沥青砂粘弹塑蠕变损伤本构模型实验研究

针对沥青砂的非线性材料特性,结合连续损伤力学理论,对传统Burgers模型进行改造,提出了粘弹塑蠕变损伤本构模型,通过对不同实验条件下沥青砂单轴蠕变试验结果的非线性拟合,获得模型参数,然后利用模型进行预测分析,得到了不同应力水平与环境温度下的蠕变曲线和损伤演化曲线,通过比较发现该文模型能够更合理地反映沥青砂加速蠕变的非线性特征,而且蠕变过程中损伤演化的速度受蠕变时间、应力水平与环境温度的影响很大。     

沥青砂粘弹特性实验研究

进行了沥青砂在不同加载应力和不同实验温度下单轴压缩蠕变实验,得到其在不同实验条件下的蠕变曲线,选择Burgers模型,编制非线性拟合程序,求得模型参数值,通过相关性分析,得到了模型参数与温度和应力函数关系式,分析了模型参数对沥青砂蠕变性能的影响,最后进行模型预测值与实验结果对比,结果表明Burgers模型能够描述沥青砂蠕变过程的第一阶段、第二阶段,反映了其粘弹特性。     

基于Eshelby等效夹杂理论的沥青混合料有效粘弹性质分析

利用Eshelby 等效夹杂理论研究了沥青混合料的单轴压缩蠕变行为。通过时间域内的Laplace 变换将问题线性化, 得到了沥青混合料的蠕变本构关系。开展了不同温度、应力水平条件下沥青砂的单轴压缩蠕变实验, 根据数据拟合了沥青砂四参量流变模型的模型参数。在此基础上, 预测了沥青混合料在不同温度、应力水平下的蠕变曲线, 分析了温度、应力水平对沥青混合料蠕变行为的影响。结果表明:在相同的应力水平下, 沥青混合料的应变和应变率都随温度的升高而增大, 并且在沥青软化点附近发生明显突变;在相同的温度下, 沥青混合料的应变和应变率都随加载应力的增加而增大。     

基于不可逆内变量热力学的岩石材料粘弹-粘塑性本构方程

岩石材料的粘弹性和粘塑性变形是与时间相关的能量耗散行为。在Rice不可逆内变量热力学框架下,引入两组内变量分别用来描述在粘弹性和粘塑性变形过程中材料的内部结构调整。通过给定比余能的具体形式和内变量的演化方程,推导出内变量粘弹-粘塑性本构方程。粘弹性本构方程具有普遍性,能涵盖Kelvin-Voigt和Poynting-Thomson在内的经典粘弹性模型的本构方程。并指出热力学力与应力呈线性关系是组合元件模型为线性模型的根本原因。粘塑性本构方程能较好地刻画岩石材料在粘塑性变形过程中的硬化现象。对模拟岩石的模型相似材料进行单轴加卸载蠕变试验,将蠕变过程中的粘弹性和粘塑性变形分离并根据试验数据对本构方程的材料参数进行辨识。试验数据和理论曲线对比结果表明该文提出的本构方程能很好地模拟材料的蠕变行为。该类型的本构方程能为岩石工程的长期稳定性的预测、评价以及加固分析提供基础。     

小冲孔试样弹塑性蠕变损伤的有限元分析

综合考虑了小冲孔试样实验过程中的弹性、塑性及蠕变等因素,采用了Mises型流动法则和改进的K-R蠕变损伤本构方程,建立了ABAQUS有限元模型,模拟计算了550℃三种载荷条件下,Cr5Mo试样的弹塑性蠕变过程,分析试样的中心蠕变挠度、蠕变应变随时间的变化规律,当量应力的分布情况,以及试样的蠕变损伤与位置的关系,同时分析了挠度与应力对网格的敏感性。结果表明:模拟结果与实验结果很接近,且与单轴实验的应变曲线十分相似;整个试样在中心损伤比较严重,呈现明显局部化特征;讨论了试样网格的敏感性,试样半径方向0mm~1mm之内应力变化幅度比较大(除约束点),并且应力的最大值也出现在这个区域,在这个区域进行网格细化提高计算的精确性,而其他区域采用粗网格减少计算成本,达到精确性和经济性的平衡。径向上网格大小对挠度影响比较大,应力对网格大小不敏感。     

PVC涂层膜材料不同应力下非线性蠕变特性的预测

对方平组织为基布的PVC (polyvinylchloride)涂层膜材料进行了经、纬两方向五种应力条件下的拉伸蠕变试验.利用七参数(四个蠕变柔量与三个推迟时间)广义Kelvin-Voigt线性粘弹性蠕变模型对试验数据进行了分析,从中发现:对于单个应力,线性粘弹性蠕变模型具有很好的拟合效果,而对于多个应力,三个推迟时间参数保持不变,并以此为基础,将七参数广义Kelvin-Voigt线性粘弹性蠕变模型修改为适合膜材料的七参数(四个蠕变柔量与三个推迟时间)非线性粘弹性和十四参数(八个蠕变柔量与六个推迟时间)非线性粘弹塑蠕变模型,并把十四参数非线性粘弹塑蠕变模型合并为四个蠕变柔量与三个推迟时间的七参数非线性模型,再将这两种非线性模型中的三个推迟时间设定为与线性粘弹性蠕变模型相同,同时还设定非线性部分均来自于四个蠕变柔量,最后借助四阶多项式对不同应力与对应的蠕变柔量进行了拟合,结果发现:该方法可以很好预测其他应力的蠕变特性.     

复合材料蠕变本构关系及实验测定Ⅱ实验测定

在前一部分,本文得到复合材料蠕变的本构关系,在此基础上,本文进一步分析了复合材料蠕变本构关系的具体形式,实验测得了长纤维增强复合材料在蠕变、恢复两个阶段的应变,以用来确定本构关系中的待定参数,考虑到本构关系为复杂的非线性方程,本文提出了用离散变量和最小二乘法联合的方法确定参数,进而拟合蠕变本构关系的理论公式,分离出了蠕变过程中的弹性变形、粘弹性变形和粘塑性变形,对本构关系中的几个参函数,本文根据有限的实验数据拟合了其函数。      

复合材料蠕变本构关系及实验测定 Ⅱ实验测定

在前一部分,本文得到复合材料蠕变的本构关系,在此基础上,本文进一步分析了复合材料蠕变本构关系的具体形式,实验测得了长纤维增强复合材料在蠕变、恢复两个阶段的应变,以用来确定本构关系中的待定参数,考虑到本构关系为复杂的非线性方程,本文提出了用离散变量和最小二乘法联合的方法确定参数,进而拟合蠕变本构关系的理论公式,分离出了蠕变过程中的弹性变形、粘弹性变形和粘塑性变形,对本构关系中的几个参函数,本文根据有限的实验数据拟合了其函数.     

用T积分研究硬聚氯乙烯粘塑性裂纹扩展

采用作者建立的硬聚氯乙烯的粘塑性本构模型,以CT试样为研究对象进行粘塑性有限元分析,并在此基础上编程计算T积分,然后再用T积分与实验所得的裂纹扩展速率关联.研究结果显示,描述蠕变延性材料和蠕变脆性材料粘塑性裂纹扩展速率的控制参量是不同的.T积分不适合作为硬聚氯乙烯粘塑性裂纹扩展的控制参量.     

添加气膜孔对镍基单晶合金DD6蠕变寿命的影响

本研究在外加应力为200 MPa、温度为1 000℃的条件下,对镍基单晶合金DD6带气膜孔试样(飞秒激光环形扫描与旋切扫描相结合加工而成)和不带气膜孔试样进行了蠕变实验。实验结果显示,带气膜孔试样的蠕变寿命显著短于不带气膜孔试样的蠕变寿命。借助扫描电镜和透射电镜对试样蠕变前、后的微观结构进行观测发现:不带气膜孔试样的变形机理为位错在塑性比较好的基体相内(γ相)均匀滑移;而带气膜孔试样由于变形主要集中在孔周围,位错大量增殖交叉,从而在γ/γ'两相界面处产生极大的应力集中,直接将强化相即脆硬相(γ'相)剪切贯穿,导致裂纹快速扩展,合金的蠕变寿命显著缩短。     

New achievements in visco-elastoplastic constitutive model and temperature sensitivity of glasphalt

This paper focuses on the experimental investigation on temperature sensitivity and visco-elastoplastic behaviour of glasphalt. Classic Burgers creep model could only describe the viscoelastic behaviour of materials before the third creep-phase, so a viscoplastic string is added in series with classic Burgers model in order to predict the visco-elastoplastic behaviour of glasphalt. In this research, the effects of loading stress and temperature on creep behaviour of glasphalt under dynamic loading are investigated. In addition, some methods were used to solve model parameters and then predictions from proposed model were compared with experimental results. It was shown that creep testing curves coincided well with theoretic curves, validating that modified Burgers model can completely characterise creep behaviour of glasphalt. Besides, temperature sensitivity of glasphalt was evaluated by using indirect tensile stiffness modulus test, and stiffness modulus behaviour model of glasphalt was presented based on the experimental results and numerical analysis.     

Modeling of viscoplastic rate-dependent hardening-softening behavior of hot mix asphalt in compression

The objective of this paper is to model viscoplastic rate-dependent hardening-softening behavior that is experimentally observed from hot mix asphalt (HMA) under repetitive creep and recovery loading in compression. A differential equation is utilized to incorporate the effects of the stress history into yield stress, and an internal variable representing rate dependence in the equation is set as a function of the viscoplastic strain rate to address the change in rate dependence of the material due to gradual hardening. Also, a separate rate-dependent function concept is adopted to describe the difference in rate dependence of the yield stress during unloading and loading. The developed viscoplastic model is applied using the time–temperature superposition principle and shows good agreement with the measured viscoplastic responses of HMA under repetitive creep and recovery loading with various load levels and rest periods.     

Anisotropic viscoplasticity constitutive model for directionally solidified superalloy

Abstract

The macroscopic deformation behaviour of a Ni-based directionally solidified (DS) superalloy was experimentally investigated, and an anisotropic constitutive model of the material was developed. Monotonic and creep tests were performed on uniaxial test specimens machined from DS plates so that the angle between the loading direction and the solidified grain direction varied between 0 and 90°. Tension-torsion creep tests were also conducted to examine the anisotropic behaviour under multiaxial stress conditions. The material exhibited marked anisotropy under elastic and viscous deformation conditions, whereas it showed isotropy under plastic deformation conditions of high strain rates. Then crystal plasticity analyses were carried out to identify slip systems under creep loading conditions, assuming the anisotropic creep behaviour of the DS material. A viscoplastic constitutive model for expressing both the anisotropic elasticity-viscosity and the isotropic plasticity was proposed. The elastic constants were determined using a self-consistent approach, and viscous parameters were modelled by crystal plasticity analyses. The calculation results obtained using the constitutive model were compared with the experimental data to evaluate the validity of the model. It was demonstrated that the constitutive model could satisfactorily describe the anisotropic behaviour under uniaxial and multiaxial stress conditions with a given set of material parameters.     

Development of viscoplastic strains during creep in continuous fibre GMT composites

This work is part of a larger study aimed at characterizing the viscoelastic–viscoplastic behavior of a continuous fiber glass mat thermoplastic composite. The purpose of this paper is to experimentally and numerically decouple the viscoplastic strains from total creep response. This enabled the characterization of the evolution of viscoplastic strains as a function of time, stress and loading cycles. The separation also allowed viscoplastic strain development to be corresponded with the progression of failure mechanisms such as interfacial debonding and matrix cracking which were captured in situ. This was achieved by performing creep tests at seven stress levels between 20 and 80 MPa. For each stress level, a series of creep-recovery tests were performed on single specimen for increasingly longer durations from 1 to 24 h. Stress and time dependence of viscoplastic strains were determined experimentally. Using part of the data generated, a viscoplastic model was developed following a method proposed by Nordin. The model had excellent agreement with experimental results for all stresses and times considered. In multiple loading cycles, the viscoplastic strain development is accelerated with increasing number of cycles at higher stress levels. The results further verify the technique for numerical separation of viscoplastic strains proposed in an earlier work. Finally, it was found that the development viscoelastic strains during creep are affected by the previous viscoplastic strain history.     

Viscoplastic BEM fracture analysis of creeping metallic cracked structures in plane stress using complex variable techniques

This paper extends a boundary element approach developed earlier by the present author to the solution of viscoplastic fracture problems arising in creeping cracked metallic structural components undergoing plane stress deformation conditions. By using a plane stress viscoplastic formulation this work adopts the procedure of Lyness and Moler to express the derivative of displacement rate as the ratio of the imaginary part of the displacement rate to the imaginary part of a complex variable representing the coordinates of a collocation point. In this manner the evaluation of creep stresses at internal points is greatly simplified. The results thus obtained are highly accurate in comparison with conventional BEM. The application of the present BEM approach is illustrated by obtaining creep stress and strain distribution for center cracked and standard CT fracture specimens subjected to remote tensile and shear stress loading under plane stress deformation conditions.     

Nonlinear behavior of bumper foams under uniaxial compressive cyclic loading

Theoretical and experimental studies on the nonlinear behaviors of bumper foams under cyclic loading are carried out in this paper. To study the compressible materials, the incompressible viscoelastic model proposed by Rajagopal and Srinivasa in 2000 is modified and expressed as a function of the principal stretches. The modified model is used to describe bumper foams for the first time. Besides, in order to better predict the nonlinear process of bumper foams under cyclic loading, a new compressible viscoplastic model is proposed, which is expressed separately as the invariants of stretches and the principal stretches. Then the compressible viscoelastic model and the compressible viscoplastic model are used to describe the response of bumper foams under cyclic loading with constant and variable amplitudes, respectively. The experimental results demonstrate that the compressible viscoelastic model and the compressible viscoplastic model are both suitable to describe the response of bumper foams under cyclic loading, the new proposed compressible viscoplastic model is more suitable to describe the deformation at the end of each cycle.     

Experimental and numerical investigation of the creep behaviour of Ni‐based superalloy GH4169 under varying loading

The high‐temperature creep experiment of Ni‐based superalloy GH4169 under the constant loading and varying loading conditions was conducted by using the round bar specimens. The creep time‐strain curves under different loading conditions were obtained to study the high‐temperature creep behaviour of GH4169 superalloy. At the same time, the longitudinal and lateral sections near the fracture of creep specimens were observed by the optical microscope, and the specimens with smaller grain corresponded to the larger creep strain rate. In view of the dispersion of the creep curves, the corresponding data processing method was put forward, and on this basis, a model that can describe the 3 stages of creep with certain physical meaning was established. The simulation results are in good agreement with the experimental results, especially the creep deformation under the varying loading condition. The predicted results of the relative time hardening model are closer to the experiment compared with time hardening and strain hardening model. The creep model is realized by the user's material subroutine code in a commercial FEM software package, which can be used as the basis of creep analysis for engineering structures.