A Normalized Stress-Strain Model for Soft Clays

-A nonlinear model for the stress-strain behaviour of normally consolidated clays is presented based on the experimental results. It is indicated that the volume strain under pure shear is a power function of stress ratio and the normalized stress-strain curve is a standard hyperbola. According to the model, the coefficient of pore pressure induced by shear stress and the critical stress ratio which governs the influence of the negative dilatancy are suggested. It is shown by some triaxial tests that the proposed model can be used to study the negative dilatancy and to describe the stress-strain-pore pressure adequately for soft clays.

Finite Element Modeling of Stress Strain Curve and Micro Stress and Micro Strain Distributions of Titanium Alloys— A Review

Most of the alloys like titanium, steel, brass, copper, etc., are used in engineering applications like automobile, aero- space, marine etc., consist of two or more phases. If a material consists of two or more phases or components it is very difficult to predict the properties like mechanical and other properties based on simple laws such as rule of mixtures. Titanium alloys are capable of producing different microstructures when it subjected to heat treatments, so much of money and time are squandering to study the effect of microstructure on mechanical properties of titanium alloys. This squandering can be reduced with the help of modeling and optimization techniques. There are many modeling tech- niques like Finite element method, Mat lab, Mathematical modeling etc. are available. But Finite element method is widely used for prediction because of capable of producing distributions of stresses and strains at any different loads. From the literature it is observed that there is a good agreement between the calculated and measured stress strain curves. This review paper describes the effect of volume fraction and grain size of alpha phase on the stress strain curve of the titanium alloys. It also can predict the effect of strength ratio on stress strain curve by using FEM. This informa- tion will be of great use in designing and selecting the titanium alloys for various engineering applications.

MODELLING OF TENSILE STRESS-STRAIN CURVE OF WOVEN FABRICS

A general shape of tensile stress-strain curves of woven fabrics is first recognised by puttingtested and predicted results together.An exponential function with two parameters is then selectedfor the prediction of tensile stress-strain relationship.The predicted results by using the proposedfunction show excellent agreement with experimental data.

Application of the Mechanical Threshold Stress Model to Large Strain Processing

Large-strain deformations introduce several confounding factors that affect the application of the Mechanical Threshold Stress model. These include the decrease with the increasing stress of the normalized activation energy characterizing deformation kinetics, the tendency toward Stage IV hardening at high strains, and the influence of crystallographic texture. Minor additions to the Mechanical Threshold Stress model are introduced to account for variations of the activation energy and the addition of Stage IV hardening. Crystallographic texture cannot be modeled using an isotropic formulation, but some common trends when analyzing predominantly shear deformation followed by uniaxial deformation are described. Comparisons of model predictions with measurements in copper processed using Equal Channel Angular Pressing are described.

Development of a time-dependent energy model to calculate the mining-induced stress over gates and pillars

Generally, longwall mining-induced stress results from the stress relaxation due to destressed zone that occurs above the mined panel. Knowledge of induced stress is very important for accurate design of adjacent gateroads and intervening pillars which helps to raise the safety and productivity of longwall mining operations. This study presents a novel time-dependent analytical model for determination of the longwall mining-induced stress and investigates the coefficient of stress concentration over adjacent gates and pillars. The model is developed based on the strain energy balance in longwall mining incorporated to a rheological constitutive model of caved materials with time-varying parameters. The study site is the Tabas coal mine of Iran. In the proposed model, height of destressed zone above the mined panel, total longwall mining-induced stress, abutment angle, induced vertical stress, and coefficient of stress concentration over neighboring gates and intervening pillars are calculated. To evaluate the effect of proposed model parameters on the coefficient of stress concentration due to longwall mining, sensitivity analysis is performed based on the field data and experimental constants. Also, the results of the proposed model are compared with those of existing models. The comparative results confirm a good agreement between the proposed model and the in situ measurements. According to the obtained results, it is concluded that the proposed model can be successfully used to calculate the longwall mining-induced stress. Therefore, the optimum design of gate supports and pillar dimensions would be attainable which helps to increase the mining efficiency.

The Complete Stress-strain Curve of Recycled Aggregate Concrete under Uniaxial Compression Loading

The mechanical performance of recycled aggregate concrete （RAC） is investigated. An experiment on the complete stress-strain curve under uniaxial compression loading of RAC is carried out. The experimental results indicate that the peak stress, peak strain, secant modulus of the peak point and original point increase with the strength grade of RAC enhanced. On the contrary, the residual stress of RAC decreases with the strength grade enhancing, and the failure of RAC is often broken at the interface between the recycled aggregate and the mortar matrix. Finally, the constitutive model of stress-strain model of RAC has been constituted, and the results from the constitutive model of stress-strain meet the experiment results very well.

Strain-stress relation in macromolecular microsphere composite hydrogel

This paper investigates the strain-stress relation for the macromolecular microsphere composite （MMC） hydrogel. The novel point is to present the strain-stress model, which is based on the microscopic mixed entropy set up in the previous work and the Flory-Rehner elastic energy. Then, the numerical result of the strain-stress model is shown, which is completely consistent with the chemical experiment. Moreover, the theoretical relation of the strain-stress depends on the microscopic parameters of the MMC hydrogel. Therefore, it is a way to investigate the relation of macroscopic properties and microscopic structures of soft matters. This approach can be extended to other soft matters,

The model developed for stress-induced structural phase transformations of micro-crystalline silicon films

The nanoindentations were applied to island-shaped regions with metal-induced Si crystallizations. The experimental stress-strain relationship is obtained from the load-depth profile in order to investigate the critical stresses arising at various phase transitions. The stress and strain values at various indentation depths are applied to determine the Gibbs free energy at various phases. The intersections of the Gibbs free energy lines are used to determine the possible paths of phase transitions arising at various indentation depths. All the critical contact stresses corresponding to the various phase transitions at four annealing temperatures were found to be consistent with the experimental results.

Residual stress modeling of thin wall by laser cladding forming

The residual stress generated in the laser cladding could lead to undesirable distortions or even crack formation. In order to better understand the evolution/yielding process of stress field,a 3 D finite-element thermo-mechanical model was established for the laser cladding formation of thin wall with the 17-4 PH powder on the FV520( B) steel. The temperature field was firstly analyzed,based on which the stress field and strain field of the laser cladding forming process were analyzed.In order to validate the prediction,the final residual stress field in the obtained thin wall was tested by X-ray diffraction in comparison with the predicted results.

Euler-Lagrange Elasticity: Differential Equations for Elasticity without Stress or Strain

Differential equations to describe elasticity are derived without the use of stress or strain. The points within the body are the independent parameters instead of strain and surface forces replace stress tensors. These differential equations are a continuous analytical model that can then be solved using any of the standard techniques of differential equations. Although the equations do not require the definition stress or strain, these quantities can be calculated as dependent parameters. This approach to elasticity is simple, which avoids the need for multiple definitions of stress and strain, and provides a simple experimental procedure to find scalar representations of material properties in terms of the energy of deformation. The derived differential equations describe both infinitesimal and finite deformations.

A work-hardening and softening constitutive model for sand:modified plastic strain energy approach

The paper describes an energy-based constitutive model for sand, which is modified based on the modified plastic strain energy approach, represented by a unique relationship between the modified plastic strain energy and a stress parameter, independent of stress history. The modified plastic strain energy approach was developed based on results from a series of drained plastic strain compression tests along various stress paths on saturated dense Toyoura sand with accurate stress and strain measurements. The proposed model is coupled with an isotropically work-hardening and softening, non-associtated, elasto-plastic material description. The constitutive model concerns the inherent and stress system-induced cross-anisotropic elastic deformation properties of sand. It is capable of simulating the deformation characteristics of stress history and stress path, the effects of pressure level, anisotropic strength and void ratio, and the strain localization.

Constitutive model depending upon temperature and strain rate of carbon constructional quality steels

The basic factors relating to the rheological stress in the constitutive equations were introduced.Carbon constructional quality steels were regarded as a kind of elastic-viscoplastic materials under high temperature and the elastic-viscoplastic constitutive models were summarized.A series of tension experiments under the same temperature and different strain rates,and the same strain rate and different temperatures were done on 20 steel,35 steel and 45 steel.52 groups of rheological stress-strain curves were obtained.The experimental results were analyzed theoretically.The rheological stress constitutive models of carbon steels were built combining the strong points of the Perzyna model and Johnson-Cook model.Comparing the calculation results conducted from the model with the experiment results,the results proves that the model can reflect the temperature effect and strain rate effect of carbon constructional quality steels better.

A Displacement Based Analytical Model to Determine Residual Stress Components in a Finite Elastic Thin Plate with Hole-Drilling Method

In order to measure the residual stress components in an elastic thin plate, the hole-drilling strain-gage method has been used. This method enables to determine the relation between the magnitudes and directions of the principal stresses and the strain relaxation about the hole. In the existing analytical models based on stress field, the formulations associated with the hole-drilling method are based on the assumption of an infinite plate, this may cause some errors for a finite plate and it’s difficult to validate these solutions by FE methods. Furthermore, in the composite, the displacement field is continuous but the stress field is not necessarily continuous, the displacement field based method has to be used. In the present paper an analyt-ical model based on a displacement field described by a function with coefficients to determine for a finite round thin plate is presented. The coefficients used in the displacement field are independent on the three residual stress components, and they are determined by minimization of the internal strain energy during the hole-drilling process. Once the coefficients in the dis-placement field are determined, three strains measured in three radial directions are utilized to determine the three residual stress components. The proposed analytical model can be also adapted to infinite plate by assuming that the diameter of the round plate tends to infinite.

巷道围岩应力应变仿真系统设计与教学探索

为了获得采动条件下巷道围岩应力应变演化特征,通过复变函数换算结合工程实践经验,运用科学计算软件编程对巷道围岩受力模型进行求解,获得巷道围岩主应力和主应力集中系数分布规律。采用MATLAB对巷道受力模型进行模拟演算,获得工程开采参数与巷道围岩应力应变的关系规律,由此设计了一种采动影响下巷道围岩应力应变模拟仿真系统。该系统可对多种巷道断面进行计算和模拟,进而呈现出多种信息仿真演示,解决了以往软件适用巷道类型单一、应力求解不具体且结果呈现单一化缺点,可为回采巷道支护设计提供相应的理论依据,有利于矿山工程技术人员及实验室技术人员开展模拟仿真教学。

再生微粉活性粉末混凝土单轴受压应力-应变曲线研究

为了进一步了解再生微粉活性粉末混凝土(RPC)的受压性能,通过试验分析并结合有限元模拟,研究了不同配比再生微粉RPC的单轴受压应力-应变曲线。利用有限元软件Ansys在细观层次上建立了随机分布钢纤维模型,建立的随机钢纤维模型能较好地反映实际情况。进而采用数值模拟的方法分析了钢纤维体积掺量对RPC受压应力-应变曲线的影响。研究结果表明,随着再生微粉对水泥替代率的增加,峰值应力减小,峰值应变和弹性模量略有降低但变化不显著。钢纤维的加入提高了再生微粉RPC的峰值应力、峰值应变和弹性模量,且随着钢纤维掺量增加,峰值应力增长,弹性模量有一定增长,峰值应变变化不明显。

混凝土内部受压力学性能试验与本构分析

为了精准测量受压混凝土内部应力-应变关系,通过在丙烯酸树脂杆上布置电阻式应变片,测量得到混凝土试块内部应变,研发大量程微型土压力盒实现混凝土内部应力测量。将相关测试装置预埋在混凝土试块内部,完成混凝土试块单轴压缩试验,获得精准的混凝土内部应力-应变关系数据。结果表明,混凝土内部本构模型相对于传统本构模型具有良好的延性,且峰值强度远高于传统本构模型。结合国内外几种具有代表性的混凝土本构方程对试验进行了有限元模拟,针对应力-应变曲线、峰值强度及峰值应变进行了对比分析。分析结果表明:实测的混凝土内部应力-应变曲线合理准确,相关测量方法具有良好的精度和可行性;基于我国规范模型的有限元模拟结果与试验结果吻合最好,可进一步推广使用。

基于液压胀破测试方法的聚合物防水膜多轴拉伸性能表征

本研究设计了一种基于液压胀破的多轴拉伸测试装置和数据处理方法,以准确表征聚合物防水膜的拉伸性能。测试结果显示:随着防水膜厚度的增加,拉伸峰值应力降低;随着试件直径的增加,拉伸峰值应力增加;与单轴拉伸相比,多轴拉伸避免了“颈缩”效应,断裂伸长率降低;防水膜的应力-应变曲线符合线性应变强化弹塑性模型,并可通过多轴拉伸试验拟合得到相关参数。结论表明,多轴拉伸方法结果更接近实际工况下的应力-应变响应,可为合理评估防水膜在工程环境中的力学性能提供参考和借鉴。

人工冻土分数阶导数应力-应变指数模型参数确定及验证

人工冻土可看成是理想固体和理想流体以某种比例进行的勾兑,其力学特性既不符合胡克定律,也不遵守牛顿黏性定律,而是遵守介于它们之间的某种关系,分数阶导数能够很好地描述这种勾兑效应。对合肥膨胀土在不同冻结温度下进行单轴压缩试验,得到冻结温度对应力-应变的影响规律。将分数阶导数引入指数模型,将它改进为人工冻土单轴压缩下应力-应变分数阶指数模型。通过对建立的模型两边取自然对数,得到不同温度下应力-应变线性方程组,求解建立方程组确定出分数阶导数模型参数。为进一步验证所建立模型的适用性,引用一组南京冻结粉质黏土三轴剪切试验,在分数阶系数中考虑围压的影响,将它改进为能考虑围压影响的应力-应变分数阶指数模型。将改进的人工冻土应力-应变分数阶指数模型的计算结果与试验结果进行比较,结果表明:计算结果与试验结果吻合度较高,能准确地预测单轴压缩、三轴压缩剪切应力-应变曲线的变化趋势。改进后的分数阶导数模型参数较少且有明确的物理意义,便于工程应用。当前的模型仅仅适用于应变硬化型,为了能进一步描述应变软化型的力学特性,下一步将在模型中考虑损伤进而建立应变软化型分数阶指数模型。同时,如何在模型中反映冻土的结构性与各向异性对应力-应变的影响也将是下一步的研究内容。

Principle of Interaction between Plastic Volumetric and Shear Strains and the Constitutive Model for Geotechnical Materials

Here is proposed the principle of interaction between plastic volumetric and shear strains, revealing the main origin of generating the complexity and variety of deformations for geotechnical materials. Here are also explained the manners of the interaction between plastic volumetric and shear strains and the conditions of generating shear dilatancy. It is demonstrated that dependency of the stress path exists and is a combination of effects of this interaction. According to this principle, it is theoretically proved that the space critical state line exists, and is unique and independent of the stress history. Based on this principle, the constitutive models that are able completely and accurately to characterize the basic behavior features for geotechnical materials have been constructed within the framework of thermodynamics. What is determined is a general expression of the constitutive relation as well as the inequality of the dissipative potential increment for obeying the second law of thermodynamics.

超高性能海水海砂混凝土单轴受压应力-应变关系

为研究超高性能海水海砂混凝土(Ultra-High Performance Seawater Sea-sand Concrete,UHPSSC)轴压应力-应变关系,探究钢纤维体积掺量对UHPSSC轴压力学性能和轴压应力-应变全曲线的影响,使用Instron 1346万能材料试验机测得UHPSSC应力-应变全曲线,借助Pycharm曲线拟合程序与已有混凝土应力-应变全曲线模型进行拟合与修正.结果表明:钢纤维掺量越高,轴压作用下UHPSSC的破坏程度越小,抗压强度和峰值应变也随之增加;已有模型拟合的UHPSSC应力-应变全曲线上升段与试验曲线比较相近,下降段后期差距较大;通过修正下降段形状参数,得到了更适用于描述UHPSSC应力-应变全曲线的修正模型,并验证该修正模型的适用范围.