ANALYSIS OF THE LOCALIZATION OF DAMAGE AND THE COMPLETE (STRESS-STRAIN) RELATION FOR MESOSCOPIC HETEROGENEOUS ROCK UNDER UNIAXIAL TENSILE LOADING

The mechanical behavior of rock under uniaxial tensile loading is different from that of rock under compressive loads. A micromechanics-based model was proposed for mesoscopic heterogeneous brittle rock undergoing irreversible changes of their microscopic structures due to microcrack growth. The complete stress-strain relation including linear elasticity, nonlinear hardening,rapid stress drop and strain softening was obtained. The influence of all microcracks with different sizes and orientations were introduced into the constitutive relation by using the probability density function describing the distribution of orientations and the probability density function describing the distribution of sizes. The influence of Weibull distribution describing the distribution of orientations and Rayleigh function describing the distribution of sizes on the constitutive relation were researched. Theoretical predictions have shown to be consistent with the experimental results.

Tensile Properties and Prediction Model of Recombinant Bamboo at Different Temperatures

The destruction of recombinant bamboo depends on many factors,and the complex ambient temperature is an important factor affecting its basic mechanical properties.To investigate the failure mechanism and stress–strain relationship of recombinant bamboo at different temperatures,eighteen tensile specimens of recombinant bamboo were tested.The results showed that with increasing ambient temperature,the typical failure modes of recombinant bamboo were flush fracture,toothed failure,and serrated failure.The ultimate tensile strength,ultimate strain and elastic modulus of recombinant bamboo decreased with increasing temperature,and the ultimate tensile stress decreased from 154.07 to 96.55 MPa,a decrease of 37.33%,and the ultimate strain decreased from 0.011 to 0.008,a decrease of 26.57%.Based on the Ramberg-Osgood model and the pseudo‒elastic design method,a predictive model was established for the tensile stress–strain relationship of recombinant bamboo considering the temperature level.The model can accurately evaluate the tensile stress–strain relationship of recombinant bamboo under different temperature conditions.

EFFECT OF DENTIN TUBULES ON THE MECHANICAL PROPERTIES OF DENTIN. PART Ⅰ: STRESS-STRAIN RELATIONS AND STRENGTH CRITERION

As known, there is a large number of dentin tubules in dentin. These tubules have varying radii and are shaped into radially parallel pattern. The anisotropy of microstructure of dentin shows that dentin should be treated as a ma- terial of varying transverse isotropy. In this Part, the elastic stress-strain relations and the quadratic strength criterion are established in the form of having varying transverse isotropy, in the framework of micromechanics to take into account of the effect of the microstructures-dentin tubules. Simplified forms for isotropic and ho- mogeneous cases, as well as the corresponding plane stress form of the stress-strain relations are also given. These theoretical models are very well supported by the experiments shown later in the continued paper (Part Ⅱ).

Changes of Stress-Strain Relationship of Rabbit Femoral Vein after Simulated Weightlessness

Objective: To observe the effect of simulated weightlessness on stress-strain relationship and the structural change of rabbit femoral vein. Methods: After seting up the Head-Down Tilt (-20°) (HDT) model to simulate weightlessness, 24 healthy male New-Zealand Rabbits were randomly divided into HDT-21d group, HDT-10d group and control group, with 8 in each. Femoral venous strips and rings were used to make uniaxial tensile test of the longitudinal and circumferential specimens of the vessels. At last we observed the microstructure of femoral vein wall in 3 groups. Results : With the increasing of load stress, both longitudinal and circumferential strains of vein samples from 3 groups increased significantly (P<0. 01). With the decrease of unload stress, strains decrease obviously (P<0. 01). The unloaded longitudinal and the circumferential strain from 3 groups increased much than those of the loaded. Under the same stress (longitudinal 0-2. 0 g, circumferential 0. 5-1. 0 g) , HDT-21d group and HDT-10d group increased obviously in tlie longitudinal or circumferential strain (load and unload) than control, and HDT-21d increased much than that of HDT-10d. The contents and structures of femoral vein walls of HDT-rabbits changed significantly. Some endotheli-um cells of femoral vein became short, columnar or cubic even fell off. Smooth muscle layers became thinner. Conclusion:The compliance of femoral venous increased significantly after weightlessness-simulation and increased much obviously after 21d-HDT than that of 10 d. The structure of femoral vein wall changed obviously. The changes may be one reason for the increase of femoral vein compliance.

Tensile stress-strain behavior of metallic alloys

Tensile stress-strain curves of five metallic alloys,i.e.,SKH51,STS316L,Ti-6Al-4V,Al6061and Inconel600were analyzed to investigate the working hardening behavior.The constitutive parameters of three constitutive equations,i.e.,the Hollomon,Swift and Voce equations,were compared by using different methods.A new working hardening parameter was proposed to characterize the working hardening behavior in different deformation stages.It is found that Voce equation is suitable to describe stress-strain curves in large strain region.Meanwhile,the predicting accuracy of ultimate tensile strength by Voce equation is the best.The working hardening behavior of SKH51is different from the other four metallic alloys.

Effects of curing age on compressive and tensile stress-strain behaviors of ecological high ductility cementitious composites

To obtain the design parameters of the structure made by ecological high ductility cementitious composites(Eco-HDCC),the effects of curing age on the compressive and tensile stress-strain relationships were studied.The reaction degree of fly ash,non-evaporable water content and the pH value in pore solution were calculated to reveal the mechanical property.The results indicate that as the curing age increases,the peak compressive strength,peak compressive strain and ultimate tensile strength of Eco-HDCC increase.However,the ultimate compressive strain and ultimate tensile strain of Eco-HDCC decrease with the increase in curing age.Besides,as the curing age increases,the reaction degree of fly ash and non-evaporable water content in Eco-HDCC increase,while the pH value in the pore solution of Eco-HDCC decreases.Finally,the simplified compressive and tensile stress-strain constitutive relationship models of Eco-HDCC with a curing age of 28 d were suggested for the structure design safety.

Textile Environmental Conditioning: Effect of Relative Humidity Variation on the Tensile Properties of Different Fabrics

With the aim that to confirm the need for humidity control in the environment in which textile sample are visually and instrumentally analyzed, three different pre-conditioned fabrics sample of cotton, polyester and silk were treated at a fix temperature of 21?C. The relative humidity adjusted to four levels: 55%, 65%, 75% and 85% RH for a conditioning time of 24 hours as specified in ASTM D-1776-98. It has been observed that as the relative humidity increase from 55% to 85% cotton increase its tensile strength, silk losses its strength and there was no significant change observed in the tensile strength of polyester fabric.

Novel Ring Compression Test Method to Determine the Stress-Strain Relations and Mechanical Properties of Metallic Materials

Although there are methods for testing the stress-strain relation and strength,which are the most fundamental and important properties of metallic materials,their application to small-volume materials and tube components is lim-ited.In this study,based on energy density equivalence,a new dimensionless elastoplastic load-displacement model for compressed metal rings with isotropy and constitutive power law is proposed to describe the relations among the geometric dimensions,Hollomon law parameters,load,and displacement.Furthermore,a novel test method was developed to determine the elastic modulus,stress-strain relation,yield and tensile strength via ring compression test.The universality and accuracy of the method were verified within a wide range of imaginary materials using finite element analysis(FEA),and the results show that the stress-strain curves obtained by this method are consistent with those inputted in the FEA program.Additionally,a series of ring compression tests were performed for seven metallic materials.It was found that the stress-strain curves and mechanical properties predicted by the method agreed with the uniaxial tensile results.With its low material consumption,the ring compression test has the potential to be as an alternative to traditional tensile test when direct tension method is limited.

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.

ANALYSIS OF THE LOCALIZATION OF DAMAGE AND THE COMPLETE STRESS-STRAIN RELATION FOR MESOSCOPIC HETEROGENEOUS BRITTLE ROCK SUBJECTED TO COMPRESSIVE LOADS

A micromechanics-based model is established. The model takes the interaction among sliding cracks into account, and it is able to quantify the effect of various parameters on the localization condition of damage and deformation for brittle rock subjected to compressive loads. The closed-form explicit expression for the complete stress-strain relation of rock containing microcracks subjected to compressive loads was obtained. It is showed that the complete stress-strain relation includes linear elasticity,nonlinear hardening,rapid stress drop and strain softening.The behavior of rapid stress drop and strain softening is due to localization of deformation and damage. Theoretical predictions have shown to be consistent with the experimental results.

Stress-strain relationship for reactive powder concrete with recycled powder under uniaxial compression

The recycled powder(RP)from construction wastes can be used to partially replace cement in the preparation of reactive powder concrete.In this paper,reactive powder concrete mixtures with RP partially replacing cement,and natural sand instead of quartz,are developed.Standard curing is used,instead of steam curing that is normally requested by standard for reactive powder concrete.The influences of RP replacement ratio(0%,10%,20%,30%),silica fume proportion(10%,15%,20%),and steel fiber proportion(0%,1%,2%)are investigated.The effects of RP,silica fume,and steel fiber proportion on compressive strength,elastic modulus,and relative absorption energy are analyzed,and theoretical models for compressive strength,elastic modulus,and relative absorption energy are established.A constitutive model for the uniaxial compressive stress-strain relationship of reactive powder concrete with RP is developed.With the increase of RP replacement ratio from 0% to 30%,the compressive strength decreases by 42% and elastic modulus decreases by 24%.

Multi-response optimization of process parameters in friction stir welded AM20 magnesium alloy by Taguchi grey relational analysis

The purpose of this paper is to optimize the process parameter to get the better mechanical properties of friction stir welded AM20 magnesium alloy using Taguchi Grey relational analysis(GRA).The considered process parameters are welding speed,tool rotation speed,shoulder diameter and plunging depth.The experiments were carried out by using Taguchi's L18 factorial design of experiment.The processes parameters were optimized and ranked the parameters based on the GRA.The percentage influence of each process parameter on the weld quality was also quantified.A validation experimental run was conducted using optimal process condition,which was obtained from the analysis,to show the improvement in mechanical properties of the joint.This study also shows the feasibility of the GRA with Taguchi technique for improvement in welding quality of magnesium alloy.

THE CONSTITUTIVE RELATION OF CRACK-WEAKENED ROCK MASSES UNDER AXIAL-DIMENSIONAL UNLOADING

An accurate and efficient numerical method for solving the crack-crack interaction problem is presented. The method is mainly by means of the dislocation model, stress superposition principle and Chebyshev polynomial expansion of the pseudo-traction. This method can be applied to compute the stress intensity factors of multiple kinked cracks and multiple rows of periodic cracks as well as the overall strains of rock masses containing multiple kinked cracks under complex loads. Many complex computational examples are given. The dependence of the crack-crack interaction on the crack configuration, the geometrical and physical parameters, and loads pattern, is investigated. By comparison with numerical results under confining pressure unloading, it is shown that the crack-crack interaction under axial-dimensional unloading is weaker than those under confining pressure unloading. Numerical results for single faults and crossed faults show that the single faults are more unstable than the crossed faults. It is found from numerical results for different crack lengths and different crack spacing that the interaction among kinked cracks decreases with an increase in length of the kinked cracks and the crack spacing under axial-dimensional unloading.

TENSILE TEST AND PHYSICAL MODEL OF NiTi SHAPE MEMORY ALLOY

The tensile stress-strain curves of NiTi wires are obtained by tensile experiments under different heat treatments. A phenomenological physical model based on hysteresis element method is developed to describe the experimentally determined stress-strain curves of shape memory alloy (SMA) wires. Numerical simulations are made. Simulation results show that：(1) a series of unusual changes on physical and mechanical properties of SMA wires occur when martensitic, especially R (rhombohedral) phase transformation emerge. The stress-strain relation of SMA wires is highly non-linear; (2) there are no notable yielding phenomena before NiTi wires are broken; (3) numerical results obtained by the physical model are in good agreement with experimental data.

Study of experiment method for triaxial frost heaving constitutive relation of frozen soil

A new experiment method is introduced to study the relations between frost heaving strain and stress of soil or fracture rock under different moisture contents, temperatures and stress states. Based on experiments, a new triaxial frost heaving stress-strain relation is presented aiming at tunnel in cold regions. The experimental equipments are strain-controlled and low temperature buildup, and different restrained conditions are controlled by changing the stiffness of a test-force-ring. Then the frost heaving strain and stress of soil or fracture rock can be obtained under different restrained conditions, and the frost heaving stress-strain relation can be got by regression analysis of some feature points. Experiments of saturated sandy soil conducted by this method show that the triaxial frost heaving stress-strain relation could be expressed by logarithmic curves, and the frost heaving stress changes linearly with the logarithm of the frost heaving strain. The stronger the constraint, the smaller the frost heaving strain and the larger the frost heaving stresses. The frost heaving stresses would tend to a limit value with increasing the constraint intensity. The larger the confining pressure, the larger the frost heaving strain and stress.

Tensile behaviors of ecological high ductility cementitious composites exposed to interactive freeze-thaw-carbonation and single carbonation

To explore the tensile property parameters in the structural design of bridge deck link slabs made by ecological high ductility cementitious composites (Eco-HDCC), the tensile properties of Eco-HDCC exposed to interactive freeze-thaw-carbonation cycles and single carbonation cycles were studied. The carbonation front of Eco-HDCC was determined by X-ray diffraction and differential scanning calorimetry-thermal gravimetric methods. Results indicate that the carbonation front of Eco-HDCC after interaction tests is deeper than that of Eco-HDCC after single carbonation tests. In addition, the ultimate tensile strength for Eco-HDCC shows an increasing trend after the interaction of 1 to 5 cycles compared with that of virgin specimens, while the ultimate tensile strength decreases after the interaction of 10 to 15 cycles. For single carbonation tests, the ultimate tensile strength of Eco-HDCC increases as cycles increase. After being subjected to interaction and single carbonation environments, both the ultimate tensile strain and tensile strain energy of Eco-HDCC decrease as cycles increase, and the decrease degrees of Eco-HDCC after interaction cycles are larger than those of Eco-HDCC after single carbonation. For general consideration, the tensile stress-strain relationship of Eco-HDCC after the interaction of 15 cycles can be adopted in the design of bridge deck link slabs for the purpose of safety.

Dependence of mechanical properties on the site occupancy of ternary alloying elements in γ'-Ni3Al: Ab initio description for shear and tensile deformation

The site occupancy behavior of ternary alloying elements inγ'-Ni3Al(a key strengthening phase of commercial Ni-based single-crystal superalloys)can change with temperature and alloy composition owing to the effect of entropy.Using a total-energy method based on density functional theory,the dependence of tensile and shear behaviors on the site preference of alloying elements inγ'-Ni3Al were investigated in detail.Our results demonstrate that Fe,Ru,and Ir can significantly improve the ideal tensile and shear strength of theγ'phase when occupying the Al site,with Ru resulting in the strongest enhancement.In contrast,elements with fully filled d orbitals(i.e.,Cu,Zn,Ag,and Cd)are expected to reduce the ideal tensile and shear strength.The calculated stress-strain relationships of Ni3Al alloys indicate that none of the alloying elements can simultaneously increase the ideal strength of theγ'phase for both Ni1-site and Ni2-site substitutions.In addition,the charge redistribution and the bond length of the alloying elements and host atoms during the tensile and shear processes are analyzed to unveil the underlying electronic mechanisms.

生理性拉应力通过Nell-1/Ihh信号通路对ATDC5软骨细胞分化的调控作用

目的:探讨生理性拉应力对软骨细胞分化的调控作用,并阐明其相关信号通路机制。方法:体外培养软骨ATDC5细胞,应用四点弯曲细胞力学加载仪对其施加生理性拉应力,首先分为对照组和拉应力组(2 000μstrain/2 h组),另分为不同力值(1 000、2 000和3 000μstrain)加力时间为2 h和力值为2 000μstrain不同加力时间(1、2和4 h)组,同时设未加力的细胞为对照组,采用实时荧光定量PCR (RT-qPCR)法检测各组细胞中Ⅱ型胶原(Col-Ⅱ)、Ⅹ型胶原(Col-Ⅹ)、聚集蛋白聚糖(Aggrecan)、性别决定区Y框蛋白9 (SOX9)、血管内皮生长因子(VEGF)、增殖细胞核抗原(PCNA)、Nel样1型分子(Nell-1)、Runt相关转录因子2 (Runx2)、印度刺猬因子(Ihh)、补缀同源物1 (Ptch-1)、GLI家族锌指蛋白1 (Gli-1)和刺猬因子相互作用蛋白1 (Hhip-1) mRNA表达水平,采用Western blotting法检测各组细胞中Nell-1、Runx2和Ihh蛋白表达水平。ATDC5细胞分为对照组、环巴胺组、拉应力组和环巴胺+拉应力组,采用RT-qPCR法检测各组细胞中Nell-1、Ihh、Ptch-1、Gli-1和Hhip-1 mRNA表达水平,采用Western blotting法检测各组细胞中Nell-1和Ihh蛋白表达水平。结果:与对照组比较,2 000μstrain/2 h组细胞中Col-Ⅱ、 Col-Ⅹ、 Aggrecan、 SOX9、VEGF和PCNA mRNA表达水平均明显升高(P<0.01)。在对细胞施加2 000μstrain不同加力时间(1、2和4 h)或不同力值(1 000、2 000和3 000μstrain) 2 h的拉应力后,与对照组比较,随时间的延长或力值的增加其他各组细胞中Runx2 mRNA表达水平逐渐升高(P<0.01),Nell-1、Ihh、 Ptch-1、 Gli-1和Hhip-1 mRNA表达水平逐渐升高(P<0.01),且在2 000μstrain/2 h时达到最高,随后出现回落但仍明显高于对照组(P<0.01)。Western blotting检测,各组细胞中Nell-1、Runx2和Ihh蛋白表达水平与mRNA表达水平变化趋势一致。环巴胺预处理后,与对照组比较,环巴胺组细胞中Ihh、Ptch-1、Gli-1和Hhip-1 mRNA表达水平均明显降低(P<0.01),拉应力组和环巴胺+拉应力组细胞中Nell-1、Ihh、Ptch-1、Gli-1和Hhip-1 mRNA表达水平明显升高(P<0.01);与环巴胺组比较,环巴胺+拉应力组细胞中Nell-1、Ihh、Ptch-1、Gli-1和Hhip-1 mRNA表达水平明显升高(P<0.01);与拉应力组比较,环巴胺+拉应力组细胞中Ihh、Ptch-1、Gli-1和Hhip-1 mRNA表达水平明显降低(P<0.01)。与对照组比较,环巴胺组细胞中Ihh蛋白表达水平明显降低(P<0.01),Nell-1蛋白表达水平差异无统计学意义(P>0.05),拉应力组和环巴胺+拉应力组细胞中Nell-1和Ihh蛋白表达水平明显升高(P<0.01);与环巴胺组比较,拉应力组和环巴胺+拉应力组细胞中Nell-1和Ihh蛋白表达水平明显升高(P<0.01);与拉应力组比较,环巴胺+拉应力组细胞中Nell-1和Ihh蛋白表达水平差异均无统计学意义(P>0.05)。结论:在生理性拉应力刺激下,Nell-1可在上游激活Ihh信号通路,进而调控ATDC5软骨细胞的分化。

基于小冲杆试验评价钛合金的高温拉伸性能

基于能量密度等效理论,提出依据塑性弯曲阶段载荷-位移曲线,获取均匀连续、各向同性的钛合金圆片试样应力-应变曲线的小冲杆试验新方法;采用该方法预测得到不同名义屈服强度和硬化指数组合工况下假想材料的应力-应变数据,并与有限元预设曲线进行对比;基于小冲杆试验获得的TC4合金在室温、高温(400℃)下的载荷-位移曲线,采用上述方法获得该合金的真应力-真应变曲线,并与单轴拉伸真应力-应变曲线进行对比。结果表明:预测得到的假想材料应力-应变数据与有限元预设曲线之间的符合优度均高于0.96;预测获得的室温和400℃下TC4合金的真应力-真应变曲线与单轴拉伸曲线之间的符合优度均高于0.97,屈服强度和抗拉强度的最大相对误差分别为4.86%和4.63%,验证了该新方法的有效性。

Isogeometric Analysis of Hyperelastic Material Characteristics for Calcified Aortic Valve

This study explores the implementation of computed tomography(CT)reconstruction and simulation techniques for patient-specific valves,aiming to dissect the mechanical attributes of calcified valves within transcatheter heart valve replacement(TAVR)procedures.In order to facilitate this exploration,it derives pertinent formulas for 3D multi-material isogeometric hyperelastic analysis based on Hounsfield unit(HU)values,thereby unlocking foundational capabilities for isogeometric analysis in calcified aortic valves.A series of uniaxial and biaxial tensile tests is executed to obtain an accurate constitutive model for calcified active valves.To mitigate discretization errors,methodologies for reconstructing volumetric parametric models,integrating both geometric and material attributes,are introduced.Applying these analytical formulas,constitutive models,and precise analytical models to isogeometric analyses of calcified valves,the research ascertains their close alignment with experimental results through the close fit in displacement-stress curves,compellingly validating the accuracy and reliability of the method.This study presents a step-by-step approach to analyzing themechanical characteristics of patient-specific valves obtained fromCT images,holding significant clinical implications and assisting in the selection of treatment strategies and surgical intervention approaches in TAVR procedures.