岩石拉压蠕变试验仪研制及应用

为了探索岩石在周期性恒定拉伸、压缩荷载作用下的蠕变行为,结合杠杆原理,设计研制了一种岩石杠杆式拉伸、压缩蠕变试验仪。该仪器具有挂重质量小、可方便切换拉、压荷载等特点。首先,通过标定好的数显式拉压荷重传感器对该试验仪拉伸、压缩荷载进行了校正,得出试验仪压缩、拉伸荷载杠杆扩力比分别为81.29倍与59.46倍,挂重质量与施加在岩样荷载呈线性关系(压缩作用下线性关系相关系数R2=0.99975,拉伸作用下R2=0.9991),荷载施加稳定。最后,采用该试验仪对红砂岩进行了单轴恒定拉、压循环荷载下的蠕变试验,探讨了受荷岩样拉压蠕变规律。上述成果丰富了岩石蠕变测试与研究内容,有助于岩石力学试验测试的发展。     

Application of a discrete polycrystal model to the analysis of cyclic straining in copper

A discrete polycrystal model, designed to simulate a metal aggregate macro-element, is applied to the study of cyclic straining in copper. The numerical method of solution (an adaptation of the “finite element method”) incorporates a convergent discrete Green's function within the constrained minimum principle which governs the (crystallographic) plastic shear increments at each load step. Isothermal elastic moduli of copper crystals and Taylor's hardening rule with constant hardening modulus are used in the calculations. Numerical results are obtained for macroscopic elastic properties, cyclic stress-strain curves (which indicate the contribution of aggregate heterogeneity to macroscopic hardening), macroscopic plastic work, and residual (latent) strain energy through four loading cycles between fixed macrostrain limits. Other estimates for elastic properties also are included, and all results are compared, both qualitatively and quantitatively, with published experiments. The predictions of the model are in general satisfactory.     

An Investigation of Fatigue Characteristics of Copper Film

Tensile and fatigue behaviors of the copper film coated by tin (Sn) were investigated considering S-N relationships and scanning electron microscope (SEM) observation of fracture surfaces. The fatigue behavior was investigated considering the effect of load ratio, R. The specimen of 2000 μm width, 8000 μm length and 15.26 μm thickness was fabricated by etching process. Tensile properties were measured using the micro-tensile testing system and in-plane electronic speckle pattern interferometric (ESPI) system for measuring the tensile strain during the test. The fatigue tests of the film were carried out in load-control mode with 40 Hz at three different stress ratios of 0.05, 0.3 and 0.5. The S-N curves, including the slope of the curve and fatigue limit, at the respective stress ratios were obtained. These curves were dependent on the load ratio. Empirical relationships indicating the dependency of the fatigue limit and S-N curve on the load ratio were suggested in this study. SEM observation of the tensile fracture surface showed that the cross-sectional area of the testing section was necked in the direction of the film thickness (i.e. parallel to the substrate surface normal) and some ductile dimples in the fracture surface were present. The fracture of the copper film under cyclic loading was progressed in the transgranular fracture mode.     

单晶体和双晶体微观层次变形行为的有限元分析

从微观层次上研究金属材料的变形行为,将位错引入到本构关系中,用硬化函数描述材料的硬化规律,考虑了变形的率相关性,采用三维模型用大变形有限单元法对单晶体在单向拉伸载荷和循环载荷作用下的变形行为、双晶体在单向拉伸作用下滑移系的开动进行了模拟计算,得到了与实验一致的计算结果。     

A study on the compression sensibility and mechanical model of carbon fiber reinforced cement smart material

Under low compressive stress, carbon fiber reinforced cement (CFRC) exhibits the compressive sensibility for its inner deformation. The curve measured in the course of specimen suddenly loaded by a constant load and unloaded suddenly exhibits the variation of resistivity with time, and the curves measured under cyclic loads exhibits the variation of resistivity with cyclic stresses. Based on the results of experiment, the tunnel conduction effect and the structure of hardened cement paste, a mechanical model for CFRC is proposed and the rules of deformation when the specimen is loaded constantly or cyclically are also inferred. These rules can well explain the mechanism of CFRC' s compressive sensibility.Financed by National Natural Science Foundation of China Key Project No. 59432061.     

轮胎钢丝帘线拉伸力学性能的实验研究

对两种结构类型的钢丝帘线在单向拉伸和循环加载下的力学性能进行了实验研究。结果表明：帘线的结构形式对拉伸力学性能（包括摩擦能量损耗）有显著影响。不考虑载荷很小的情况时，普通结构帘线可近似为弹脆性材料，高伸长结构帘线却可以产生较大的塑性变形；循环载荷下，普通结构帘线几乎没有摩擦能量损耗，而高伸长结构帘线不仅有明显的摩擦能量损耗，而且随着循环应力幅值的增加，摩擦能量损耗呈加速增加的趋势；在循环应力幅值较大时高伸长结构帘线还出现了类似于高温下循环硬化金属材料的循环蠕变现象。文章还讨论了高伸长结构帘线由于塑性变形而引起循环蠕变现象的机理。     

Digital Image Correlation Study of Plastic Deformation and Fracture in Fully Martensitic Steels

Plastic deformation and fracture of two grades of fully martensitic steel are investigated with a miniature tensile stage, a custom image acquisition algorithm and digital image correlation. The image acquisition algorithm controls the camera framing rate according to user defined load, displacement and timing thresholds. This provides a greater number of images captured during periods of rapid load change over small displacements. True stress–true strain curves reveal substantial differences in material ductility and failure behavior. Fracture surfaces are examined using scanning electron microscopy and energy dispersive spectroscopy to provide insight into differences in the tensile behaviors observed for these steels.     

Elevated-temperature creep and relaxation of torsion-tension members

Based on the assumption that each material satisfies the condition for isotropic hardening for a von Mises material, an incremental solution is developed to predict axial-strain creep curves and maximum shearing-strain creep and relaxation curves for solid circular torsion-tension members subjected to proportionate and nonproportionate stepped loading including creep in tension and relaxation in torsion. Test data are obtained from torsion-tension members made either of annealed OFHC copper at 800°F (427°C) or hot-rolled SAE 1045 steel at 950°F (510°C). The loading histories include either four stepped proportionate load changes, four stepped nonproportionate load changes, or torsion-tension loading in which the axial load remains constant and the torsional load is allowed to relax during two loading periods of the test. Each test duration is about 100 h. Good agreement is indicated between the predicted and measured creep and relaxation curves.     

焊接接头局部塑性行为的循环压痕实验研究

基于纳米压痕技术,对转子钢焊接接头不同区域（母材、焊缝和热影响区）开展了压入位移控制的单向压痕实验和压入载荷控制的循环压痕实验研究．首先,通过压入位移控制的单向压痕实验,采用多次测试取平均值的方式获得了焊接接头各个区域的弹性模量和硬度分布特征,同时对各区域弹性模量中值点的载荷-压入深度曲线进行了分析;其次,对各个区域进行压入载荷控制的循环压痕实验,比较其压入深度随循环周次的演化特征．结果表明,焊接接头不同区域力学性能差异较大,热影响区的弹性模量、硬度、抗拉强度和抗循环变形能力最高,焊缝次之,母材最弱;三个区域在循环压痕载荷下的接触载荷-压入深度滞回环曲线均表现出类似棘轮变形的演化特征,且母材演化速度高于焊缝,高于热影响区．研究结果对汽轮机焊接转子的焊接工艺的优化、寿命预测和可靠性设计具有重要的借鉴意义．     

Inertial effects of hypoelastic cylindrical bar of grade zero under simple extension of constant velocity

In this paper an analytical solution for finite deformation at the simple extension of a hypoelastic cylindrical bar of grade zero under constant velocity is obtained. It is found that the difference between stress-strain curves at different tensile velocities is only due to the inertial terms in the solution. The axial stress due to inertia increases with the square of ratio of tensile velocities.     

Experimental investigation of cyclic and time-dependent deformation of titanium alloy at elevated temperature

A novel cyclic deformation test program was undertaken to characterize macroscopic time dependent deformation of a titanium alloy for use in viscoplastic model development. All tests were conducted at a high homologous temperature, 650 °C, where there are large time dependent and loading rate dependent effects. Uninterrupted constant amplitude tests having zero mean stress or a tensile mean stress were conducted using three different control modes: strain amplitude and strain rate, stress amplitude and stress rate, and a hybrid stress amplitude and strain rate. Strain ratcheting occurred for all cyclic tests having a tensile mean stress and no plastic shakedown was observed. The shape of the strain ratcheting curve as a function of time is analogous to a creep curve having primary, steady state and tertiary regions, but the magnitude of the ratchet strains are higher than creep strains would be for a constant stress equal to the mean stress. Strain cycles interrupted with up to eight 2-h stress relaxation periods around the hysteresis loop, including hold times in each quadrant of the stress–strain diagram, were also conducted. Stress relaxation was path-dependent and in some cases the stress relaxed to zero. The cyclic behavior of these interrupted tests was similar even though each cycle was very complex. These results support constitutive model development by providing exploratory, characterization and validation data.     

A technique for rapid two-stage dynamic tensile loading of polymers

A method for rapid two-stage dynamic-dynamic tensile loading of polymers, based on a tensile Hopkinson bar apparatus, is established. In this technique, the initial incident wave and its reflection are used to load a specimen in quick succession. Consequently, the specimen is stressed, momentarily unloaded, then reloaded until fracture. By adopting appropriate assumptions, a procedure to obtain the associated stress-strain curves for such double-stage loading is formulated. These assumptions are examined experimentally and analytically to substantiate their validity. To verify the proposed approach, a relatively rate-insensitive material, LEXAN 141 polycarbonate, was subjected to two-stage dynamic tension. The stress-strain curves obtained via the procedure established were compared with results from static loading. Favorable correlation between the two indicates that the proposed technique can be applied to the study of load history effects on the dynamic behavior of polymeric materials.     

零级次弹性圆柱杆在常速度拉伸时的惯性效应

本文给出了一个零级次弹性圆柱杆在常速度拉伸时有限变形的解析解,由其解发现在不同拉伸速度下应力-应变曲线的不同,完全是由于惯性力而引起,它使轴向应力几乎是随着拉伸速度比值的平方而增加。     

PVC-CFRP管钢筋混凝土柱抗震性能非线性有限元分析

在试验研究基础上,借助有限元分析软件,选取合理材料本构关系,建立PVC-CFRP管钢筋混凝土柱有限元分析模型,验证分析模型的正确性。对试验进行延拓分析,探讨混凝土强度等级、纵筋配筋率、CFRP条带层数及剪跨比等对试件骨架曲线、刚度退化影响规律,分析低周往复荷载作用下PVC-CFRP管钢筋混凝土柱的受力性能,揭示低周往复荷载作用下PVC-CFRP管钢筋混凝土柱的受力工作机理。     

海洋平台K型管节点的疲劳裂纹扩展分析Ⅰ:试验测试

对承受疲劳载荷的海洋平台K型管节点首先进行了静力测试,确定了沿着焊缝的热应力区的应力分布及热应力区最大应力点的位置,从而判断裂纹产生的位置;然后通过专用测试设备提供循环疲劳载荷,用ACPD(Alternating Current Potential Drop,即交流电流势能落差法)技术检测裂纹的产生和增长过程,得到裂纹最深点,用S-N曲线估算其疲劳寿命。对已有裂纹的K型管节点,用应力强度因子估计其剩余寿命。同时用测试的结果验证了S-N的准确性和可靠性。     

Spread of plasticity from a stack of cracks under mode I conditions

The paper studies the extent of plastic relaxation around the tips of an infinite sequence of slitlike cracks contained in a large elastic solid. The cracks have a constant distance of vertical separation, and the solid is deforming under tensile loading (mode I). The plastic region around each of the crack tips coplanar with the crack itself is represented by a suitable distribution of edge dislocations, which is determined from equilibrium considerations. The latter lead to a singular integral equation which is solved numerically. The solution procedure is uniformly valid for any crack spacing. Furthermore, an alternate perturbation technique is used for widely spaced cracks. Solutions are obtained as a function of the crack spacing and the applied tensile load, and the results discussed from the point of fracture initiation at stresss concentrations.     

Plastic reversed deflections of a tapered-thickness cantilever plate

The load-strain-deflection characteristics of a tapered-thickness cantilever aluminum plate representing a Naflex seal geometry were evaluated by means of high-elongation strain gages, high- and low-sensitivity photoelastic coatings and dial gages. Load was achieved by a deflection of the free end, constant across the width, and measured by a strain-gaged load cell. After deflection to approximately 5-percent tensile strain, the loading was reversed until approximately 5-percent compressive strain had been reached beyond the original position, then reversed again and loaded to approximately 10-percent tensile strain. The plane-stress to plane-strain transition, traversing from the edge of the plate, occurred quite rapidly for both elastic and plastic strains. The influence of the Bauschinger effect on the load-deflection relationship was pronounced.     

Using Micro-Raman Spectroscopy to Assess MEMS Si/SiO2 Membranes Exhibiting Negative Spring Constant Behavior

We introduce a novel micro-mechanical structure that exhibits two regions of stable linear positive and negative stiffness. Springs, cantilevers, beams and any other geometry that display an increasing return force that is proportional to the displacement can be considered to have a “Hookean” positive spring constant, or stiffness. Less well known is the opposite characteristic of a reducing return force for a given deflection, or negative stiffness. Unfortunately many simple negative stiffness structures exhibit unstable buckling and require additional moving components during deflection to avoid deforming out of its useful shape. In Micro-Electro-Mechanical Systems (MEMS) devices, buckling caused by stress at the interface of silicon and thermally grown SiO₂ causes tensile and compressive forces that will warp structures if the silicon layer is thin enough. The 1 mm² membrane structures presented here utilizes this effect but overcome this limitation and empirically demonstrates linearity in both regions. The Si/SiO₂ membranes presented deflect ~17 μm from their pre-released position. The load deflection curves produced exhibit positive linear stiffness with an inflection point holding nearly constant with a slight negative stiffness. Depositing a 0.05 μm titanium and 0.3 μm layer of gold on top of the Si/SiO₂ membrane reduces the initial deflection to ~13.5 μm. However, the load deflection curve produced illustrates both a linear positive and negative spring constant with a fairly sharp inflection point. These results are potentially useful to selectively tune the spring constant of mechanical structures used in MEMS. The structures presented are manufactured using typical micromachining techniques and can be fabricated in-situ with other MEMS devices.