梯度依赖的混凝土弹塑性非局部损伤的本构模型

以连续介质力学和不可逆热力学为基础,提出了新的混凝土弹塑性损伤本构模型.在该模型中采用了塑性应变εp、各向同性损伤标量D以及应变梯度(△)ε作为内变量,其中应变梯度反应了损伤的非局部性质,本模型是梯度依赖的非局部损伤模型,严格满足热力学的基本方程.应用梯度依赖弹塑性非局部损伤本构模型得出的结果比已有的混凝土损伤塑性模型更为合理.     

粘弹性复合材料力学性能的细观研究

利用细观力学的Eshelby等效夹杂方法研究了复合材料的粘弹性问题。通过时间域内的Laplace变换及对应性原理,将问题线性化,预报了颗粒增强复合材料的蠕变本构关系,给出了材料模量随时间、夹杂体积分数的变化规律,并与实验结果对照,吻合较好。      

冻融荷载耦合作用下岩石损伤力学特性

以寒区岩体工程为背景,针对冻融受荷岩石,从细观力学机理出发,运用宏观唯象损伤力学和非平衡统计的概念和方法,得到了以冻融循环次数和应变为损伤演化控制变量的损伤演化方程;应用推广后的应变等价原理,建立了较为真实的岩石冻融受荷损伤扩展本构关系:探讨了岩石材料细观结构损伤及其诱发的材料力学性能演化途径.研究表明:岩石的冻融损伤...     

考虑夹杂间相互影响的弱界面颗粒复合材料等效模量预测的一般细观力学方法

基于Ju和Chen提出的颗粒增强复合材料的一般细观力学方法,采用Qu给出的一阶近似修正的Eshelby张量,本文建立了考虑夹杂之间相互影响的含任意分布弱界面颗粒增强多相复合材料等效模量预测的一般细观力学方法,通过体积均匀化方法得到了一组细观力学本构方程。分别推导了忽略夹杂之间相互影响的弱界面多相复合材料宏观模量表达式以及考虑夹杂之间相互影响的弱界面两相复合材料的宏观模量表达式。此外,还给出了几种特殊复合材料体系在考虑界面弱化下的宏观等效性能参数的解析表达式,在界面粘结完好的假设下,给出的表达式均可以退化成经典的细观力学预测结果。在考虑夹杂之间相互影响的情况下,本文预测结果与实验数据吻合很好,验证了模型的有效性。     

形状记忆合金短纤维增强弹塑性基体复合材料的力学行为

基于Eshelby等效夹杂方法和Mori-Tanaka的平均化理论推导了针对SMA短纤维增强弹塑性基体复合材料的细观力学模型。利用此模型,分析了这种复合材料的力学行为,讨论了材料温度、纤维体积分数和纤维特征形状等参数对复合材料残余应力和残余应变的影响。这对复合材料的分析和设计都有重要的意义。      

基于连续损伤的岩石渗流有限元分析

该文基于连续损伤力学,建立了含损伤的岩石渗流模型,研究岩石损伤破坏并分析其渗流特性。该文采用Weibull分布函数来模拟岩石弹性模量和强度的非均质特性,利用Biot本构关系建立含孔隙结构的弹性变形方程,结合Drucker-Prager强度准则、统计强度理论和连续损伤理论推导出岩石连续损伤演化控制方程及其定解条件,进而采用有限元法对其求解。该文给出的数值算例结果表明所提出的含损伤渗流模型适用于研究岩石的渗流特性,分析结果与实验数据吻合较好,且该求解方法可以对岩石体积压裂、岩层弱面进行可靠、有效的数值分析。     

低速冲击下短纤维复合材料本构方程研究

借助ZWT非线性粘弹性一维本构关系,导出了常应变率条件下,热固性聚合物不含时间变量和积分项的、简化的、三次多项式型本构方程;采用细观力学方法,将建立在线弹性理论之上的Eshelby等效包容体理论推广应用于非线性弹性问题.在上述工作基础之上,结合应变率影响,提出了低速冲击下随机分布短纤维复合材料的一维率相关本构方程;把方程预测结果与实验结果对比,发现吻合很好,因而初步验证了所提本构方程的可靠性.     

高应变率下岩石本构特性的研究

高应变率下岩石的动态本构关系和力学特性,目前主要是利用SHPB装置等来获得．基于统计学和损伤力学的观点,建立岩石在高应变率下的动态本构模型是进一步研究爆破机理、爆炸应力波传播、爆破参数优化的基础。理论和实验证明,本文推荐的4种动态本构模型与试验结果有较好的一致性,对进一步研究岩石的动态力学性质奠定了基础．     

混凝土坝地震动力损伤分析

基于塑性损伤本构理论,将损伤变量作为内变量,在Drucker-Prager本构模型中引入损伤变量,考虑材料损伤引起的材料劲度的退化,基于非关联流动法则计算材料的塑性应变,根据材料的有效塑性应变计算损伤量,考虑到张开裂缝闭合时材料弹性劲度的恢复,推导了考虑塑性损伤的混凝土动态本构关系,并给出了内变量的计算步骤和动力方程的迭代格式。最后利用建立的动态本构模型对Koyna重力坝进行了非线性地震响应时程分析,并给出了关键时刻坝体最大受拉损伤分布,结果表明在坝颈和坝基处出现了较大的损伤,坝颈处的损伤最终形成由下游向上游的开裂破坏,这与该坝的实际震害较为一致。     

循环荷载作用下粉砂岩疲劳流变损伤模型研究

针对现有流变模型难以有效描述循环荷载作用下岩石变形及疲劳损伤演化特征等问题,开展了粉砂岩循环加卸载试验,分析了不同上限荷载下岩石的流变规律与疲劳特性。基于Kachanov蠕变损伤理论建立损伤变量,引入一个带应变触发和应力阈值的黏塑性元件,与Burgers模型串联构建循环荷载作用下岩石疲劳流变损伤模型;将正弦波应力函数替换流变微分本构方程中的恒定应力,推导岩石在循环荷载下的一维、三维微分型损伤本构方程,再根据叠加原理得到模型的黏弹塑性流变损伤方程。适用性验证表明,新建模型不仅可以精确地反映循环加卸载过程中粉砂岩的衰减、稳态流变阶段,还可以有效地描述上限荷载高于疲劳强度时的加速流变阶段。通过粉砂岩疲劳损伤流变全过程定量化分析,提出加速流变阶段的临界损伤阈值和破坏失稳判据,并给出加速流变阶段的启始时间、持续时间及疲劳寿命预测方法,模型对岩体工程长期稳定性评价具有一定的理论指导意义。     

CDM based modelling of damage and fracture mechanisms in concrete under tension and compression

Anisotropic damage evolution and crack propagation in the elastic–brittle materials is analysed by the concepts of continuum damage mechanics (CDM) and finite element method (FEM). The modified Murakami–Kamiya (MMK) model of elastic-damage material is used to describe damage anisotropy in concrete. The Helmholtz free energy representation is discussed. The unilateral crack opening/closure effect is incorporated in such a way that the continuity requirement during unloading holds. The incremental form of the stress–strain equations is developed. The general failure criterion is proposed by checking the positive definiteness of the Hessian matrix of the free energy function. The local approach to fracture (LAF) by FEM is applied to the pre-critical damage evolution that precedes the crack initiation, and the post-critical damage/fracture interaction. Crack is modelled as the assembly of failed finite elements in the mesh, the stiffness of which is reduced to zero when the critical points at stress–strain curves are reached. A concrete specimen with the pre-load, inclined crack is analysed in order to simulate different fracture mechanisms in tension or compression. The constitutive model is capable of predicting the kinked-type crack under tension and the wing-type crack under compression.     

含损伤复合材料的湿热弹性响应

依据不可逆热力学理论, 未引入任何附加假设, 建立了湿热弹性各向异性损伤复合材料的一般理论。应用建立损伤本构方程的本构泛函展开法, 推导出湿热弹性损伤材料全部本构方程的一般形式, 其中包括比自由能密度表达式、 应力-应变关系、 熵密度方程、 损伤应变能释放率表达式、 吸湿对偶力表达式、 湿-热-固-损伤耦合的热传导方程和损伤演化方程。研究表明, 在本构方程中含有若干损伤效应函数, 表征损伤对材料宏观力学性能与湿、 热性能的影响, 其具体形式可由细观力学解确定, 从而使连续损伤力学与细观损伤力学有机结合在一起。最后, 从细观力学与实验观测两个角度, 举例说明损伤效应函数与系数张量的确定方法, 为分析变温变湿环境下复合材料的损伤问题提供重要的理论依据。      

高压水射流作用下岩石的损伤模型

基于高压水射流破岩过程的分析,运用连续损伤力学和细观损伤力学理论,建立了适用于水射流破岩两个阶段的岩石损伤模型以及两个阶段岩石损伤的耦合模式。依据所建立的损伤模型,利用非线性动力有限元方法,对高压水射流破岩的全过程进行了模拟,计算结果与试验规律基本一致。     

A continuum damage mechanics model with the strain-based approach to biaxial low cycle fatigue failure

A continuum damage mechanics model for low cycle fatigue failure of initially isotropic materials under biaxial loading conditions is presented. The expression for the equivalent strain in the fatigue damage evolution equation contains the three material parameters, and the strain intensity as well as the maximum principal strain and the volume strain for amplitudes. It is shown how these material parameters can be determined from a series of basic experiments using a cruciform specimen. Particular expressions for the equivalent strain with a smaller number of material parameters and invariants are obtained. Model predictions are found to be in satisfactory agreement with the experimental low cycle fatigue data under full ranged biaxial loadings obtained in the test using a cruciform specimen.     

One damage law for different mechanisms

We consider here a general three-dimensional kinetic damage law. It uses the thermodynamic of irreversible processes formalism and the phenomenological aspects of isotropic damage. It gives the damage rate as a function of its associated variable, the strain energy density release rate and the accumulated plastic strain rate. Associated with different plastic constitutive equations, this damage law takes into account brittle damage, ductile damage, low and high cycle fatigue and creep damage. In this paper we mainly focus on creep-fatigue interaction and high cycle fatigue. Associated to a viscoplastic constitutive equation having kinematic hardening, the damage law gives the non linear creep-fatigue interaction. The agreement with experiments is good. Associated to plastic constitutive equations also having kinematic hardening but introduced in a micromechanical two scale model based on the self-consistent scheme, it models the non linear accumulation of damage induced by a succession of sequences of different amplitudes as well as the effect of the mean stress and the influence of non proportional loading.     

Stability and size-dependency of temperature-related Cauchy–Born hypothesis

In continuum mechanics, the constitutive models are usually based on the Cauchy–Born (CB) hypothesis which seeks the intrinsic characteristics of the material via the atomistic information and it is valid in small deformation. The main purpose of this paper is to investigate the temperature effect on the stability and size-dependency of Cauchy–Born hypothesis. Three-dimensional temperature-related Cauchy–Born formulations are developed for crystalline structure and the stability and size-dependency of temperature-related Cauchy–Born hypothesis are investigated by means of direct comparison between atomistic and continuous mediums. In order to control the temperature effect, the Nose–Hoover thermostat is employed. Since the Helmholtz free energy is temperature dependent; the first Piola–Kirchhoff stresses are explicitly computed as the first derivative of the Helmholtz free energy density to the deformation gradient. It is numerically shown that the validity surfaces become smaller at higher temperature, which is significant in larger specimen. It is also presented that the material stability decreases with increasing the ambient temperature.     

基于能量的弹塑性损伤实用本构模型

建立一个实用的弹塑性损伤本构模型。在有效应力空间采用经验公式计算塑性变形,能将模型塑性部分与损伤部分解耦,降低模型的数值处理复杂性,同时大大简化模型塑性应变的计算。结合不可逆热力学理论,基于损伤能量释放率建立损伤准则,损伤能量释放率由修正后的弹性Helmholtz自由能导出,模型中将弹性Helmholtz自由能分解为应力球量部分和应力偏量部分,将其应力球量部分产生的损伤取为零,同时根据应力状态引入折减系数对其应力偏量部分进行修正,使得模型能较为准确的模拟混凝土材料在双轴加载下的本构行为。将应力张量谱分解为正、负两部分以分别定义材料受拉、受压损伤演化,并采用受拉损伤变量、受压损伤变量分别模拟混凝土材料在拉、压加载下的本构特性。引入一个加权损伤变量使得模型能较准确的反映混凝土材料的“拉-压软化效应”。最后该文给出初步试验验证,证明了该文模型的有效性。