MEASUREMENT IN TENSILE TEST OF TRUE STRESS-STRAIN CURVES FOR LOW-CARBON STEEL1)

针对低碳钢拉伸的真应力-应变曲线测量问题,开发了基于单相机三维数字图像相关的实验教学系统。该系统可测量低碳钢拉伸过程中的表面三维形貌和表面应变,并由此获得低碳钢拉伸的真应力-应变曲线。该实验有助于学生深入理解塑性金属材料的力学性能,培养其科研兴趣。     

预取向半晶态高分子片晶结构形成微观机理及其应力-应变响应特性的分子动力学模拟

通过分子动力学模拟方法对不同预取向聚乙烯醇熔体(polyvinyl alcohol,PVA)形成的半晶态高分子熔体形成核结晶及拉伸过程中的应力-应变响应特性进行了系统地研究.模拟结果显示预取向度高的PVA熔体对应更快的成核动力学.通过追踪全trans伸直链长度(dtt)、成核原子维诺体积(V)和中心对称参数(S)等序参量在不同取向度熔体下的等温成核与结晶演化过程,给出了PVA熔体成核路径及形成半晶态的分子构象;通过对形成的半晶态高分子结构进一步分析,发现随着熔体取向度的增加,晶体和无定型对应的取向度也会增加,但是当应变剪切大于5时,其对应的结晶度、晶体和无定型取向度不再发生变化;通过对无定型区链结构的定义与分析,可知取向度越高的熔体对应越高的Tie链数目,随着熔体取向度的增加,Loop链的数目也会减小;通过恒速拉伸应力测试可知,所形成半晶态高分子结构力学响应会随着取向度及Tie链数增加而增加,当取向及Tie链数目相同时,应力-应变曲线形状大小也基本保持一致.     

Effect of Heat Treatment on the Stress-Strain Characteristics of Cd-Sn Eutectic Alloy

Stress-strain curves of slowly cooled and quenched Sn-33wt. alloy were studied in the temperature range from 353 to 433 K. The coefficient of logarithmic work/hardening, the fracture time, the yield stress and the fracture stress of annealed and quenched samples decreased with increasinig deformation temperature and exhibited a minimum at 393 K. The activation energy of the fracture mechanism is found to be 5.4 and 12.8 KJ/mol characterizing a dislocation mechanism for annealed and quenched samples. The X-ray analysis of the slowly cooled and quenched samples showed that the residual internal strains increase in the low temperature relaxation stage and decrease in the second stage.     

玻璃态高聚物PMMA粘弹性力学行为的率温等效关系

基于粘弹-塑性本构模型并以玻璃态高聚物PMMA在温度T=233—343K和应变率(?)=1.0 x 10~(-4)—1.0×10~(-1)s~(-1)范围内实测的应力-应变曲线及其拟合计算为依据,讨论了试验材料的率温等效性,从位移因子a_T和归一化应力-应变主曲线的存在,证实试验材料在屈服前存在率温等效关系,而在屈服后不复存在。     

Microdamage of a nonlinear elastic material in combined stress state

The structural theory of short-term damage is used to study the coupled processes of deformation and microdamage of a physically nonlinear material in a combined stress state. The basis for the analysis is the stochastic elasticity equations for a physically nonlinear porous medium. Damage in a microvolume of the material is assumed to occur in accordance with the Huber-Mises failure criterion. The balance equation for damaged microvolumes is derived and added to the macrostress-macrostrain relations to produce a closed-form system of equations. It describes the coupled processes of nonlinear deformation and microdamage of the porous material. Algorithms are developed for calculating the dependence of microdamage on macrostresses and macrostrains and plotting stress-strain curves for a homogeneous material under either biaxial normal loading or combined normal and tangential loading. The plots are analyzed depending on the type of stress state __________ Translated from Prikladnaya Mekhanika, Vol. 42, No. 11, pp. 30–39, November 2006.     

Nonlinear deformation of a particulate material in complex stress state

An algorithm is proposed to determine the effective deformation properties and stress-strain state of particulate composite materials with physically nonlinear components and complex stress state. The laws that govern the deformation of particulate composites are studied. A particulate composite is considered a two-component material of random structure. Its effective properties are determined by conditional averaging. The nonlinear equations that incorporate the physical nonlinearity of the components are solved by the method of successive approximations. The relationship between macrostresses and macrostrains is established. The effective deformation properties of a particulate composite as a function of the volume fractions of the components and stress state are studied __________ Translated from Prikladnaya Mekhanika, Vol. 42, No. 3, pp. 50–60, March 2006.     

波分离技术在混凝土冲击压缩试验中的应用

介绍了SHPB实验中的波分离技术基本原理,该方法利用两相距很近应变计测量信号分离重叠的入射波和反射波。由于两应变计的距离很近,在弹性杆可以忽略应力波传播的弥散。由于应变计的位置可以非常接近试样,减少了实验中因应力波弥散产生的误差。利用该方法对混凝土进行了冲击压缩实验研究,得到了混凝土的动态应力-应变关系。讨论了测量误差导致波分离技术的误差。     

Dynamic torsion testing of nanocrystalline coatings using high-speed photography and digital image correlation

The strength and ductility of microcrystalline and nanocrystalline tungstsen carbide-cobalt (WC-Co) cermets have been evaluated by employing a stored energy Kolsky bar apparatus, high-speed photography and digital image correlation. The test specimens were thin-walled tubular AI7075-T6 substrates 250 μm thick, coated with a 300 μm thick microcrystalline or nanocrystalline WC-Co layer with an average grain size of about 3 μm and 100 nm, respectively. Dynamic torsion experiments reported in this paper reveal a shear modulus of 50 GPa and a shear strength of about 50 MPa for both microcrystalline and nanocrystalline WC-Co coatings. The use of high-speed photography along with digital image correlation has shown that damage to the coating coincides with a significant softening on the stress-strain curve. The coating failed in mode III, and strong interactions between the crack faces were probably responsible for the increase in load after failure of the coating. The overall failure of the coating-substrate system was not brittle but rather progressive and controlled by the ductility of the aluminum substrate. A methodology for investigating damage kinetics and failure has been established. This methodology can be applied to examine the behavior of other advanced materials that can be manufactured as coatings on ductile substrates. Manufacturing coatings free of initial microcracks remains a significant challenge. Research on optimization of the spray deposition parameters should be pursued to produce high-quality nanostructured coatings that can fully exploit the benefits of nano-size grains.     

Two-dimensional buckling problem for a composite reinforced with a periodic row of collinear short fibers

A solution is found to the two-dimensional buckling problem for a composite material reinforced with a periodic row of collinear short fibers and compressed along the fibers. The problem formulation is based on the piecewise-homogeneous model and the three-dimensional theory of stability of deformable bodies. The dependence of the critical strain and buckling mode on the fiber spacing is studied for various material and geometrical characteristics of the composite components __________ Translated from Prikladnaya Mekhanika, Vol. 42, No. 6, pp. 90–100, June 2006.     

汽车薄钢板应力应变曲线及屈服轨迹的研究

采用十字形双向拉伸的实验方法对两种汽车用薄钢板BH220和SPEN进行了不同 加载路径下的双向拉伸试验，得到了不同应力状态下的应力应变关系曲线，同时，根据单位 体积塑性功相等的原则，确定了两种钢板等效塑性应变从0.2\%$\sim$2\%的实验屈服轨迹. 结果分析表明：不同加载路径下板料的应力应变关系不同，随着加载比例由单拉到等双拉状 态，板料的硬化指数逐步增大；实验屈服轨迹呈外凸性，且以等双拉为界的上下部分屈服轨 迹不对称，随着变形程度的增加，屈服轨迹向外扩大，但单拉时强化程度最小，而等双拉 时最大. 对BH220和SPEN钢板的实验屈服轨迹与几种常用理论屈服轨迹的比较发现，Hosford各向 异性屈服准则的理论轨迹与实验结果最为接近，Hill48准则与实验结果相差最大，此外一 向被视为只适用于各向同性材料的Mises准则与实验结果也较为接近，其他几个屈服准则的 理论屈服轨迹与实验点相差较大.