物理实验、有限元模拟和MLR模型的方法确定颈缩区的塑变行为,建立316LN不锈钢高温本构模型.模型中,颈缩前的真应力-真应变呈幂函数关系,颈缩发生后,较低温度时应力随着应变的增加迅速下降,而当较高温度时应力随着应变的增加而缓慢下降,真应力-真应变呈非线性关系.基于MLR模型,修正了颈缩后不同温度、不同应变速率下的真应力-真应变曲线,并将有限元模拟的颈缩区长度与实测值相对比,相对偏差为4.73%.这说明修正后的应力-应变本构模型能够准确地描述316LN的高温塑性行为.     

Evaluating Measurement Systems and Manufacturing Processes Using Three Quality Measures

Manufacturers and suppliers use quality measures calculated from dimensional data to make informed decisions regarding measurement systems and product quality. Many manufacturers and suppliers use the precision-to-tolerance ratio—scaling the standard deviation of gage error (σ_g) by the design tolerance—to approve a measurement system. Manufacturers and suppliers also use one or more measures of process capability, such as C_p that scales the tolerance by the standard deviation of the product (σ_p), to approve a manufacturing process. A measure used to assess the ability for two parties to communicate via dimension data is the correlation in repeat measurements that we derive as a function of σ_g and σ_p. By plotting the precision-to-tolerance ratio and the correlation in repeat measurements on the σ_g and σ_p axes, acceptable and nonacceptable regions for measurement systems are defined. When we add C_p (a measure of process capability) to the mix, a relationship between the three measures suggests a method for determining an acceptable level for the correlation criterion and defines additional regions. This approach of plotting the quality measurement criteria in the σ_g and σ_p axes precisely defines the quality situation and lends to improvement suggestions.     

(Photo)conductivity of conjugated oligomer films: mobile charge carrier formation by oxygen

Electrical dark and photo conductivities, (σ_d and σ_p, of vapor deposited films of -sexithiophene, 4,4′-distyrylstilbene, and titaniumoxophthalocyanine were investigated as function of temperature, exposure time to oxygen of variable pressure, irradiation time and intensity. Molecular oxygen increases σ_d and σ_p by orders of magnitude. Careful removal of oxygen at elevated temperatures under high vacuum and assistance of irradiation will reduce both σ_d and σ_p to almost negligible values. From the rise and decay kinetics of conductivity and from luminescence quenching it is concluded that oxygen must be complexed with the film constituting molecules in order to act as separation center for electrical charge. The complex is formed already in the dark, but more efficiently upon irradiation. The interaction with oxygen is essentially reversible but leads in a side reaction also to irreversibly oxidized photoproducts.     

Determining the stress-strain behaviour at large strains from high strain rate tensile and shear experiments

To characterise the high strain rate mechanical behaviour of metals, split Hopkinson bar experiments are frequently used. These experiments basically yield the force and elongation history of the specimen, reflecting not only the specimen material behaviour but also the specimen structural behaviour. Calculation of the real material behaviour from this global response is not straightforward, certainly for materials such as Ti6Al4V where due to low strain hardening, the specimen deformation is very inhomogeneous. However, for fundamental material research and constitutive material modelling, knowledge of the true effective stress versus plastic strain, strain rate and temperature is essential.In this contribution, a combined experimental-numerical approach for extraction of the strain rate and temperature dependent mechanical behaviour from high strain rate experiments is presented. The method involves the identification of the material model parameters used for the finite element simulations. The technique is applied to determine the stress-strain behaviour of Ti6Al4V using both high strain rate in-plane shear and tensile test results. For the tensile tests, even stress-strain data beyond diffuse necking are retrieved. A comparison is made between the material behaviour extracted from the tensile and the shear experiments. The material behaviour is modelled with the Johnson-Cook constitutive relation. It is found that the simultaneous use of tensile and shear tests to identify the model parameters gives a more generally applicable model. Validation of the material model and the finite element simulations is done by local strain measurements in the shear and tensile test by means of digital image correlation.     

Optical constants of polycrystalline CuGaTe2 films

Thin films of CuGaTe₂ with thicknesses in the range, 0.1–1.0 μm were deposited on Corning 7059 glass substrates by flash evaporation. The substrate temperatures, T_s, were maintained in the range 373–623 K. The transmittance of the films was recorded in the wavelength range 900–2500 nm. The dependence of the optical band gap, E_g, on substrate temperature showed that the value of E_g varied from 1.21 eV to 1.24 eV. The variation of refractive index and extinction coefficient with photon energy was studied from which the material properties such as the limiting value of dielectric constant, ε_∞, plasma frequency, ω_p, and hole effective mass, m_h^*, were evaluated as ε_∞ = 7.59, ω_p = 1.47 × 10¹⁴ and m_h^* = 1.25 m₀.     

Magnetic properties of NiSb2O4

The magnetic transition of the antimony oxide NiSb₂O₄ has been studied using heat capacity measurements and magnetic susceptibility data. The Néel temperature previously evaluated from neutron diffraction is confirmed: T_N = 45 ± 1 K. The entropy excess ΔS(20 – 60 K) associated with the transition is calculated and compared with the theoretical value. The susceptibility data are interpreted in terms of a Curie-Weiss law (θ_p = − 98K, M_eff = 3.98 μ_ß). Exchange integrals are evaluated.     

Thermo-mechanical stability of forged Ti-25Al-11Nb (at.%)

Dynamic material modeling (DMM) has been applied to the analysis of the mechanical behavior of a (Widmanstätten + primary ₂ + β) Ti-25Al-11Nb alloy to establish the flow stress-strain rate-temperature-strain criteria for stable flow.

Unstable and stable flow regimes predicted by DMM were correlated to microstructural observations. Unstable flow zones were related to transformation of orthorhombic O → ₂, kinking of the ₂ lamellae, shear band formation and coarsening of the dynamically recrystallized grain structure; stable flow regimes were shown to be associated with dissolution of the Widmanstätten ₂, coarsening of primary ₂ and dynamic spheroidization of the lamellar ₂ microconstituents.