轻粗骨料含量对混凝土抗压性能尺寸效应影响的细观数值模拟

为了研究轻粗骨料含量对轻骨料混凝土立方体抗压破坏行为与尺寸效应的影响，本文根据瓦拉文公式确定轻粗骨料数量，借助混凝土细观数值建模方法分别建立了边长为100 mm、150 mm、300 mm、450 mm的二维轻骨料混凝土细观数值模型．采用塑性损伤本构模型，通过细观数值模拟研究了不同试件尺寸及不同轻粗骨料含量的轻骨料混凝土在单轴受压下的破坏形态及宏观应力-应变关系曲线．结果表明，在相同试件尺寸下，轻粗骨料含量对轻骨料混凝土单轴抗压强度有一定影响，受压破坏时轻粗骨料颗粒破坏．比较轻粗骨料含量相同的轻骨料混凝土立方体抗压性能发现，轻骨料混凝土尺寸效应明显，单轴抗压强度随试件尺寸增大而减小，且粗骨料含量影响轻骨料混凝土的尺寸效应．此外，细观数值模拟结果表明Ba?ant尺寸效应律适用于轻骨料混凝土的抗压性能．     

基于三维细观模型的全级配混凝土静态力学性能的数值模拟

根据混凝土材料的细观组成和力学特性,研究了骨料几何形状和空间分布规律,建立全级配混凝土三维凸多面体随机细观模型,引入了混凝土细观组份材料的本构模型,分别模拟了单轴、双轴和三轴状态下混凝土的静态力学性能,并建立混凝土梁的三维宏细观分析模型,研究了三点弯曲梁的变形及裂缝扩展情况。结果表明,本文建立的细观力学模型的计算结果与实验数据吻合较好,可以较好地模拟各种复杂应力条件下混凝土的静态力学性能和损伤破坏机理。     

高温后高强高性能混凝土双轴压力学性能

利用大型静动真三轴试验机,进行了常温20${^\circ}$C以及200${^\circ}$C$\sim $ 600${^\circ}$C\,6个温度等级高温后高强高性能混凝土在7种应力比双轴压应力状态下的强度与变形试验.测得了双轴主压方向的静态强度、峰值应变与应力应变曲线,剖析了温度和应力比对单、双轴压强度与峰值应变发展趋势的影响规律性以及试件破坏形态. 试验结果表明:随温度的升高,高强高性能混凝土的单轴压减摩强度并不一定降低;双轴压强度相对于单轴压强度的提高倍数取决于应力比、不同温度等级后的高强高性能混凝土``脆硬性'. 提出了带有温度和应力比参数的Kupfer-Gerstle破坏准则公式.      

混凝土拉伸断裂的细观数值分析

根据混凝土试件拉伸和三点弯曲的物理模型,用梁-颗粒模型BPM 2D(B eam-Particle M ode l)模拟了混凝土拉伸和三点弯曲试件微裂纹的萌生、扩展直至试件宏观破坏的全过程。在梁-颗粒模型中用三种类型梁单元形成混凝土细观数值模型,每种类型梁单元的力学性质均按韦伯(W e ibu ll)分布随机赋值以模拟混凝土细观结构的非均匀性。数值模拟结果给出了混凝土拉伸应力-应变曲线和三点弯曲载荷-位移曲线,以及混凝土试件破坏过程最大应力分布图和裂纹扩展图。数值模拟结果显示混凝土破坏过程实际上就是微裂纹萌生、扩展、贯通,直到宏观裂纹产生导致混凝土失稳断裂的过程。通过对数值模拟结果的分析,揭示出混凝土在拉伸条件下裂纹尖端的拉应力集中是裂纹扩展的动力,混凝土组成材料力学性质的非均匀性是造成裂纹扩展路径曲折的重要原因。     

考虑细观变形特征的镁合金宏观本构模型及数值模拟

为了能有效描述镁合金宏观各向异性塑性行为,考虑了滑移、孪生、去孪生三种细观变形模式的特点,给出了相应的硬化函数;根据VonMises屈服准则,发展了一种镁合金宏观本构模型及其迭代算法。模型将变形模式的开启与晶粒取向相关联,同时针对镁合金孪生变形时引起的晶粒重新定向问题,描述了一种晶向偏转的方法。在此基础上编写了ABAQUS/UMAT材料用户子程序;利用开发的本构模型,开展了单轴拉伸、单轴压缩、单轴循环拉压加载条件下镁合金塑性行为的数值模拟,并对随机织构下的镁合金板材轧制过程进行了有限元仿真实验。模拟结果表明:单轴拉伸、单轴压缩和循环加载情形下的镁合金宏观硬化行为与实验结果基本吻合;轧制后镁合金板材表现出了应力-应变不均匀特性,多晶织构演化结果与实验结果基本一致。说明文中所提出的宏观本构模型、晶向偏转模型能够有效描述镁合金的宏观塑性行为和织构演化。     

基于细观力学的混凝土弹塑脆性损伤研究

将混凝土假定为一种由硬化水泥砂浆、粗骨料、界面粘结带所组成的三相复合材料,在满足骨料级配曲线算法的基础上,采用细观单元的弹塑脆性损伤本构关系,考虑材料的非均质特性,建立了基于细观力学的混凝土弹塑脆性损伤数值模型;分别研究了单轴受拉预置裂纹试样和单轴受压混凝土试样的细观弹塑脆性损伤破坏行为,并揭示了混凝土的宏观表征强度存在明显的尺寸效应,通过将计算结果与 Bazant 尺寸效应公式、单轴受压物理实验曲线进行对比,验证了模型的正确性。数值试验表明：该模型可以清晰地模拟混凝土细观塑性屈服和失效裂纹的萌生和扩展。骨料与水泥砂浆间的界面粘结带相对薄弱,在混凝土试件形成宏观损伤局部化带前,试件的屈服和破坏首先发生在骨料边缘处的界面位置,并沿着界面粘结带扩展、贯通;同时,导致宏观裂纹形成和发展的因素仍以细观单元的拉伸破坏为主。     

镁合金MB2在高应力三轴度下的拉伸破坏行为研究

通过光滑试件及不同曲率半径缺口圆柱试件的拉伸试验,实现对镁合金MB2的单向及多向应力状态加载。结合数值模拟分析,研究了不同试件在拉伸加载过程中应力状态的变化。以应力三轴度为参数,给出了镁合金MB2等效破坏应变的变化规律,在应力三轴度-等效破坏应变空间建立了镁合金MB2的失效破坏准则。利用扫描电镜对试件断口形貌进行观察,分析了导致材料宏观延性变化的微观损伤机理,对不同应力状态下镁合金MB2的失效破坏行为做出了合理解释。     

基于J-C模型的镁合金MB2动静态拉伸破坏行为

为了研究不同应力状态和应变率条件下镁合金MB2的拉伸破坏行为,利用材料试验机和分离式Hopkinson拉杆(SHTB),对镁合金MB2的光滑及缺口圆柱试件进行了动静态拉伸加载;拟合得到了镁合金MB2的动静态拉伸本构关系,建立了其修正的Johnson-Cook失效破坏准则,并对不同试件的拉伸破坏行为进行了数值模拟;利用SEM对宏观破坏模式对应的微观损伤机理进行了分析。结果表明,随着应力三轴度的增加,镁合金MB2的等效破坏应变先增大后减小,宏观破坏模式由剪切转为正拉断,微观损伤机制由混合断裂转变为韧窝断裂;而随着应变率的增加,等效破坏应变不断减小,破坏模式不发生改变。Johnson-Cook本构关系和修正后的Johnson-Cook失效破坏准则能较好地拟合动态静态拉伸实验结果并预测不同试件的杯锥形破坏特征。     

混凝土中心裂缝单轴拉伸损伤断裂数值模拟研究

基于对混凝土细观力学的认识,假定混凝土是由砂浆基质,骨料及它们之间的界面组成的三相复合材料,各组分的材料性质按照某个给定的Weibull分布来赋值,细观单元满足弹性损伤的本构关系,应用细观力学损伤模型研究了混凝土的宏观力学性质,并且通过有限元程序对中心裂缝混凝土试件在单向拉伸情况下的破坏过程进行了数值模拟.模拟结果表明,该模型可以用来研究单向载荷作用下混凝土结构的破坏机理.     

基于S准则发展的混凝土动态多轴强度准则

通过对经典强度理论或准则的强度参数分析,将强度参数转化为由混凝土的单轴压缩和单轴拉伸强度表示,结合S准则给出的混凝土单轴强度的率效应规律,得出了强度参数受应变率影响的率效应函数,进而将常用的静态多轴强度准则摩尔-库伦强度理论、松岗-中井强度理论、德鲁克-普拉格强度理论、拉得-邓肯强度准则和胡克-布朗强度准则发展为动态多轴强度准则.利用混凝土动态单轴压缩和动态单轴拉伸试验结果统计分析给出的S准则率效应参数,分析了5种强度准则中强度参数随应变率的变化规律以及取值范围.基于混凝土单轴压缩和单轴拉伸强度,分别给出了5种强度准则在子午面、偏平面和平面应力条件下的强度曲线与主应力空间中的强度曲面,对比分析了5种强度准则间的异同,以及每种强度准则随应变率的变化规律.利用混凝土材料的动态双轴和动态真三轴强度试验结果,分析评价了5种动态多轴强度准则,并且阐述了各动态多轴强度准则的应变率适用范围.德鲁克-普拉格强度理论与试验结果相差甚远,不适于描述混凝土材料的强度规律.动态双轴加载时,在拉压区各强度准则差别不大,都可较好地描述试验规律;在压压区各强度准则差别较大,松岗-中井强度理论与试验结果吻合最好.在动态真三轴比例加载时,摩尔-库伦强度理论和胡克-布朗强度准则无法考虑中主应力的影响;松岗-中井强度理论和拉得-邓肯强度准则都可较好地描述试验规律.      

混凝土动态双轴拉压破坏准则细观数值模拟研究

正常服役期内的混凝土结构往往处于复杂应力状态,并且不可避免地会受到偶发动力载荷作用.对于复杂载荷作用下的混凝土力学性能研究,破坏准则是基础.受试验设备等条件限制,现有的动态双轴拉压破坏准则形式复杂、缺乏更高应变率和侧应力比范围且尚未综合考虑应变率和侧应力比的耦合作用.为进一步提出适用范围更高且更准确的混凝土动态双轴拉压...     

Dynamic multi-axial behavior of shape memory alloy nanowires with coupled thermo-mechanical phase-field models

The objective of this paper is to provide new insight into the dynamic thermo-mechanical properties of shape memory alloy (SMA) nanowires subjected to multi-axial loadings. The phase-field model with Ginzburg–Landau energy, having appropriate strain based order parameter and strain gradient energy contributions, is used to study the martensitic transformations in the representative 2D square-to-rectangular phase transformations for FePd SMA nanowires. The microstructure and mechanical behavior of martensitic transformations in SMA nanostructures have been studied extensively in the literature for uniaxial loading, usually under isothermal assumptions. The developed model describes the martensitic transformations in SMAs based on the equations for momentum and energy with bi-directional coupling via strain, strain rate and temperature. These governing equations of the thermo-mechanical model are numerically solved simultaneously for different external loadings starting with the evolved twinned and austenitic phases. We observed a strong influence of multi-axial loading on dynamic thermo-mechanical properties of SMA nanowires. Notably, the multi-axial loadings are quite distinct as compared to the uniaxial loading case, and the particular axial stress level is reached at a lower strain. The SMA behaviors predicted by the model are in qualitative agreements with experimental and numerical results published in the literature. The new results reported here on the nanowire response to multi-axial loadings provide new physical insight into underlying phenomena and are important, for example, in developing better SMA-based MEMS and NEMS devices     

STRENGTH CRITERION FOR PLAIN CONCRETE UNDER MULTIAXIAL STRESS BASED ON DAMAGE POISSON＇S RATIO

A new unified strength criterion in the principal stress space has been proposed for use with normal strength concrete （NC） and high strength concrete （HSC） in compressioncompression-tension, compression-tension-tension, triaxial tension, and biaxial stress states. The study covers concrete with strengths ranging from 20 to 130 MPa. The conception of damage Poisson＇s ratio is defined and the expression for damage Poisson＇s ratio is determined basically. The failure mechanism of concrete is illustrated, which points out that damage Poisson＇s ratio is the key to determining the failure of concrete. Furthermore, for the concrete under biaxial stress conditions, the unified strength criterion is simplified and a simplified strength criterion in the form of curves is also proposed. The strength criterion is physically meaningful and easy to calculate, which can be applied to analytic solution and numerical solution of concrete structures.     

STRENGTH CRITERION FOR PLAIN CONCRETE UNDER MULTIAXIAL STRESS BASED ON DAMAGE POISSON’S RATIO

A new unified strength criterion in the principal stress space has been proposedfor use with normal strength concrete (NC) and high strength concrete (HSC) in compression-compression-tension, compression-tension-tension, triaxial tension, and biaxial stress states. Thestudy covers concrete with strengths ranging from 20 to 130 MPa. The conception of damagePoisson's ratio is defined and the expression for damage Poisson's ratio is determined basically.The failure mechanism of concrete is illustrated, which points out that damage Poisson's ratiois the key to determining the failure of concrete. Furthermore, for the concrete under biaxialstress conditions, the unified strength criterion is simplified and a simplified strength criterion inthe form of curves is also proposed. The strength criterion is physically meaningful and easy tocalculate, which can be applied to analytic solution and numerical solution of concrete structures.     

STRENGTH CRITERION FOR PLAIN CONCRETE UNDER MULTIAXIAL STRESS BASED ON DAMAGE POISSON''S RATIO

A new unified strength criterion in the principal stress space has been proposed for use with normal strength concrete (NC) and high strength concrete (HSC) in compressioncompression-tension, compression-tension-tension, triaxial tension, and biaxial stress states. The study covers concrete with strengths ranging from 20 to 130 Mpa. The conception of damage Poisson's ratio is defined and the expression for damage Poisson's ratio is determined basically.The failure mechanism of concrete is illustrated, which points out that damage Poisson's ratio is the key to determining the failure of concrete. Furthermore, for the concrete under biaxial stress conditions, the unified strength criterion is simplified and a simplified strength criterion in the form of curves is also proposed. The strength criterion is physically meaningful and easy to calculate, which can be applied to analytic solution and numerical solution of concrete structures.     

Nonlinear granular micromechanics model for multi-axial rate-dependent behavior

Constitutive equations for class of materials that possess granular microstructure can be effectively derived using granular micromechanics approach. The stress–strain behavior of such materials depends upon the underlying grain scale mechanisms that are modeled by using appropriate rate-dependent inter-granular force–displacement relationships. These force–displacement functions are nonlinear and implicit evolutions equations. The numerical solution of such equation under applied overall stress or strain loading can entail significant computational expense. To address the computations issue, an efficient explicit time-integration scheme has been derived. The developed model is then utilized to predict primary, secondary and tertiary creep as well as rate-dependent response under tensile and compressive loads for hot mix asphalt. Further, the capability of the derived model to describe multi-axial behavior is demonstrated through generations of biaxial time-to-creep failure envelopes and rate-dependent failure envelopes under monotonic biaxial and triaxial loading. The advantage of the approach presented here is that we can predict the multi-axial effects without resorting to complex phenomenological modeling.     

脆性材料动态强度应变率效应

通过求解波动方程,结合有限结构-时间破坏准则,得到了动态载荷下脆性材料单轴拉伸强度应变 率效应的解析表达式。分析结果表明:材料动态强度的应变率效应具有明显的结构响应特征,即材料动态强 度除与应变率和静态参数相关外,还显著地依赖于外载荷结构及其与材料间的相互作用,因而动态强度不是 表征材料动态破坏的内禀材料性质;此外,由于在不同的外载荷条件下材料将表现出不同的动态强度,这是导 致实验结果离散性大的内在因素。     

拉伸载荷下混凝土体分形内聚模型

在综合考虑混凝土试件微断裂面分形分布及断面分形演化基础上,提出了体分形内聚模型,用于描述混凝土试件在准静态拉伸载荷作用下的破坏行为。数值计算结果与现有实验数据吻合较好。此外,采用该模型分析了混凝土骨料级配对材料软化性质的影响。结果表明,在拉伸载荷作用下,混凝土骨料级配越均匀,材料的软化特征越明显。