Coupled Mechanical and Chemical Damage in Calcium Leached Cementitious Structures

Long-term durability of concrete structures must be faced both from the point of view of cracking and physical degradations. In this paper, the relevance and the sensitivity of an existing constitutive relation aimed at modeling mechanical and chemical damage is examined. This constitutive relation is based on a scalar continuum damage model. The chemical degradation mechanism is calcium leaching. It is observed that the model predictions, i.e., the lifetime of cement-based beams subjected to leaching, are very sensitive on the tensile strength and fracture energy of the sound material. The existing model predicts the response of bending beams subjected to various states of leaching prior to any mechanical loading. The simulation of the size effect tests shows that the mechanical internal length and the damage threshold of the material cannot be considered to be constant. The internal length ought to decrease and the damage threshold should increase.     

初始温度条件下全尾胶结膏体损伤本构模型

膏体充填料到达采场初始温度不同是矿山存在的普遍现象,不同初始温度条件下膏体力学特性及应力-应变关系直接影响到矿山采充周期及相邻采场开采时贫化指标.通过对初始温度为2、20、35和50℃的硬化膏体进行单轴抗压强度试验,获得不同初始温度下充填体应力-应变演化曲线.根据理论推导和试验结果,建立了不同初始温度下膏体损伤本构模型,通过本构模型参数回归,提出膏体温度-时间耦合损伤本构模型.最后,采用Comsol数值模拟软件,将温度-时间耦合损伤本构模型嵌入solid mechanics模块,对单轴抗压试验进行数值模拟,模拟应力-应变曲线与试验结果较为吻合,验证了所提出本构模型的可靠性.     

Experimental Investigation of Bending Fatigue Response of Grouted Stay Cables

An experimental investigation was conducted to determine the susceptibility of grouted stay cables to bending fatigue damage. The results from twelve, bending fatigue tests are reported in this paper. Fretting of adjacent wires within a single strand was the dominant cause of bending fatigue damage in the grouted stay-cable specimens. This damage tended to be concentrated at the ends of the specimens and at locations where concentrated loads were applied to the stays. The laboratory tests indicated that the risk of bending fatigue damage was low at the tension rings, along the free length of the stays, and in the vicinity of unintentionally crossed strands.     

Thermomechanical fatigue,oxidation, and Creep: Part II. Life prediction

A life prediction model is developed for crack nucleation and early crack growth based on fatigue, environment (oxidation), and creep damage. The model handles different strain-temperature phasings(i.e., in-phase and out-of-phase thermomechanical fatigue, isothermal fatigue, and others, including nonproportional phasings). Fatigue life predictions compare favorably with experiments in 1070 steel for a wide range of test conditions and strain-temperature phasings. An oxide growth (oxide damage) model is based on the repeated microrupture process of oxide observed from microscopic measurements. A creep damage expression, which is stress-based, is coupled with a unified constitutive equation. A set of interrupted tests was performed to provide valuable damage progression information. Tests were performed in air and in helium atmospheres to isolate creep damage from oxidation damage.     

块石含量对充填体残余承载阶段损伤演化影响研究

块石胶结充填体中块石含量是决定承载能力的关键因素,在RMT-150C材料试验机上进行不同块石含量的尾砂胶结充填体单轴压缩实验。试验结果显示了不同块石含量胶结充填体承载过程中残余承载阶段应力-应变变化关系。利用损伤力学理论建立了不同块石含量的损伤本构方程及损伤演化方程。对比损伤-应变关系曲线,发现随着块石含量的增加,块石胶结充填体应力峰值所对应的损伤值逐渐增加。分析认为,块石含量对充填体试件残余承载阶段的损伤增长速率的影响规律是块石含量较低（10%、20%）时加速损伤增长,块石含量较高（30%）时抑制损伤增长。     

单轴压缩下充填体损伤本构模型研究

为了揭示充填体材料在单轴荷载下的损伤机理,首先对充填体材料在单轴荷载下的应力—应变曲线进行分析,再以损伤变量作为影响充填体材料力学特性的内变量,基于统计损伤理论、最大拉应力准则和应变等效假说,推导了充填体材料在单轴荷载下的经典损伤本构模型.由于充填体试件在初始阶段存在压密过程,提出了压密系数并将其引入到经典损伤本构模型...     

不同应力上限和加载速率下的黄砂岩疲劳特性研究

为研究黄砂岩单轴疲劳加载的特性,开展了不同应力上限和加载速率下的单轴疲劳荷载试验。试验结果表明：黄砂岩的疲劳试验曲线受到单轴压缩应力—应变曲线的控制,疲劳极限变形与峰后对应变形一致;砂岩疲劳过程的不可逆变形和耗散能密度均具有三阶段演化规律,依据倒“S”型损伤模型,验证了黄砂岩疲劳损伤三阶段演化规律;分析认为三阶段规律的本质是砂岩的塑性变形和内部孔隙微裂纹生成以及扩展速度的不同所呈现的结果。研究表明应力上限和加载速率对疲劳寿命有显著影响,根据所得应力—寿命公式,可以估计砂岩在一定条件下的疲劳寿命。     

静载荷与循环冲击组合作用下岩石损伤本构模型研究

建立合理的损伤本构模型,对研究岩石在静载荷与循环冲击组合作用下的动态力学性能具有重要的意义. 首先,基于静载荷与循环冲击组合作用下岩石的损伤累积演化模型,结合岩石弹性模量与累积损伤变量的关系,将循环冲击次数引入岩石损伤本构关系中. 接着,假设岩石微元体强度服从Weibull分布,将统计损伤模型和黏弹性模型相结合,并根据Drucker-Prager破坏准则,建立有轴压有围压时循环冲击作用下岩石的动态损伤本构模型.进而,利用岩石动静组合加载试验数据验证损伤本构模型的正确性和可行性. 最后,研究模型中参数对循环冲击过程中应力-应变曲线的影响规律.结果表明,建立的岩石损伤本构模型较好地与试验数据吻合,能较好地反映具有静载荷的岩石在循环冲击过程中的本构关系.岩石的弹性模量、黏性系数等参数对循环冲击时岩石的应力-应变曲线影响较大;而岩石的摩擦角和泊松比等对具有三维静载岩石的冲击疲劳应力-应变曲线影响较小. 循环冲击过程中,岩石的非均匀度逐渐增加. 研究结果有益于完善静载荷与循环冲击组合作用下岩石动态疲劳力学理论体系.      

Competing Modes for Crack Initiation from Non-metallic Inclusions and Intrinsic Microstructural Features During Fatigue in a Polycrystalline Nickel-Based Superalloy

Cyclic fatigue experiments in the high and very high cycle fatigue regimes have been performed on a René 88DT polycrystalline nickel-based superalloy. The microstructural configurations that favor early strain localization and fatigue crack initiation at high temperature from 400 °C to 650 °C have been investigated. Competing failure modes are observed in the high to the very high cycle fatigue regime. Fatigue cracks initiate from non-metallic inclusions and from intrinsic internal microstructural features. Interestingly, as stresses are reduced into the very high cycle regime, there is a transition to initiation only at crystallographic facets. At higher stress in the high cycle fatigue regime, a significant fraction of specimens initiate cracks at non-metallic inclusions. This transition is analyzed with regard to microstructural features that favor strain localization and accumulate damage early during cycling.     

双轴等拉低周疲劳实验装置

本文给出能在一般单向加载液压疲劳试验机上做双轴等拉低周疲劳实验研究的双向拉伸装置,并对有关影响因素进行了讨论。
本试验是在百吨液压低周疲劳试验机上进行的。双向拉体装置已经受近百万次疲劳载荷,其中交变载荷幅围5.5,t亦经受近20万次。在使用过程,各构件完好合手使用要求,将十字试样进行低周疲劳断裂试验,成功地开出合格裂纹,用它进行超速条件下叶轮材料性能研究亦取得初步结果。可以认为它适用于平而应力集中问题低周疲劳性能的研究。若使用液压高、中周疲劳试验机,它还可用于双轴载荷下裂纹试样裂纹扩展的研究。
本文指出了双向拉伸装置有待改进的方面。     

Closed-Form Solutions for Flexural Response of Fiber-Reinforced Concrete Beams

A constitutive law for fiber-reinforced concrete materials consisting of an elastic perfectly plastic model for compression and an elastic-constant postpeak response for tension is presented. The material parameters are described by using Young’s modulus and first cracking strain in addition to four nondimensional parameters to define postpeak tensile strength, compressive strength, and ultimate strain levels in tension and compression. The closed-form solutions for moment-curvature response are derived and normalized with respect to their values at the cracking moment. Further simplification of the moment-curvature response to a bilinear model, and the use of the moment-area method results in another set of closed-form solutions to calculate midspan deflection of a beam under three- and four-point bending tests. Model simulations are correlated with a variety of test results available in literature. The simulation of a three- and four-point bending test reveals that the direct use of uniaxial tensile response underpredicts the flexural response.     

基于动载荷谱的齿轮弯曲疲劳寿命预测

为了探讨齿轮弯曲疲劳寿命计算问题,将齿轮疲劳总寿命分为两个阶段,即疲劳裂纹萌生寿命和裂纹扩展寿命.通过ADAMS软件仿真实验齿轮的工作情况,使其接近真实状况,得到齿轮载荷谱.根据齿轮载荷谱,利用有限元ANSYS软件分析在齿轮齿根危险截面处的最大应力.采用断裂力学、雨流法和Miner疲劳损伤累积模型,对考虑动载荷情况下的齿轮弯曲疲劳寿命进行预测,推导了齿根裂纹萌生期和扩展期的疲劳寿命计算公式.在高频疲劳试验机上对算例齿轮进行了双齿脉动加载齿根弯曲疲劳寿命实验研究,理论计算结果与实验结果基本吻合,验证了本文理论分析的正确性.     

Testing and Modeling of Soil-Structure Interface

An accurate modeling of soil-structure interfaces is very important in order to obtain realistic solutions of many soil-structure interaction problems. To study the mechanical characteristics of soil-structure interface, a series of direct shear tests were performed. A charged-coupled-device camera was used to observe the sand particle movements near the interface. It is shown that two different failure modes exist during interface shearing. Elastic perfect-plastic failure mode occurs along the smooth interface, while strain localization occurs in a rough interface accompanied with strong strain-softening and bulk dilatancy. To describe the behavior of the rough interface, this paper proposes a damage constitutive model with ten parameters. The parameters are identified using data from laboratory interface shear tests. The proposed model is capable of capturing most of the important characteristics of interface behavior, such as hardening, softening, and dilative response. The interface behaviors under direct and simple shear tests have been well predicted by the model. Furthermore, the present model has been implemented in a finite element procedure correctly and calculation results are satisfactory.     

Low-cycle fatigue of X 2 NiCoMo 18 12 and X 10 NiCrAITi 32 20

The low-cycle fatigue and short crack-growth behaviour of an ultra-high strength maraging steel and a high-nickel austenitic steel have been studied. The tests were performed in a laboratory environment at room temperature in push-pull cycles with constant strain amplitudes of 0.25 to 2 %. Short crack-growth investigations were performed using bending specimens loaded by a resonance bending machine. Crack growth was measured using the replica method. The cyclic and tension stress strain behaviour has been compared. Fatigue tests were interpreted according to the recommended practice of ASTM-E 606-80. First microcracks were observed after 10 to 70 % of the fatigue life.     

Kinetics of small fatigue cracks in steel during cyclic loading

The kinetics of development of fatigue microcracks in a high-strength 08Kh14AN4MD steel is studied when a cantilever specimen with several notches is subjected to rotational bending tests. The specific feature of the tests consists in the fact that, when the specimen is loaded at a constant load, different stress amplitudes are realized in different notches. As a result, after the sample has failed across the section with the maximum stress, a longitudinal polished section of the specimen contains fatigue cracks having nucleated in the mouths of the other notches at lower stress amplitudes. A relation between inflection points in the fatigue curve and the conditions of local and developed yield in a notch has been established. Fatigue fracture mechanisms at a superhigh number of loading cycles are studied, and their relation to the structure near a crack is found. A rapid method for estimating the fatigue limit at a superhigh number of loading cycles is proposed.     

Crack Propagation in Flexural Fatigue of Concrete

In this paper the behavior of concrete subjected to flexural fatigue loading is studied. Notched concrete beams were tested in a three-point bending configuration. Specimens were subjected to quasi-static cyclic and constant amplitude fatigue loading. The cyclic tests were performed by unloading the specimen at different points in the postpeak part of the quasi-static loading response. Low cycle, high amplitude fatigue tests were performed to failure using four different load ranges. The crack mouth opening displacement was continuously monitored throughout the loading process. Crack propagation caused by quasi-static and fatigue loads is described in terms of fracture mechanics. It is shown that the crack propagation in the postpeak part of the quasi-static load response is predicted using the critical value of the mode I stress intensity factor (KIC). The ultimate deformation of the specimen during the fatigue test is compared with that from the quasi-static test; it is demonstrated that the quasi-static deformation is insufficient as a fatigue failure criterion. It is observed that crack growth owing to constant-amplitude fatigue loading comprises two phases: a deceleration stage when there is a decrease in crack growth rate with increasing crack length, followed by an acceleration stage where the rate of crack growth increases at a steady rate. The crack length where the rate of crack growth changes from deceleration to acceleration is shown to be equal to the crack length at the peak load of the quasi-static response. Analytical expressions for crack growth in the deceleration and acceleration stages are developed, wherein the expressions for crack growth rate in the deceleration stage are developed using the R-curve concept, and the acceleration stage is shown to follow the Paris law. It is observed that the crack length at failure for constant amplitude fatigue loading is comparable to that of the corresponding load in the postpeak part of the quasi-static response. Finally, a fracture-based fatigue failure criterion is proposed.     

Prediction of thermomechanical fatigue lives in metal matrix composites

To identify the role of silicon carbide participate reinforcement on high-temperature thermomechanical fatigue behavior of Al 2xxx-T4, experiments have been conducted under thermomechanical out-of-phase and in-phase loading conditions. A general constitutive representation, based on Eshelby’s inclusion theory, is used for the determination of volumetric average stresses and strains under cyclic loading of the metal matrix composite. This constitutive representation is used with a life prediction model, based on the matrix stress-strain behavior, which predicts contributions of fatigue, creep, and environmental damages to failure under both isothermal and thermomechanical fatigue loading. In isothermal fatigue experiments at 200 °C and 300 °C, pure fatigue damage and creep damage are the dominant damage mechanisms in the short-life regime. In the long-life regime, however, the stress levels are too low to induce considerable creep damage; so, oxidation damage becomes dominant. When fatigue damage is dominant, the model predicts a decrease in life, based on strain range, with increasing volume fraction of reinforcement. Based on stress range, improved fatigue lives are predicted with increasing volume fraction of reinforcement. The reinforced alloy exhibits longer lives when compressive hydrostatic stresses in the matrix at the high-temperature end of the cycle reduce the creep damage. Temporarily Director, Mechanics and Materials Program, National Science Foundation, Washington, DC 20550     

High-temperature mechanical behavior and hot rolling of AA705X

High-temperature mechanical behavior and processing performance of 705X aluminum alloys is examined, employing a combination of mechanical testing, microscopy, and computational modeling. We perform hot uniaxial compression tests over a range of temperatures and strain rates and fit the data to power-law constitutive models. These models are supported and expanded by microscopy and calorimetry, which help to elucidate the operating deformation mechanisms and examine damage evolution. The mechanical behavior constitutive relations are implemented in a finite-element code to simulate the hot rolling process. The results of the rolling simulation are used to predict final product crystallographic texture, which is compared with experimental electron backscattered diffraction measurements for model validation. Finally, we propose a parameter to characterize the development of damage during processing. This work provides a solid foundation for the design of thermomechanical processing of these alloys to maximize yield and optimize process performance.     

Application of Fiber Reinforced Polymer Overlays to Extend Steel Fatigue Life

An experimental and analytical study was conducted to investigate the effectiveness of applying carbon fiber reinforced polymer (CFRP) overlays to steel fatigue tension coupons to prolong fatigue life. Specimens were either notched or center hole specimens and tested in uniaxial tension. Variables studied were CFRP system, bond length, bond area, one and two sided applications, and applications prior or subsequent to crack propagation. Two sided applications were very effective, prolonging fatigue life by as much as 115%. Similar application of CFRP materials subsequent to crack propagation extended the remaining fatigue life by approximately 170% without any other means of crack arrest. The method therefore showed promise as both a preventive technique and repair method. The epoxy performance was critical to the effectiveness of the system, with all failures initiated by debonding of the CFRP. Overlays were most effective when the system was applied directly to the potential crack trajectory. One-sided applications introduced bending and did not significantly improve performance. CFRP materials with a moderate modulus of elasticity performed best. FEM was performed to estimate stress factors and effectiveness of overlays.     

Numerical Simulation of Damage Localization in Polyester Mooring Ropes

This paper presents the derivation of a mechanical model to estimate the effects of damage on the response of ropes. Damage can be represented through a degradation of the properties of individual rope elements, and it can also include the complete rupture of one or more elements. The general assumptions made to estimate the length over which damage propagates along the length of a rope and how this length is considered in modeling damaged rope behavior are explained. Consistent with tests on damaged polyester (PET) mooring ropes, numerical simulations demonstrate the existence of strain localization around the failure region and, due to degradation of rope element properties, damage localization as well. This damage localization causes the premature failure of rope elements, reducing the maximum load capacity and maximum failure strain that a damaged rope is capable of resisting relative to that of an intact rope. The proposed model suggests that some of the variables that affect damaged rope behavior are the degree of damage present at a given cross-section, the location of broken rope elements, and the length over which damage propagates along the rope length. Experimental data are used to validate the model.