滑轨导向式静/动态双轴拉伸实验技术

基于液压伺服高速加载系统,发展了一种材料双轴拉伸力学性能测试技术。利用锥面接触导向驱动方法,把加载锤竖直方向的驱动力转化为水平方向的双轴驱动力,从而实现对十字形试样平面双轴加载。借助有限元数值模拟手段优化了锥面接触角和十字形试样尺寸。当接触锥角为45°时,既有较好的水平驱动转化效率,同时又保持较小的接触力,确保水平驱动加载各组件在弹性变形范围内,可多次重复使用。确定了加载臂狭缝个数、狭缝与减薄区边缘长度和标距段厚度等试样设计关键参数,在十字形试样测试标距段内实现了均匀变形。设计了测力夹持一体化导杆和非接触光学全场应变测试系统,准确获得了试样的应力和应变。利用此平面双轴拉伸加载装置,开展2024-T351铝合金板单轴拉伸实验和激光探测同步性验证实验,验证装置设计的可行性;开展铝合金板材在不同加载速率下的双轴拉伸实验,得到在双轴加载下铝合金板材应力应变曲线,并与单轴加载下实验结果进行了对比分析。     

Dynamic Tensile Testing of Soft Materials

Determination of dynamic tensile response of soft materials has been a challenge because of experimental difficulties. Split Hopkinson tension bar (SHTB) is a commonly used device for the characterization of high-rate tensile behavior of engineering materials. However, when the specimen is soft, it is challenging to design the necessary grips, to measure the weak transmitted signals, and for the specimen to achieve dynamic stress equilibrium. In this work, we modified the SHTB on the loading pulse, the equilibrium-monitoring system, and the specimen geometry. The results obtained using this modified device to characterize a soft rubber indicate that the specimen deforms under dynamic stress equilibrium at a nearly constant strain rate. Axial and radial inertia effects commonly encountered in dynamic characterization of soft materials are also minimized.     

New equipment for cyclic biaxial testing

Methods of achieving biaxial-stress states in fatigue tests are reviewed. A new design of equipment that produces five distinct biaxial-stress states by simultaneous direct pressurization and axial loading of thinwalled cylindrical specimens is described. Four variations of stress state are obtained by the use of two sizes of specimen and by reversal of the pressurizing connections. The fifth state is obtained by direct pressurization without axial load. The actual magnitudes of stress in the specimen are computed from the output of the load cell in the reaction frame in which the biaxial-testing device is mounted. Additional stress ratios are obtained using standard uniaxial and torsional cyclic-testing arrangements. The initial program using this equipment was to investigate the effects of biaxiality on the cyclic properties and low-cycle fatigue behavior of normalized 1018 mild steel, under fully reversed constant-amplitude strain control. Control was achieved using a servo-controlled, electrohydraulic testing system with one of the two clipon strain transducers, that were mounted on the specimen, providing the controlling electrical signal. The system allowed direct recording of the stress-strain hysteresis loops in both principal directions. The behavior of the equipment and the modes of failure of the specimens are described and some test data are presented. The range of application and limitations of the equipment for further cyclic biaxial testing are discussed.     

Fatigue life of dovetail joints: Verification of a simple biaxial model

A biaxial fatigue specimen, designed to model the state of stress of the blade to disk joint in a typical gas turbine, has been analyzed and tested. The method of analysis was shown to predict the location of the worst damage and of crack initiation. This paper describes the analysis and testing of a simplified model that may be used in preference to the previous one.     

一种FRP累积损伤模型及其在结构疲劳寿命估算中的应用

推荐了一种应变损伤累积模型，能够考虑单向板面内多轴应力和平均应力的影响。只需要单向板在确定应力比下的若干典型疲劳试验结果，就可以预测相同材料体系多向层压结构在不同应力比的循环载荷下的疲劳寿命，有助于降低试验成本和工作量。研究了适用于多向层压结构剩余强度估算和疲劳寿命预测的步骤和程序。针对碳纤维／树脂基T300／QY8911复合材料，试验测定了三组典型单轴循环应力（[0]16拉－拉、[90]16拉－拉和[0/90]4S剪－剪）下的S－N曲线。以此为输入，预测四种多向铺层板在各种拉－拉循环应力下的疲劳寿命，寿命预测结果和相应的试验结果吻合良好。采用了保持计算和试验的载荷／强度比相对等值的方法来近似抵消层合效应对疲劳寿命的影响。强调了进一步发展能够定量估计层间应力影响与分层扩展过程的疲劳损伤模型的重要性。     

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.     

A new direct biaxial testing machine for anisotropic materials

A screw-driven new biaxial testing machine for the realization of experimental investigations on anisotropic sheet materials, such as composite plates or rolled sheet metals, is presented. The described mechanical concept and servocontrol system allow cruciform specimens to be subjected to large strain biaxial tensile and compressive tests without kinematic incompatibilities. Moreover, for the proper implementation of biaxial tensile tests, the specific problems linked to the anisotropic properties of the investigated materials are taken into account; therefore, for the first time, the biaxial machine is supplied with the original ‘off-axes testing device,’ consisting of hinged fixtures with knife-edges at each arm of the cruciform specimen. A recently developed optimization method for the optimal design of flat tensile cruciform specimens is shortly reviewed. Numerical simulations illustrate the decisive superiority of the optimized specimen compared with specimen designs proposed in the literature, as well as the necessity to use the ‘off-axes’ testing technique in biaxial tests on anisotropic materials.     

Viscoelasticity of brain corpus callosum in biaxial tension

Computational models of the brain rely on accurate constitutive relationships to model the viscoelastic behavior of brain tissue. Current viscoelastic models have been derived from experiments conducted in a single direction at a time and therefore lack information on the effects of multiaxial loading. It is also unclear if the time-dependent behavior of brain tissue is dependent on either strain magnitude or the direction of loading when subjected to tensile stresses. Therefore, biaxial stress relaxation and cyclic experiments were conducted on corpus callosum tissue isolated from fresh ovine brains. Results demonstrated the relaxation behavior to be independent of strain magnitude, and a quasi-linear viscoelastic (QLV) model was able to accurately fit the experimental data. Also, an isotropic reduced relaxation tensor was sufficient to model the stress-relaxation in both the axonal and transverse directions. The QLV model was fitted to the averaged stress relaxation tests at five strain magnitudes while using the measured strain history from the experiments. The resulting model was able to accurately predict the stresses from cyclic tests at two strain magnitudes. In addition to deriving a constitutive model from the averaged experimental data, each specimen was fitted separately and the resulting distributions of the model parameters were reported and used in a probabilistic analysis to determine the probability distribution of model predictions and the sensitivity of the model to the variance of the parameters. These results can be used to improve the viscoelastic constitutive models used in computational studies of the brain.     

混凝土动态双轴拉压破坏准则细观数值模拟研究

正常服役期内的混凝土结构往往处于复杂应力状态,并且不可避免地会受到偶发动力载荷作用.对于复杂载荷作用下的混凝土力学性能研究,破坏准则是基础.受试验设备等条件限制,现有的动态双轴拉压破坏准则形式复杂、缺乏更高应变率和侧应力比范围且尚未综合考虑应变率和侧应力比的耦合作用.为进一步提出适用范围更高且更准确的混凝土动态双轴拉压...     

功能材料双轴拉伸十字板试件的优化设计

在对Demmerle和Boehler提出的一个基于试件实验区应力标准差的数学判据进行修正的基础上,将这一判据应用于有限元方法中,通过 ABAQUS有限元软件计算出试件实验区的应力分布,结合Powell优化设计方法,实现了对各向同性形状记忆合金材料双轴拉伸十字板试件的最优化设计.优化设计所得试件的应力位移分布图与原有的Kelly试件迸行了比较,结果表明经过优化的试件在满足双轴试验要求方面有了明显的改进.同时验证了在各向同性材料下优化所得的试件同样适用于各向异性材料.最后,对形状记忆合金相变过程在试验区中引起的应力应变变化进行了数值模拟,其结果表明优化试件完全能够满足记忆合金材料双轴实验的要求.     

EFFECT OF SPECIMEN ASPECT RATIO ON FATIGUE LIFE OF CLOSED CELL Al-Si-Ca ALLOY FOAM

Quasi-static and compressive fatigue tests on the closed cell Al-Si-Ca alloy foam specimens with three different aspect ratios were performed. It turned out that the onset of cyclic shortening of foam with a lower aspect ratio took place earlier and the fatigue strength was lower compared with the specimen with a higher aspect ratio, although all the dimensions of specimen satisfied the seven times the cell size criterion, while the quasi-static stress-strain curves were almost same having same Young＇s modulus, yield stress and plateau stress. Therefore, the seven times the cell size criterion for the quasi-static compression behavior was not applicable to the fatigue analysis of Al-Si-Ca alloy foam.     

一种考虑拉伸保持效应的多轴疲劳寿命模型

通过定义考虑拉伸保载效应的CFI因子(creep-fatigue interaction factor)，将拉伸蠕变损伤和疲劳损伤进行非线性耦合. 根据断裂实验的观察，针对拉伸主 导的裂纹萌生、扩展及破坏的多轴疲劳问题，给出了一个基于临界面方法的能量型高温多轴 疲劳寿命预测模型. 所给出的模型可对不同温度、不同载荷特点、不同保载时间的多轴疲劳 寿命进行预测，模型的材料参数不依赖于温度和载荷. 并且此方法可以很方便地推广到其它 因素主导破坏的高温多轴疲劳寿命预测. 通过拟合高温合金Udimet720Li单轴带保持时间的 低循环疲劳(low cycle fatigue, LCF)寿命试验数据，得到了材料常数. 结合黏 塑性有限元分析方法，对高温双轴带保载循环载荷下Cruciform试件的寿命进行了 预测，预测结果基本落在2倍分散带内，达到工程的要求，证明了该模型的有效性.     

复合材料后压力框加筋元件双轴拉伸试验与模拟

试验针对应用在飞机后压力框上的碳纤维织物复合材料的力学特性和设计压力框壳体加强筋的刚度匹配问题,本文设计了十字形的试件,并开展了双轴拉伸试验,测试分析了试件中心位置含有不同尺寸加强筋时对刚度和强度的影响。依照试验条件,建立有限元模型,以中心位置未加筋试件为基准,探讨了两种尺寸筋条增强效果的差异。试验与模拟结果表明:在双轴拉伸载荷作用下,帽型加强筋明显提高了试件刚度和强度;两种筋条尺寸的差异对于试件刚度和强度的影响不明显;根据重量最小的原则,在同等条件下使用较小尺寸加强筋更为合适;有限元模拟与试验结果吻合良好,验证了本文提出的试验和模拟方法可用于复杂应力条件下织物复合材料结构的力学性能分析。     

Stress Measurement Using EBSD Analysis of Grains in Copper Foil

A cyclic biaxial stress measurement method using electrodeposited copper foil was examined. The crystallographic orientations of individual grains that undergo grain growth in copper foil subjected to cyclic loading were analyzed by electron backscatter diffraction (EBSD). One of the slip directions in most of the grains corresponded to the direction of maximum shear stress when the biaxial stress ratio was negative. However, the number of grains with other orientations gradually increased as the biaxial stress ratio approached zero. On the basis of these features, we propose biaxial stress measurement using EBSD analysis of grown grains in copper foil. Our new method has excellent resolution compared with other stress-strain measurement methods since it can measure the average biaxial stress ratios in an area of 500 μm × 500 μm.     

航空涡轮盘多轴蠕变-疲劳寿命预测及对比研究

针对几种经典和新发展的蠕变-疲劳寿命模型开展综述介绍,并建立预测航空涡轮盘在循环热-机蠕变-疲劳载荷谱下蠕变-疲劳行为的数值流程,对某型航空涡轮盘的蠕变-疲劳损伤和寿命进行预测和对比．结果表明：等效应变法与临界平面法得出的疲劳损伤差距较小,等效应变法由于数值计算简单,工程适用性更强．寿命-时间分数(TF)法由于无法考虑应力松弛效应,给出了最为保守的蠕变损伤预测,其对盘体应力三轴度引起的损伤不敏感;延性耗竭法(DE)法仅以蠕变应变率作为损伤因素,虽考虑多轴蠕变因子的影响,但是给出的蠕变损伤过小;修正应变能密度耗竭(MSEDE)法综合考虑蠕变应变与应力松弛,并且考虑多轴蠕变因子与弹性跟随效应的影响,结合疲劳损伤模型可以给出合理的蠕变、疲劳损伤比例,其预测结果更加合理．     

A crystallographic model for the orientation dependence of low cyclic fatigue property of a nickel-base single crystal superalloy

ＩｎｔｒｏｄｕｃｔｉｏｎＭｕｃｈｗｏｒｋｈａｓｂｅｅｎｃａｒｒｉｅｄｏｕｔｔｏｉｎｖｅｓｔｉｇａｔｅｔｈｅｉｎｆｌｕｅｎｃｅｏｆｏｒｉｅｎｔａｔｉｏｎａｎｄｓｔｒａｉｎｒａｔｅｏｎｔｈｅｍｅｃｈａｎｉｃａｌｐｒｏｐｅｒｔｙｏｆｎｉｃｋｅｌ＿ｂａｓｅｓｉｎｇｌｅｃｒｙｓｔａｌｓｕｐｅｒａｌｌｏｙｓ .Ｉｎｐａｒｔｉｃｕｌａｒ,ｔｈｅａｎｏｍａｌｏｕｓｙｉｅｌｄｉｎｇｂｅｈａｖｉｏｒ,ｔｅｎｓｉｏｎ/ｃｏｍｐｒｅｓｓｉｏｎａｓｙｍｍｅｔｒｙａｎｄｏｒｉｅｎｔａｔｉｏｎｄｅｐｅｎｄｅｎｃｅｈａｖｅｂｅｅｎ…     

An investigation of fatigue and fretting in a dovetail joint

Specimens designed to reproduce the state of stress of the dovetail joint between blade and disk in a typical gas-turbine configuration have been tested in a 250-kN capacity biaxial fatigue machine. The materials tested were Ti-IMI 829 (disk and blade), Inco 901 and steel FV535 (blades only). The investigation highlights the importance of fretting at the disk/blade interface. A design parameter that combines fretting damage and peak stress is proposed as a basis for the life assessment of the joints.     

Inelastic behavior of an AS4/PEEK composite under combined transverse compression and shear. Part I: experiments

The nonlinear behavior in shear and transverse compression of unidirectional AS4/PEEK and their interaction are investigated experimentally. The composite is rate dependent even at room temperature and its rate exponent is similar to that of neat PEEK. The material is tested under pure shear, pure compression and under biaxial loading histories. The biaxial tests are performed in a custom facility on thin strips of the material. The facility allows freedom to choose the loading path in the biaxial stress and strain spaces of interest. Tests are performed for three biaxial loading paths. In the first, the specimen is sheared then compressed while the shear stress is held constant; in the second, the specimen is compressed then sheared while the compressive stress is held constant; and in the third, the specimen is loaded simultaneously by proportional amounts of compression and shear. It was found that the induced deformation is influenced significantly by the loading history followed. Also, initial loading in shear or compression has only a modest effect on subsequent loading of the other type. An unorthodox yielding behavior for the composite results from this lack of interaction. Finally, the stresses at failure are found to trace an elliptical path in the shear–transverse compression plane, but the failure stress state is not significantly affected by the loading path followed.     

Low-cycle fatigue of 7075-T651 aluminum alloy in biaxial bending

The uniaxial and biaxial low-cycle-fatigue life of 7075-T651 aluminum alloy in completely reversed bending is investigated. Uniaxial data is obtained from a cantilever specimen, and round and elliptical simply supported plate specimens are used for nominal strain biaxialities of 1∶1, 1∶075 and 1∶0.50 in the plane of the specimen. Experimental data is correlated on the basis of the total octahedral shearing strain range in which the dependent component strains are calculated using the “effective value” of Poisson's ratio for elastic-plastic loading. Good agreement is obtained oetween the uniaxial and biaxial data when the total octahedral shearing-strain range is plotted against the number of cycles to failure in logarithmic coordinates. Also, it is shown that, in anisotropic materials, the directional uniaxial and biaxial low-cycle properties can be predicted from fatigue properties in any one direction if the anisotropy of fracture ductility is known.