Statistical damage model for quasi-brittle materials under uniaxial tension

Based on the parallel bar system, combining with the synergetic method, the catastrophe theory and the acoustic emission test, a new motivated statistical damage model for quasi-brittle solid was developed. Taking concrete for instances, the rationality and the flexibility of this model and its parameters-determining method were identified by the comparative analyses between theoretical and experimental curves. The results show that the model can simulate the whole damage and fracture process in the fracture process zone of material when the materials are exposed to quasi-static uniaxial tensile traction. The influence of the mesoscopic damage mechanism on the macroscopic mechanical properties of quasi-brittle materials is summarized into two aspects, rupture damage and yield damage. The whole damage course is divided into the statistical even damage phase and the local breach phase, corresponding to the two stages described by the catastrophe theory. The two characteristic states, the peak nominal stress state and the critical state are distinguished, and the critical state plays a key role during the whole damage evolution course. Foundation item: Projects(90510018, 50679006) supported by the National Natural Science Foundation of China; Project(NCET-05-0413) support by the Program for New Century Excellent Talents in University     

Mechanical and damage evolution properties of sandstone under triaxial compression

To study the mechanical and damage evolution properties of sandstone under triaxial compression,we analyzed the stress strain curve characteristics,deformation and strength properties,and failure process and characteristics of sandstone samples under different stress states.The experimental results reveal that peak strength,residual strength,elasticity modulus and deformation modulus increase linearly with confining pressure,and failure models transform from fragile failure under low confining pressure to ductility failure under high confining pressure.Macroscopic failure forms of samples under uniaxial compression were split failure parallel to the axis of samples,while macroscopic failure forms under uniaxial compression were shear failure,the shear failure angle of which decreased linearly with confining pressure.There were significant volume dilatation properties in the loading process of sandstone under different confining pressures,and we analyzed the damage evolution properties of samples based on acoustic emission damage and volumetric dilatation damage,and established damage constitutive model,realizing the real-time quantitative evaluation of samples damage state in loading process.     

Experimental study on mechanical behavior of high performance concrete under multi-axial compressive stress

Experimental investigation was conducted to characterize the responses of high performance concrete(HPC) subjected to multiaxial compressive stresses. The HPC specimens were prepared with three different mix proportions, which corresponds to three different uniaxial compressive strengths. The cubic specimens with size of 100 mm for each edge were tested with servo-hydraulic actuators at different stress ratios. The principal stresses and strains of the specimens were recorded, and the failure of the cubic specimens under various stress states was examined. The experimental results indicated that the stress states and stress ratios had significant influence on the strength and deformation of HPC under biaxial and triaxial compression, especially under triaxial compression. Failure criteria were proposed for the HPC specimens under biaxial and triaxial compressive loading. The test results provided a valuable reference for obtaining multi-axial constitutive law for HPC.     

A composite material model for investigation of micro-fracture mechanism of brittle rock subjected to uniaxial compression

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混凝土三轴受压等幅疲劳力学性能试验研究

为研究混凝土在多轴疲劳应力作用下的性能,进行了0.0fc、0．1fc、0．25fc、0．4fc4个不同初始定侧压级别下的混凝土三轴压静载试验和0．1c、0．25fc两个不同初始定侧压级别下的混凝土三轴压等幅疲劳试验,疲劳试验中最小应力水平均为0．1fc,最大应力水平为1．6fc-2．5fc．根据初始定侧压下混凝土三轴压本构特征,试验中将侧压变化的拐点作为其多轴疲劳破坏的判据．试验分析表明,三轴压疲劳荷载作用下混凝土纵向最大、最小应变表现出同单轴和双轴疲劳一样的三阶段规律,而变形模量则衰减不大．由各侧压级别下的S-N曲线得到混凝土在相应工况下的疲劳强度折减系数,其研究成果可为混凝土结构疲劳设计提供参考．     

混凝土双轴压缩强度尺寸效应机理

为了研究混凝土双轴强度尺寸效应产生的机理,考虑混凝土尺寸效应及其宏观力学非线性源于其材料组成的非均质性,结合混凝土细观结构形式,从细观角度将混凝土看作由骨料颗粒、砂浆基质和界面过渡区组成的三相复合材料,建立了尺寸分别为150 mm×150 mm、300 mm×300 mm和600 mm×600 mm的数值混凝土试件.首先通过对150 mm×150 mm方形混凝土试件进行单轴压缩力学特性模拟,采用反演法确定了界面过渡区的力学参数,进而对双轴压缩加载下混凝土的破坏行为及其尺寸效应进行了分析.研究结果表明:1)损伤破坏形式随试件尺寸发生变化,试件最终破坏消耗的断裂能不同是导致混凝土双轴压缩强度产生尺寸效应的主要原因;2)混凝土单、双轴压缩强度均存在明显的尺寸效应行为,并且Bazant尺寸效应律能够较好地描述混凝土在单、双轴压缩加载下的强度尺寸效应.     

Experimental investigation on strength and deformation of plain high-strength concrete under multiaxial stress state

To investigate the strength and deformation behavior of plain high-strength concrete (HSC) under multiaxial stress states,a large static-dynamic true triaxial machine was employed,and multiaxial tests were performed on 100 mm×100 mm×100 mm cubes concrete specimens.Friction-reducing pads were three-layer plastic membranes with glycerine in-between for the compressive loading plane.The tensile loading plane of concrete samples was processed by attrition machine,and then the samples were glued up with the loading plate with structural glue.Failure modes of specimens were described.The principal static compressive strengths,strains at the peak stress and stress-strain curves were measured,and the influence of stress ratios on them was analyzed as well.Experimental results show that the ratio of the compressive strength σ3f over the uniaxial compressive strength fc depends on brittleness-stiffness of concrete besides stress state and stress ratios.The formula of Kupfer-Gerstle’s and Ottosen’s failure criterion for plain HSC under biaxial compression and multiaxial stress state is proposed respectively.     

Experimental and theoretical analysis on the procedure for estimating geo-stresses by the Kaiser effect

Acoustic emission tests of the core specimens retrieved from boreholes at the depth over 1000 m in Hongtoushan Copper Mine were carried out under uniaxial compressive loading, and the numerical test was also done by using the rock failure process analysis (RFPA2D) code, based on the procedure for estimating geo-stresses by the Kaiser effect under uniaxial compression. According to the statistical damage mechanics theory, the Kaiser effect mechanism was analyzed. Based on these analyses, it is indicted that ...     

混凝土损伤弹性本构关系及断裂判据

以混凝土单轴拉压损伤为基础,给出了单元弹性损伤本构关系,分析了裂纹尖端损伤微裂区尺寸,定义了有效应力强度因子,给出了Ⅰ型裂纹损伤断裂判据。     

Failure of rock under dynamic compressive loading

Split Hopkinson Pressure Bar（SHPB） test was simulated to investigate the distribution of the first principal stress and damage zone of specimen subjected to dynamic compressive load. Numerical models of plate-type specimen containing cracks with inclined angles of 0°,45° and 90° were also established to investigate the crack propagation and damage evolution under dynamic loading. The results show that the simulation results are in accordance with the failure patterns of specimens in experimental test. The interactions between stress wave and crack with different inclined angles are different; damage usually appears around the crack tips firstly; and then more damage zones develop away from the foregoing damage zone after a period of energy accumulation; eventually,the damage zones run through the specimen in the direction of applied loading and split the specimen into pieces.     

橡胶混凝土受压破坏机理的余能基面力元分析

建立橡胶混凝土二维混合随机骨料模型，基于余能原理基面力元法对橡胶混凝土的细观力学性能进行分析，获得了应力-应变曲线图、破坏过程图和最大主应力、应变云图，模拟了橡胶混凝土破坏过程，分析了橡胶混凝土受压破坏机理，并探究了橡胶颗粒的不同粒径及掺量对橡胶混凝土抗压强度的影响。结果表明，抗压强度随着橡胶掺量的提高，明显降低。相同掺量下，采用大粒径的橡胶颗粒可以使抗压强度略有提高。破坏最开始发生于橡胶颗粒较为密集区域，故而在配置橡胶混凝土时应尽量将橡胶颗粒分散均匀。该模型为分析并预测橡胶混凝土的细观力学性能提供了新方法。     

超高应变率下岩石的破坏机理

通过对大理石的轻气炮试验进行三维数值仿真重现,探索了岩石在超高应变率下的破坏机理.大理石试样中两个高速应力计的实测应力信号被用作数值模拟的匹配目标,结果表明,在高速冲击压缩载荷下,细观静水压破坏是大理石的主要破坏机制,而不是通常认为的细观剪切或拉伸破坏.此外,为再现不同冲击速度下大理石的物理实验观测数据,提出了新的考虑...     

单轴受压下再生混凝土的损伤特性研究

利用超声检测技术,以超声波波速的衰减来度量损伤,对单轴受压条件下不同再生粗骨料取代率再生混凝土的损伤进行实时超声试验。通过试验研究了不同再生粗骨料取代率下再生混凝土损伤随单轴压应力的变化规律.试验结果表明再生粗骨料取代率对单轴压应力下混凝土损伤影响显著,且不应忽视混凝土在极限压应力以前的损伤。在考虑单轴压应力、再生混凝土初始强度影响因素下,建立了再生混凝土损伤关系模型。     

冲击荷载下的新浇混凝土损伤特性及强度试验研究

以新浇混凝土养护过程会受到各类工程振动的作用与影响为工程背景,结合静载试验与冲击动载试验,得出冲击荷载影响下新浇混凝土的强度随龄期的变化规律。通过基于Weibull分布的统计损伤模型得出混凝土关于龄期的动态损伤本构模型。采用SHPB压杆装置进行混凝土试件破坏试验,得出在混凝土不同龄期的冲击动载下,混凝土抗压强度与静载下抗压强度的比值均为2:1左右;随着龄期的增长,混凝土试件受冲击破坏的破碎程度越来越小。采用SHPB压杆装置对不同龄期的混凝土做加载损伤试验,得出了冲击荷载下混凝土最薄弱龄期区段及其强度降低率达到峰值的龄期。     

混凝土多级等幅疲劳的变形发展规律试验研究

首先试验验证和揭示了混凝土单级和两级等幅疲劳损伤（变形）发展三阶段规律的普遍性和级间相似性。其次,根据混凝土疲劳变形发展三阶段规律的具体特征以及考虑变幅增量与当前损伤状态的耦合效应,由试验数据得到了单级和两级等幅疲劳损伤第二阶段规范化疲劳变形及其发展速率的经验公式;最后将上述公式推广到了混凝土的多级等幅疲劳的情况。     

Failure Patterns and Energy Analysis of Shaft Lining Concrete in Simulated Deep Underground Environments

The failure patterns and energy evolution of three types of shaft lining concrete subjected to static and dynamic loading were reported.The energy and damage characteristics of concrete were determined by means of a uniaxial hydraulic servo machine,acoustic emission (AE) equipment,a split Hopkinson pressure bar (SHPB) and an ultrasonic wave analyser.The experimental results indicate that the confluence of multiple cracks forms a penetrating cross section in normal high-strength concrete (NHSC) under the condition of static loading,while the elastic energy that surges out at failure can cause tremendous damage when subjected to dynamic loading.A single crack was split into multiple propagation directions due to the presence of fibres in steel fibre-reinforced concrete (SFRC);adding fibre to concrete should be an effective way to dissipate energy.The non-steam-cured reactive powder concrete (NSC-RPC) designed in this paper can store and dissipate more energy than normal concrete,as NSC-RPC exhibits a strong ability to resist impact.Applying NSC-RPC to the long-service material of a shaft lining structure in deep underground engineering is quite effective.     

三轴受压状态混凝土当量损伤本构模型

本文根据Ｈ－Ｔ－Ｃｈｅｎ的多轴受压状态下的破坏准则,引用了混凝土损伤力学的概念,提出了在多轴受压状态下的当量损伤模型。可以模拟混凝土在多轴受压状态下的应力应变关系,得到等效单轴受压的应力应变全过程曲线,并且可求得在不同侧压力情况下混凝土的极限抗压强度。本模型不仅可模拟混凝土多轴受力的应力应变过程,并为拱坝非线性全过程分析了依据。     

Experimental investigation of rigid confinement effects of radial strain on dynamic mechanical properties and failure modes of concrete

In this study, to confirm the effect of confining pressure on dynamic mechanical behavior and failure modes of concrete, a split Hopkinson pressure bar dynamic loading device was utilized to perform dynamic compressive experiments under confined and unconfined conditions. The confining pressure was achieved by applying a lateral metal sleeve on the testing specimen which was loaded in the axial direction. The experimental results prove that dynamic peak axial stress, dynamic peak lateral stress,and peak axial strain of concrete are strongly sensitive to the strain rate under confined conditions.Moreover, the failure patterns are significantly affected by the stress-loading rate and confining pressure.Concrete shows stronger strain rate effects under an unconfined condition than that under a confined condition. More cracks are created in concrete subjected to uniaxial dynamic compression at a higher strain rate, which can be explained by a thermal-activated mechanism. By contrast, crack generation is prevented by confinement. Fitting formulas of the dynamic peak stress and dynamic peak axial strain are established by considering strain rate effects(50–250 s~(-1)) as well as the dynamic confining increase factor(DIFc).     

弱胶结泥质软岩的三向压缩损伤特性

针对西部矿区弱胶结软岩在复杂应力状态下的损伤行为,采用三轴压缩试验和等效应变原理,得到泥岩在三轴压缩下损伤变量的演化规律.基于weibull分布,引入修正系数λ建立可考虑泥岩残余阶段变形的统计损伤本构模型.结果分析表明：在三轴压缩下损伤变量呈先减小后增大的变化趋势,损伤演化过程没有水平段,表明受压时泥岩没有实际意义上的线弹性阶段;损伤变量减增变化的转折点恰好为全应力-应变曲线的屈服点和残余阶段起始点,说明该类泥岩的破坏起始于屈服点;当围压<3 MPa时,损伤起始点随围压增大而后移,当围压>3 MPa时,损伤初始点有前移的趋势,高围压下泥岩的峰前塑性提高,产生整体塑性剪切破坏;本构模型能够较准确地描述泥岩破坏的三阶段.     

Dynamic mechanical property of hybrid fiber reinforced concrete(HFRC)

The uniaxial compressive response of steel-polypropylene hybrid fiber reinforced concrete(HFRC) and steel fiber-reinforced concrete(SFRC) was analyzed under high strain rate loading with a 74 mm diameter split Hopkinson pressure bar(SHPB).The experimental investigation focused on recorded data and resulted in distinguishing the strain rate that mobilized different ductility of steel-polypropylene hybrid fiber reinforced concrete(HFRC) and steel fiber-reinforced concrete(SFRC).64 specimens of HFRC and SFRC with higher static compressive strength were tested at the strain rates changing from 20 to 120 s-1.The static compressive strength and dynamic stress-strain curves of the two materials were obtained at 4 different strain rates and the failure stress,and peak strain and peak toughness were also analyzed.The results show that HFRC has quite good dynamic mechanical property and clear strain-rate effect,and the failure mechanism of HFRC and SFRC was also compared based on the specimens’ failure modes in static and dynamic compressive tests.