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1.
准脆性工程材料及结构在外力作用下,不仅引起内部缺陷变化和微裂纹的出现及发展,且使得其结构承载能力降低或性能劣化.在其材料失效过程中常存在裂缝与断裂损伤过程区.为研究材料细观缺陷或微裂纹与宏观破坏的规律,通过细观力学方法,对于代表性体积单元RVE中的圆饼型微裂纹的尺寸与密度变化,探讨其宏观断裂过程区力学参量与损伤之间的量化关系.借助宏观断裂过程区的黏聚裂纹模型,将损伤单元RVE嵌入到宏观裂缝端部的断裂过程区中,对其进行联接细观损伤到宏观破坏的力学多尺度研究.文中也通过实验数据,对其理论计算结果进行了算例的讨论与分析.  相似文献   

2.
杨卫  张宿林 《力学季刊》1997,18(3):189-195
微裂纹串接为宏观灾难性裂纹的过程取决于相邻微裂纹的强相互作用。微裂纹的分布规律影响材料的强度和韧性。本文研究简单的共线微裂纹构型,确定由于微裂纹长度和韧度尺寸的统计分布所产生的影响。研究结果预计了脆性材料的尺度效应,即对于相同密度的微裂纹分布,大尺寸构件的强度比小尺寸构件要低。计算还表明脆性体的强度随微裂纹分布函数标准方差的增加而减小。  相似文献   

3.
脆性材料在双向应力下的断裂实验与理论分析   总被引:5,自引:0,他引:5  
包亦望 《力学学报》1998,30(6):682-689
研究了脆性材料在双向应力下的断裂特性和失效机理,特别是在平行于裂纹的应力对临界断裂参数的影响方面进行了实验上和理论上的研究.采用玻璃、陶瓷等脆性材料进行了平面双向拉伸和单向拉伸试验,并对实验结果进行比较.观测直通裂纹的启裂和扩展过程,证明了双向应力对裂纹驱动力有明显影响,讨论了裂纹扩展的应变准则.  相似文献   

4.
左宏  陈宜亨 《力学学报》1999,31(4):493-497
通过对微裂纹屏蔽不同来源的分析及计算,发现在各向同性脆性材料中,残余应力释放引起的微裂纹对主裂尖产生最大屏蔽效应时该微裂纹的倾角与最大张应力的方向没有明显的对应关系.在Hutchinson[1]所指出的屏蔽效应的第二个来源中,还应计及微裂纹形成引起的远场应力在微裂纹处产生的应力场的释放从而导致应力场的再分布.  相似文献   

5.
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。  相似文献   

6.
微裂纹模型是研究砂土变形中应变局部化问题的微观力学模型。本文在前文的基础上建立了有限平面的徽裂纹模型的基本方程。数值解析表明,微裂纹模型可再现应变局部化及应变软化现象。同时,本文简要地讨论了边界约束、尺寸效应及侧压对应变局部化的影响。  相似文献   

7.
混凝土断裂过程区的虚拟裂纹粘聚力奇异性   总被引:4,自引:0,他引:4  
混凝土断裂过程区视为具有粘聚阻力作用的虚拟裂纹,其非线性断裂和尺寸效应特性是与该虚拟裂纹粘聚力分布规律密切相关的。通过得到的粘聚应力分布函数解析结果,对该粘聚力分布特征的分析得知,在基于断裂过程区之外用线弹性场的力学模型上,该粘聚力随距离虚拟裂纹尖点的靠近,仍具有平方根奇异性。从而本文提出一个能够反映裂纹发展状态的粘聚应力奇异性强度参数,它是无粘聚力的线弹性裂纹应力强度因子和表征裂纹张开位移分布多项式参数的函数;因此,该参数可以作为混凝土非线性断裂的一个参量。文中就已有断裂试验测试结果进行了算例分析和相应的讨论。  相似文献   

8.
本文建立了金属中裂纹形核的一个理论模型,通过理论分析,讨论了氢对裂纹形核的影响。定义了与材料晶粒和脆性沉淀物尺寸有关的材料影响参数。结果表明,本文提出的模型有更明确的物理意义,环境中的氢参加裂纹形核和生长的三个阶段。  相似文献   

9.
将包含众多不同规则微裂纹的混凝土材料视为等效微裂纹系统,结合细观微裂纹动力律与波动理论,确定了裂纹演化与应力波传播的内在关系;根据有核长大思想和能量耗散理论,建立了一种便于工程应用的混凝土类准脆性材料拉伸损伤演化方程,并由混凝土单轴拉伸实验确定了相关材料参数;进行了混凝土平板撞击和内爆炸所引起的波传播和层裂问题的数值计算。结果表明:混凝土靶板的自由面速度时程曲线呈现明显周期性震荡,震荡周期与应力波在裂片中往返一次的时间基本相同;混凝土靶板损伤空间分布和损伤云纹图与已有实验结果一致,验证了本文所建立的微裂纹型拉伸损伤模型的科学实用性。  相似文献   

10.
在强动载作用下,脆性材料的碎裂问题是一个重要的研究课题,而脆性材料在冲击拉伸载荷下的力学行为的实验研究相对较匮乏.提出了一种动态拉伸断(碎)裂的液压膨胀环实验技术,可用于准脆性/脆性材料的动态拉伸.利用该技术对有机玻璃(PMMA)圆环试件进行了不同膨胀速度下的动态碎裂实验研究.从回收碎片的断口形貌和碎片内部残余裂纹观察可知试件的破碎由环向拉伸应力造成,碎片断口处发出的稀疏波会将周围的拉伸应力卸载,从而抑制其他裂纹的进一步发展.利用超高速相机记录了试件的膨胀碎裂过程,利用DISAR激光速度干涉仪获得了试件外表面粒子的径向膨胀速度历史,通过试件上的应变片获得了试件的应变历史和断裂应变.实验结果表明:在拉伸应变率150~500 s~(-1)范围,材料的动态断裂应变低于准静态加载下的断裂应变,体现出"动脆"现象;随着加载应变率的提高,PMMA材料的碎片尺寸减小;无量纲化的PMMA圆环的平均碎片尺寸介于韧性碎裂模型和脆性碎裂模型的预测数值之间,反映出材料的准脆性特性.  相似文献   

11.
We attempt the identification, study and modeling of possible sources of size effects in concrete structures acting both separately and together. We are particularly motivated by the interplay of several identified scaling lengths stemming from the material, boundary conditions and geometry. Methods of stochastic nonlinear fracture mechanics are used to model the well published results of direct tensile tests of dog-bone specimens with rotating boundary conditions. Firstly, the specimens are modeled using microplane material law to show that a large portion of the dependence of nominal strength on structural size can be explained deterministically. However, it is clear that more sources of size effect play a part, and we consider two of them. Namely, we model local material strength using an autocorrelated random field attempting to capture a statistical part of the complex size effect, scatter inclusive. In addition, the strength drop noticeable with small specimens which was obtained in the experiments is explained by the presence of a weak surface layer of constant thickness (caused e.g., by drying, surface damage, aggregate size limitation at the boundary, or other irregularities). All three named sources (deterministic-energetic, statistical size effects, and the weak layer effect) are believed to be the sources most contributing to the observed strength size effect; the model combining all of them is capable of reproducing the measured data. The computational approach represents a marriage of advanced computational nonlinear fracture mechanics with simulation techniques for random fields representing spatially varying material properties. Using a numerical example, we document how different sources of size effects detrimental to strength can interact and result in relatively complex quasibrittle failure processes. The presented study documents the well known fact that the experimental determination of material parameters (needed for the rational and safe design of structures) is very difficult for quasibrittle materials such as concrete.  相似文献   

12.
This paper extends the theoretical framework presented in the preceding Part I to the lifetime distribution of quasibrittle structures failing at the fracture of one representative volume element under constant amplitude fatigue. The probability distribution of the critical stress amplitude is derived for a given number of cycles and a given minimum-to-maximum stress ratio. The physical mechanism underlying the Paris law for fatigue crack growth is explained under certain plausible assumptions about the damage accumulation in the cyclic fracture process zone at the tip of subcritical crack. This law is then used to relate the probability distribution of critical stress amplitude to the probability distribution of fatigue lifetime. The theory naturally yields a power-law relation for the stress-life curve (S-N curve), which agrees with Basquin's law. Furthermore, the theory indicates that, for quasibrittle structures, the S-N curve must be size dependent. Finally, physical explanation is provided to the experimentally observed systematic deviations of lifetime histograms of various ceramics and bones from the Weibull distribution, and their close fits by the present theory are demonstrated.  相似文献   

13.
A heterogeneous material model based on macro-mechanical observations is proposed for simulation of fracture in steel projectiles during impact. A previous experimental study on the deformation and fracture of steel projectiles during Taylor bar impact tests resulted in a variety of failure modes. The accompanying material investigation showed that the materials used in the impact tests were heterogeneous on scales ranging from microstructure as investigated with SEM to variation in fracture strains from tensile tests. A normal distribution is employed to achieve a heterogeneous numerical model with respect to the fracture properties. The proposed material model is calibrated based on the tensile tests, and then used to independently simulate the Taylor bar impact tests. A preliminary investigation showed that the simulations are sensitive to assumptions regarding the anvil behaviour and friction properties. A flexible anvil and a yield-limited friction law are shown to be necessary to correctly reproduce the experimental behaviour. The proposed model is then shown to be capable of correctly reproducing all fracture modes but one, and also predict critical impact velocities for projectile fracture with reasonable accuracy. Fragmentation at velocities above the critical velocity is not well reproduced due to excessive element erosion. Measures to make the element erosion process more physical are proposed and discussed with their respective drawbacks. The use of a simple fracture criterion in combination with an element erosion technique accentuates the effect of distributing the fracture parameter.  相似文献   

14.
15.
Engineering structures must be designed for an extremely low failure probability such as 10−6, which is beyond the means of direct verification by histogram testing. This is not a problem for brittle or ductile materials because the type of probability distribution of structural strength is fixed and known, making it possible to predict the tail probabilities from the mean and variance. It is a problem, though, for quasibrittle materials for which the type of strength distribution transitions from Gaussian to Weibullian as the structure size increases. These are heterogeneous materials with brittle constituents, characterized by material inhomogeneities that are not negligible compared to the structure size. Examples include concrete, fiber composites, coarse-grained or toughened ceramics, rocks, sea ice, rigid foams and bone, as well as many materials used in nano- and microscale devices.This study presents a unified theory of strength and lifetime for such materials, based on activation energy controlled random jumps of the nano-crack front, and on the nano-macro multiscale transition of tail probabilities. Part I of this study deals with the case of monotonic and sustained (or creep) loading, and Part II with fatigue (or cyclic) loading. On the scale of the representative volume element of material, the probability distribution of strength has a Gaussian core onto which a remote Weibull tail is grafted at failure probability of the order of 10−3. With increasing structure size, the Weibull tail penetrates into the Gaussian core. The probability distribution of static (creep) lifetime is related to the strength distribution by the power law for the static crack growth rate, for which a physical justification is given. The present theory yields a simple relation between the exponent of this law and the Weibull moduli for strength and lifetime. The benefit is that the lifetime distribution can be predicted from short-time tests of the mean size effect on strength and tests of the power law for the crack growth rate. The theory is shown to match closely numerous test data on strength and static lifetime of ceramics and concrete, and explains why their histograms deviate systematically from the straight line in Weibull scale.Although the present unified theory is built on several previous advances, new contributions are here made to address: (i) a crack in a disordered nano-structure (such as that of hydrated Portland cement), (ii) tail probability of a fiber bundle (or parallel coupling) model with softening elements, (iii) convergence of this model to the Gaussian distribution, (iv) the stress-life curve under constant load, and (v) a detailed random walk analysis of crack front jumps in an atomic lattice. The nonlocal behavior is captured in the present theory through the finiteness of the number of links in the weakest-link model, which explains why the mean size effect coincides with that of the previously formulated nonlocal Weibull theory. Brittle structures correspond to the large-size limit of the present theory. An important practical conclusion is that the safety factors for strength and tolerable minimum lifetime for large quasibrittle structures (e.g., concrete structures and composite airframes or ship hulls, as well as various micro-devices) should be calculated as a function of structure size and geometry.  相似文献   

16.
The recent rewriting of the Ba?ant’s size effect law (Morel, 2008) which has suggested the existence of an additional asymptotic regime for intermediate structure sizes is now compared to numerical simulations of fracture of geometrically similar notched structures of different sizes extending over 2.4 decades. The quasibrittle fracture behavior is simulated through cohesive zone model (bilinear softening) using a constant set of cohesive parameters whatever the specimen size D is. The R-curves resulting from the load–displacement responses are estimated and appear as size-independent. On this basis, the different asymptotic regimes expected for the size effect on fracture properties at peak load such as the relative crack length, the resistance to crack growth and the nominal strength are shown in fair agreement with the size effect observed on the results obtained from numerical simulations.  相似文献   

17.
18.
A heterogeneous fracture approach is presented for modeling asphalt concrete that is composed of solid inclusions and a viscous matrix, and is subjected to mode-I loading in the fracture test configuration. A heterogeneous fracture model, based on the discrete element method (DEM), is developed to investigate various fracture toughening mechanisms of asphalt materials using a high-resolution image processing technique. An energy-based bilinear cohesive zone model is used to model the crack initiation and propagation of materials, and is implemented as a user-defined model within the discrete element method. Experimental fracture tests are performed to investigate various fracture behavior of asphalt concrete and obtain material input parameters for numerical models. Also, bulk material properties are necessary for each material phase for heterogeneous numerical models; these properties are determined by uniaxial complex modulus tests and indirect tensile strength tests. The main objective of this study is to integrate the experimental tests and numerical models in order to better understand the fracture mechanisms of asphaltic heterogeneous materials. Experimental results and numerical simulations are compared at different test conditions with excellent agreement. The heterogeneous DEM fracture modeling approach has the potential capability to understand various crack mechanisms of quasi-brittle materials.  相似文献   

19.
For composites and adhesive joints, the determination of the cohesive zone parameters from Double Cantilever Beam specimens loaded with pure moments is now well established and documented. However, for quasibrittle materials used in Civil Engineering such as concrete or wood, the difficulty to apply a pure bending moment lies inappropriated the method used for composites. Nevertheless, the one-to-one correspondence which exists between the R-curve and the softening curve is here revisited and adapted for any kind of specimen geometry and for the bilinear approximation of the softening function, well-known to successfully describe the failure of a wide group of quasibrittle materials. It is shown that even though the connections between the cohesive parameters and the ‘equivalent LEFM’ R-curve are geometry and material dependent, their trends are preserved whatever the specimen geometry and the material are. The outline of a general estimation procedure of the cohesive zone parameters funded on the equivalent LEFM R-curve is proposed.  相似文献   

20.
In phase field fracture models the value of the order parameter distinguishes between broken and undamaged material. At crack faces the order parameter interpolates smoothly between these two states of the material, which can be regarded as phases. The crack evolution follows implicitly from the time integration of an evolution equation of the order parameter, which is coupled to the mechanical field equations. Among other phenomena phase field fracture models are able to reproduce crack nucleation in initially sound materials. For a 1D setting it has been shown that crack nucleation is triggered by the loss of stability of the unfractured, spatially homogeneous solution, and that the stability point depends on the size of the considered structure. This work numerically investigates to which extend size effects are reproduced by the 2D phase field model. Exemplarily, a finite element study of the hole size effect is performed and the simulation results are compared to experimental data.  相似文献   

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