延性材料全阶段单轴本构关系获取方法

材料拉伸直至破坏的单轴本构关系对于材料与结构的大变形分析和裂纹问题中的变形行为研究有重要意义。该文提出一种将实验与数值分析相结合获取材料拉伸直至颈缩破断全程本构关系的新方法——TF法(tensile test-finite element method)。通过构造初始微小缺陷实现拉伸试样的颈缩有限元模拟;应用自制对中夹具,结合DIC(digital image correlation)方法和应变传感器来测量材料拉伸全程的标距位移、试样颈缩根部直径和颈缩区轮廓线;应用逐步逼近原理并借助开发的ANSYS APDL命令流程序实现材料全程单轴真实本构关系求解的迭代计算。应用TF方法得到的全程材料本构关系对SS316L和T225NG合金拉伸试样的颈缩行为进行有限元模拟分析,结果表明:颈缩区两端的载荷-位移曲线、最小截面直径-位移曲线和颈缩区轮廓线的数值分析结果与实验结果吻合良好;对SS316L小曲率半径漏斗试样的载荷-位移曲线的模拟结果也与实验结果吻合良好。还给出SS316L和T225NG合金两种材料的Ramberg-Osgood全程单轴本构模型参数与等效破断应力应变,讨论两种材料单轴拉伸试样破断时颈缩根部横截面的应力分布。     

2.5D C/SiC复合材料连续损伤本构模型

基于连续损伤力学建立了一种包含拉伸与剪切损伤变量的2.5D C/SiC复合材料连续损伤本构模型。分别开展了拉伸和剪切试验,获得应力-应变曲线,并通过拟合试验曲线获得各损伤变量的演化参数。采用子程序技术将本构模型嵌入商用有限元软件ANSYS,应用有限元法计算了材料的应力-应变曲线。考虑了拉剪损伤耦合效应,计算了偏轴拉伸情况下的应力-应变曲线。结果表明:沿经纱拉伸、沿纬纱拉伸以及面内剪切的应力-应变曲线与试验结果吻合,最大偏差依次为4.30%、3.09%及3.73%;偏轴拉伸计算与试验应力-应变曲线也吻合较好。      

TA17合金薄片材料毫小试样疲劳性能研究

关键工程结构、小尺寸零部件和焊接区的疲劳寿命评估中往往无法采用传统大试样进行疲劳试验,因此本文提出了一种采用毫米级别薄片试样获取材料循环本构关系和低周疲劳寿命的新方法。在Care原位试验机上完成毫米级别薄片漏斗试样的加载工装和低周疲劳试验的基础上,通过变幅对称循环试验和等辐循环试验分别实现了材料循环本构关系和低周疲劳性能的获取。该文提出了一种对不同幂律材料和不同几何尺寸构型均具有良好普适性的材料循环本构关系预测模型,并通过有限元实现了模型准确性的正反向预测验证。将循环本构关系用于有限元计算中,给出了薄片漏斗试样漏斗两侧名义应力、名义应变和漏斗根部真实应力、真实应变的转换方程,进而预测材料的低周疲劳寿命。该文完成了TA17合金等直圆棒试样和1.2 mm厚度薄片漏斗试样的对称变幅循环试验和多级等辐循环试验。由模型预测获得的TA17合金循环本构关系与等直圆棒试样的试验结果比较表明:两种曲线的弹性段和0.009 mm/mm~0.011 mm/mm应变段吻合良好,在弹塑性过渡段（0.004 mm/mm~0.009 mm/mm）模型预测结果最大相对误差小于9%。根据两组应力和应变转换方程获得的漏斗试样材料代表性体积单元疲劳寿命和Manson-Coffin寿命预测模型与等直圆棒试样试验结果均吻合良好。     

基于等效全程单轴本构关系的应力三轴度分析

该文利用有限元辅助测试(Finite element Aided Testing,FAT)方法测得了四种工程材料的等效全程单轴本构关系,基于此全程本构关系对拉伸过程中漏斗试样的应力三轴度演化进行了有限元分析,并同基于Bridgman应力修正后的单轴本构关系得到的漏斗试样漏斗根部截面中心处应力三轴度计算公式结果进行了比较。得到了四种材料的等效全程单轴本构关系、破断应变、破断应力和漏斗根部截面上应力三轴度分布随截面中心Von-Mises等效应变的变化规律。同时基于三轴度理论还讨论了漏斗型试样的拉伸破断机理以及断口的形貌。     

钢/聚氨酯夹层结构动态压缩力学性能与本构模型研究

基于霍普金森压杆(SHPB)装置,对高应变率下的钢/聚氨酯夹层结构进行了动态压缩实验,通过对不同厚度比的试样进行比较,建立了夹层结构的动态本构模型。实验结果表明,钢/聚氨酯夹层结构对应变率较为敏感,其屈服强度随应变率的增大而增大,且屈服强度的增幅随应变率的提高会有一定幅度的上升。基于Johnson-Cook模型,考虑厚度比对结果的影响,建立适用于钢/聚氨酯夹层结构的动态本构模型,并通过试验分析,确定了模型参数。利用实测应力应变曲线拟合的方法得到的钢/聚氨酯夹层结构的动态压缩本构模型,能描述不同厚度比的钢/聚氨酯夹层结构在高应变率下的应力应变关系,具有较高的精度。     

高强钢TRB高温下热流变特性

为了准确仿真高强钢板热冲压成形过程,获得高强钢高温下的材料本构关系模型,利用Gleeble3500热模拟试验机在不同温度和应变速率下对不同厚度的高强钢B1500HS钢板进行了单向拉伸试验,获得各种工艺条件下的应力-应变曲线,并基于变形抗力数学模型,引入板材厚度参数,通过最小二乘法进行数据拟合获得高强钢TRB高温下的材料本构关系.利用试验结果对本构关系模型进行的拟合验证表明,拟合程度较好,说明建立的材料本构关系能很好地描述高强钢TRB在高温下的应力-应变关系.     

新型高温合金材料建模及涡轮盘成形工艺模拟

采用Gleeble-3800对一种新型变形高温合金材料GH4066进行热物理模拟测试,获得了该材料在温度为800, 900,1000,1100,1150℃,应变速率为0.0003,0.001,0.01,0.1,1,10 s~(-1)的不同变形工艺条件下高温流动应力特征。基于实验数据与唯象学模型,建立了该材料的本构关系模型;通过对不同温度、不同应变速率条件下的材料试样进行微观组织观察与晶粒尺寸测试,建立了材料的动态再结晶和晶粒长大模型;将材料本构关系、峰值应力应变、动态再结晶以及晶粒长大模型嵌入有限元软件中进行该材料涡轮盘锻造成形工艺的模拟计算,给出了该材料涡轮盘热锻造成形的合理参数范围。通过对材料模型的准确度验证,建立了一种综合实验与计算的材料模型构建及涡轮盘锻造工艺参数确定的方法。     

复杂应力状态下超细晶材料的塑性

本工作通过三维非接触全场应变测量系统(DIC)和有限元模拟(FEM)对比研究了超细晶铜和粗晶铜在拉伸过程中颈缩处的应变和应力分布,比较了在颈缩后的复杂应力状态下超细晶铜和粗晶铜延伸率的差异,实验发现虽然超细晶铜与粗晶铜在工程应力-应变曲线上的断裂延伸率相差较大,但两者颈缩区域的延伸率相当。有限元模拟揭示颈缩区域为三向应力状态,实验设计的缺口试样进一步验证复杂应力状态下超细晶材料的塑性变形能力与粗晶材料相当。复杂应力状态下,超细晶铜内部更多的滑移系被激发,使得超细晶铜的塑性变形能力与粗晶铜相当。     

瓦楞纸板本构模型参数识别方法

目的 解决瓦楞纸板的本构模型参数识别难题,建立一个数学函数较为简单的本构方程.方法 采用正弦函数和正切函数分别表征应力-应变曲线的波动部分和压实部分,正弦正切函数组合构建瓦楞纸板本构方程,利用参数识别软件得到瓦楞纸板本构模型中的参数.结果 实验结果得出,应力-应变曲线与理论应力-应变曲线的平均相对误差均小于5％.结论 本构模型中的参数比现有文献中的本构参数数量少,利用软件能简化本构模型参数识别过程,提高了计算效率.参数识别软件的运用为瓦楞纸板力学应用提供了方便.     

应力三轴度和应变率对6063铝合金力学性能的影响及材料表征

对6063铝合金试样在不同应力三轴度和不同应变率下进行拉伸试验,得到了该合金在这两种情况下的力学性能.研究结果表明:随着应力三轴度的减小,材料的等效弹性模量、等效屈服应力减小,但等效断裂应变增大;随着应变率逐渐增大,材料的屈服强度和断裂强度略有增大;断裂应变明显减小;抗拉强度基本不变.Johnson-cook本构模型及其断裂应变模型可以用来描述6063铝合金在不同三轴应力度和不同应变率下的本构及失效关系.通过材料表征,得出了Johnson-cook本构模型及其断裂应变模型中各个参数,为有限元(ABAQUS)模拟提供帮助.     

Modeling fracture of fiber reinforced polymer

In the present paper 3D rate sensitive constitutive model for modeling of laminate composites is presented. The model is formulated within the framework of continuum mechanics based on the principles of irreversible thermodynamics. The matrix (polyester resin) is modeled by employing a 3D rate sensitive microplane model. For modeling of fibers (glass) a uni-axial constitutive law is used. The fibers are assumed to be uniformly smeared-out over the matrix. The formulation is based on the assumption of strain compatibility between matrix and fibers. Total stress tensor is additively decomposed into the contribution of matrix and fibers, respectively. To model de-lamination of fibers, the matrix is represented by periodically distributed initial imperfection over the pre-defined bands, which are parallel to fibers. Physically, this assumption accounts for the matrix-fiber interface in a smeared way. The input parameters of the model are defined by the mechanical properties of matrix and fibers (elastic properties, strength and fracture energy), the volume fraction of fibers and by their spatial orientation. The model is implemented into a 3D finite element code. To assure mesh objective results crack band method is employed. The model is first calibrated using a few basic test results. Subsequently, the model is validated with several numerical examples for specimens loaded in uni-axial tension, uni-axial compression and shear. Comparison between numerical and test results shows that the proposed model is able to predict the resistance and failure mode of complex fiber-reinforced composite for different orientation of fibers and different loading conditions with sufficient accuracy. Finally, based on the qualitative type of the finite element analysis, it is demonstrated that the strain rate dependency becomes more important when the angle between the fiber and load direction increases.     

FE Analysis of Shearing of Granular Bodies in a Direct Shear Box

A plane strain analysis of a deformation and stress field in cohesionless granular bodies during shearing in a direct shear tester was performed with a finite element method on the basis of a hypoplastic constitutive law enhanced by polar quantities: rotations, curvatures, couple stresses, and a mean grain diameter used as characteristic length. The constitutive law takes into account the effect of pressure, void ratio, direction of deformation rate, mean grain diameter, and grain roughness on the material behavior. The FE calculations were carried out with a different initial void ratio, vertical load, mean grain diameter, and specimen length. Attention was focused on the size effect caused by the size of microstructure related to the specimen dimensions and the effect of side boundaries on the shear zone formation. The FE results show that the thickness of the shear zone increases with increasing initial void ratio, pressure level, mean grain diameter, and specimen length. Due to the effect of boundary conditions, the thickness changes along a horizontal midsection (it is widest in the mid-region).     

Elastic-plastic analysis of a stationary crack under cyclic loading and effect of overload

A recently proposed elastoplastic constitutive model has been implemented in a finite element code to study crack front behaviour under variable loading. The importance of proper modelling of a material's behaviour becomes evident when a variable loading condition is considered. We present stress, strain and displacement distribution along a stationary crack front for constant amplitude cyclic loading with an overload cycle. The analysis predicts a decreased tensile stress and damage accumulation following an overload.     

Reliability analysis of metal‐composite adhesive joints under debonding modes I,II, and I/II using the results of experimental and FEM analyses

In the present study, the experimental and finite element (FE) analyses are first carried out to investigate the deboning behavior of metal‐composite adhesive joints under modes of I and mode II loading. To conduct an FE on the debonding propagation, cohesive zone method (CZM), as well as maximum nominal stress and energy criteria, is applied. In the reliability analysis, to achieve the probability of debonding growth (PODG), limit state functions are formulated based on the energy release rate. To that end, the first‐order reliability method (FORM), the second‐order reliability method (SORM), and the Monte Carlo simulation (MCS) are used to calculate the PODG. The effect of initial debonding length on the PODG in all mentioned modes is investigated. Results obtained from reliability analysis reveal that the random variables including the initial debonding length, width, and thickness are the most sensitive variables to ascertain the PODG.     

The effect of carbon nanotube orientation and content on the mechanical properties of polypropylene based composites

Finite element method (FEM) is used to predict the tensile and compressive stress–strain curves of single wall carbon nanotube (SWCNT) reinforced polypropylene (PP) composites. The numerical simulations, using shell and tetrahedron elements, are first carried out to investigate the effect of SWCNT orientation on the mechanical properties of the nano-composites. Second, the Grunfest–Young constitutive equation is selected to determine the effect of strain rate and solve the finite element program to analyze the mechanical behavior of the nano-composites. Third, the effect of SWCNT volume fraction is studied. In all cases, the shear and normal stresses distribution along the nanotube axis are investigated and compared with the macroscopic tensile or compressive stresses on the composites. At the same time, the stresses of the interface between SWCNT and the matrix along the loading direction are analyzed. Finally, the effects of SWCNT orientation, content and strain rate on the strength of the nano-composites are studied. From the results obtained, it was shown that strain rate can substantially affect the tensile and shear stresses of the composites, but do not significantly influence the initial tensile or compressive elastic moduli. This is especially the case for SWCNT orientation angles less than 30° and volume fractions higher than 0.74%.     

Measurement of residual stresses in a multi-layer explosive welded joint with successive milling technique

In the multi-layer welded joint of titanium-tantalum (Ti-5Ta/Ti-5Ta/Ta/substrate of stainless steel (SUS304) the second layer of plate Ti-5Ta is 4mm thick, and the third plate Ta is only 1 mm thick. It is almost impossible to measure the stresses near the weld with cutting strip technique. Using a successive milling technique the inplane elastic strain releases normal to the thickness direction are measured. With the finite element method (FEM) inherent strain distribution along thickness z-direction is evaluated according to the elastic strain releases. Subsequently, assuming that the inherent strains (plastic strains resulting from the welding process) are the initial strains of the FEM analysis for the welded residual stresses, these are used further to evaluate the residual stress distributions along the thickness z-direction in the multi-layer explosive welding joint.     

Plastic flow and ductile rupture of a 2198 Al–Cu–Li aluminum alloy

Plasticity and fracture mechanisms of a 2198 Al–Cu–Li thin sheet alloy having a thickness equal to 6 mm are investigated. Two heat treatments are studied: T351 and T851. Mechanical tests are carried out on flat specimens including smooth tensile samples and U-notched specimens. Test data are used to identify the parameters of constitutive equations describing plastic anisotropy. The microscopic fracture surfaces of the different specimens are observed using scanning electron microscopy. Smooth and notched samples exhibit a slant fracture surface. Two microscopic fracture mechanisms are identified: fibrous fracture involving grain boundary decohesion and dimple fracture. Observed fracture modes depend on specimen geometry (notches increase stress triaxiality and favor dimple fracture) but also on loading direction. Loading along the rolling direction leads to predominant fibrous fracture. Reducing sheet thickness to 2 mm also favors fibrous fracture. Finally a localization indicator based on Rice’s analysis of bifurcation is used to analyse finite element simulations and predict observed fracture plane orientations.     

循环荷载下奥氏体型和双相型不锈钢材料本构关系研究

为研究循环荷载下不锈钢材料的本构关系,对奥氏体型S30408不锈钢和双相型S220503不锈钢材料进行了单调拉伸和大应变超低周循环加载试验。采用三种常用的单调拉伸本构模型对所得应力-应变曲线进行拟合,得到相应单调荷载下材料本构参数;采用Ramberg-Osgood本构模型对循环骨架曲线进行拟合,得到材料循环强化参数;利用Chaboche塑性本构模型,标定了两种材料的循环本构参数。结果表明:在单调拉伸荷载下,G-R-O本构模型更适用于拟合不锈钢材料的单调拉伸本构;在循环荷载下,不锈钢材料滞回曲线饱满,且随着应变增大,两种材料在加载后期均表现出了明显的循环强化现象;Ramberg-Osgood本构模型对骨架曲线拟合较好,有限元计算结果和试验滞回曲线吻合度高;表明该文标定出的强化参数、循环本构参数可用于结构体系地震响应分析之中,为准确分析不锈钢结构在地震作用下的受力性能提供参考。     

Creep behavior and life assessment of anisotropic bicrystals with a void and without void in different kinds of grain boundaries

Based on crystallographic theory, a creep constitutive relationship and a life predictive model have been presented. The crystallographic creep constitutive relationship has been implemented as a user subroutine ’CRPLAW' to MACR. Bicrystal models containing a void in the grain boundary and bicrystal model without void have been studied by the finite element method. Different loading direction has been studied in order to show the influence of relative direction of loading to grain boundary on the creep behavior of the bicrystals. The numerical results of bicrystal models show that there are a high stress gradient and stress concentration near the void and grain boundary. The existing of the void has strong influence on creep durability life of the crystal. The stress distribution and creep strain characterization are dependent on the crystallographic orientations of the two crystals and the grain boundary direction as well as the existing of the void and loading directions. It is shown that the bicrystal model of the loading direction perpendicular to the grain boundary has the highest creep strain and creep damage, while that model of the of the loading direction parallel to the grain boundary has the minimum. This above conclusion is also same to the growth of void.