Theoretical analysis of geometrical and mechanical parameters in plain woven structures

An analytic solution for the estimation of structural parameters and initial tensile modulus of plain woven fabrics under uniaxial tensile loading in their linear elastic domain of deformation is presented. For this purpose, a new approach in straight line geometry with a parallel segment to the fabric plane and an inclined segment at the weave intersection in 3D form is proposed which leads to the theoretical estimation of all the structural parameters of plain woven fabrics with saw-tooth geometry. Defining and applying of JJ₂ Ratio in the model enable us to modify the geometrical model and estimate the value of structural parameters considering the history of samples influenced mainly by its manufacturing process. The strain energy method and Castigliano’s theorem are used for the mechanical analysis of the structure. The elasticity, bending, shearing, and compression rigidity of yarns are incorporated into the model. It has been shown that predicting the geometrical and mechanical parameters of woven fabrics before production are possible if and only if the crimp value of the fabrics can be estimated before their production. The proposed theory is validated and compared by applying into some experimental data and a previous model.     

Charactersitics of Stress-strain Curve of High Strength Steel Fiber Reinforced Concrete under Uniaxial Tension

A whole of 110 specimens divided into 22 groups were tested with varying the volume fraction of steel fibers and the matrix strength of these specimens. The stress-strain behaviors of four types of steel fiber reinforced concrete （SFRC） under uniaxial tension were studied experimentally. When the matrix strength and the fiber content increase, the tensile stress and tensile strain vary differently according to the fiber type. The mechanisms of reinforcing effect for different types of fiber were analyzed and the stress-strain curves of the specimens were plotted. Some experimental factors for stress or strain of SFRC were given. A tensile toughness modulus Re0.5 was introduced to evaluate the toughness characters of SFRC under uniaxial tension. Moreover, the formula of the tensile stress-strain curve of SFRC was regressed. The theoretical curve and the experimental ones fit well, which can be used for references in construction.     

Unified analytical stress- strain curve for quasibrittle geomaterial in uniaxial tension, direct shear and uniaxial compression

Considering strain localization in the form of a narrow band initiated just at peak stress, three analytical expressions for stress - strain curves of quasibrittle geomaterial （such as rock and concrete） in uniaxial tension, direct shear and uniaxial compression were presented, respectively. The three derived stress - strain curves were generalized as a unified formula. Beyond the onset of strain localization, a linear strain-softening constitutive relation for localized band was assigned. The size of the band was controlled by internal or characteristic length according to gradient-dependent plasticity. Elastic strain within the entire specimen was assumed to be uniform and decreased with the increase of plastic strain in localized band. Total strain of the specimen was decomposed into elastic and plastic parts. Plastic strain of the specimen was the average value of plastic strains in localized band over the entire specimen. For different heights, the predicted softening branches of the relative stress- strain curves in uniaxial compression are consistent with the previously experimental results for normal concrete specimens. The present expressions for the post-peak stress - deformation curves in uniaxial tension and direct shear agree with the previously numerical results based on gradient-dependent plasticity.     

A Comparative Study of Several Material Models for Prediction of Hyperelastic Properties: Application to Silicone-Rubber and Soft Tissues

Abstract:  The correct modelling of constitutive laws is of critical importance for the analysis of mechanical behaviour of solids and structures. For example, the understanding of soft tissue mechanics, because of the nonlinear behaviour commonly displayed by the mechanical properties of such materials, makes common place the use of hyperelastic constitutive models. Hyperelastic models however, depend on sets of variables that must be obtained experimentally. In this study the authors use a computational/experimental scheme, for the study of the nonlinear mechanical behaviour of biological soft tissues under uniaxial tension. The material constants for seven different hyperelastic material models are obtained via inverse methods. The use of Martins's model to fit experimental data is presented in this paper for the first time. The search for an optimal value for each set of material parameters is performed by a Levenberg–Marquardt algorithm. As a control measure, the process is fully applied to silicone-rubber samples subjected to uniaxial tension tests. The fitting accuracy of the experimental stress–strain relation to the theoretical one, for both soft tissues and silicone-rubber (typically nonlinear) is evaluated. This study intents also to select which material models (or model types), the authors will employ in future works, for the analysis of human soft biological tissues.     

巷道开挖效应下围岩损伤演化规律及试验研究

为了研究巷道开挖卸荷引起的围岩损伤演化规律及力学响应机制,采用RMT-150试验机开展了白砂岩试件三轴加载-卸载-单轴再加载力学特性试验,建立了岩石三维损伤本构模型。根据损伤岩石单轴加载应力-应变曲线,分析其破坏模式,确定了单轴再加载强度、波速与初始轴压加载比的非线性关系。结果表明:(1)三轴应力作用下,岩石损伤演化过程分为5个阶段,即损伤稳定减小阶段、无损伤阶段、损伤稳定增长阶段、损伤加速增长阶段、损伤破坏阶段;(2)单轴再加载强度、波速与初始轴压加载比呈现3次非线性相关关系,从环向应变规律来看,损伤岩石表现出塑性硬化和塑性软化两种状态;(3)损伤岩石以张拉破坏为主,随着损伤变量值增加,破坏形式逐渐向剪切滑移破坏转化。通过对岩石损伤演化规律的研究,为工程围岩稳定性判别及预警提供理论依据。     

无纺织物单向拉伸孔径变化试验与理论预测对比分析

无纺织物作为反滤材料,常处于单向受拉工作状态。单向拉伸引起无纺织物孔径变化,易导致反滤工程失效。通过控制织物应变的干筛试验,定量测试了无侧限单向拉应变逐级增大的过程中,两种不同厚度短纤针刺无纺织物的孔径分布曲线变化。采用干筛试验结果,对现有两种体系的单向应变下无纺织物孔径预测理论解进行验证:一类是佘巍等效孔径O95理论解,一类是Rawal孔径分布曲线理论解。通过对比两种理论解对各级拉应变下的O95值预测,归纳二者的预测误差规律,从理论假设出发分析误差原因。同时采用前人图像法测得的热粘无纺织物O95变化验证两类理论解。两种理论解均能较好地预测无侧限单向拉应变下无纺织物O95的变化规律。O95随单向拉应变呈近似线性减小的规律。对于O95变化斜率的预测,佘巍解较准确,Rawal解偏大。对于O95数值预测可结合两类理论解给出变化范围。     

OpenSees混凝土单轴本构关系二次开发

由于其强大的非线性分析功能,OpenSees越来越广泛地被应用于结构的非线性分析当中。混凝土本构模型是钢筋混凝土结构非线性分析的基础。详细介绍了在OpenSees中添加新单轴材料本构的基本原理。Mander模型是应用最为广泛的箍筋约束混凝土本构模型之一,但OpenSees中尚未集成Mander模型。以Mander模型为例,介绍了箍筋约束混凝土单轴滞回本构关系的开发过程,并成功地将其集成到OpenSees中。最后用三个算例验证了新添加本构关系的实用性。     

浸水时间对饱水岩石损伤破坏过程中声发射特征影响的试验

为研究不同浸水时间饱水岩石损伤破坏过程中的力学特性和声发射特征,对天然及不同浸水时间的饱水闪长岩进行单轴压缩力学试验和声发射试验,研究单轴应力状态下其力学特性和声发射特征,结合累积声发射数与损伤变量一致的观点,建立了基于浸水时间的饱水岩石声发射损伤模型。结果表明:饱水岩石的单轴抗压强度和弹性模量随浸水时间的增加呈指数的变化规律逐渐减小,浸水60 d后其值均趋于稳定;在加载的各个阶段不同浸水时间的饱水岩样均有声发射事件产生,声发射事件率与岩样的应力-应变关系趋势均具有较好的一致性;声发射累积数的变化规律与岩样内部损伤演化规律具有一致性。     

基于数字体散斑法煤样内部三维应变场的测量

煤岩内部三维应力应变场的测量对冲击地压、气体吸附及煤与瓦斯突出的机理研究具有重要意义,但在实验室中很少有方法能够直接测量煤岩内部三维应力应变场。采用新近发展的数字体散斑法与微焦点工业CT相结合的方法分别测量了单轴压缩和CO2气体吸附过程中煤样内部的变形,由变形获得煤样内部的三维应变场。研究表明,煤样内部天然结构可以作为携带变形信息的散斑结构,用于变形测量。通过单轴压缩煤样内部等效应变及体积应变的可视化,直观地显示出试件内部应变局部化区域孕育发展的过程,煤样的变形破坏具有明显的应变局部化特征,应变局部化区域与煤样最终破坏断裂区域位置一致。利用DVSP法测得了煤样吸附CO2过程中的三维应变场,并与应变片测量结果进行了对比,两者具有较好的一致性。通过体积应变的分布图直观揭示出煤样在吸附气体过程中存在膨胀区与压缩区,由于非均质性造成内部变形不均匀。