Modeling of Fatigue Crack Growth Closure Considering the Integrative Effect of Cyclic Stress Ratio,Specimen Thickness and Poisson's Ratio

Key components of large structures in aeronautics industry are required to be made light and have long enough fatigue lives.It is of vital importance to estimate the fatigue life of these structures accurately.Since the FCG process is affected by various factors,no universal model exists due to the complexity of the mechanisms.Most of the existing models are obtained by fitting the experimental data and could hardly describe the integrative effect of most existing factors simultaneously.In order to account for the integrative effect of specimen parameters,material property and loading conditions on FCG process,a new model named integrative influence factor model（IIF） is proposed based on the plasticity-induced crack closure theory.Accordingly to the predictions of crack opening ratio（γ） and effective stress intensity factor range ratio（U） with different material under various loading conditions,predictions of γ and U by the IIF model are completely identical to the theoretical results from the plane stress state to the plane strain state when Poisson＇s ratio equals 1/3.When Poisson＇s ratio equals 0.3,predictions of γ and U by the IIF model are larger than the predictions by the existing model,and more close to the theoretical results.In addition,it describes the influence of R ratios on γ and U effectively in the whole region from-1.0 to 1.0.Moreover,several sets of test data of FCG rates in 5 kinds of aluminum alloys with various specimen thicknesses under different loading conditions are used to validate the IIF model,most of the test data are situated on the predicted curves or between the two curves that represent the specimen with different thicknesses under the same stress ratio.Some of the test data slightly departure from the predictions by the IIF model due to the surface roughness and errors in measurement.Besides,based on the analysis of the physical rule of crack opening ratios,a relative thickness of specimen is defined to describe the influence of material property,specimen thickness and so forth on FCG characteristics conveniently.In conclusion,the relative thickness of specimen simplifies the expression of FCG characteristic and provides a general parameter to analyze the fatigue characteristics of different materials with various thicknesses under different loading conditions.The IIF model describes the integrative effect of existing influence factors explicitly and quantitatively,and provides a helpful tool for fatigue property estimation of practical component and experiment design.

Critical Cyclic Stress Ratio of Undisturbed Saturated Soft Clay in the Yangtze Estuary under Complex Stress Conditions

There exists a critical cyclic stress ratio when sand or clay is subjected to cyclic loading. It is an index dis-tinguishing stable state or failure state. The soil static and dynamic universal triaxial and torsional shear apparatus de-veloped by Dalian University of Technology in China was employed to perform different types of tests on saturated soft marine clay in the Yangtze estuary. Undisturbed samples were subjected to undrained cyclic vertical and torsional coupling shear and cyclic torsional shear after three-directional anisotropic consolidation with different initial consoli-dation parameters. The effects of initial orientation angle of major principal stress, initial ratio of deviatoric stress,initial coefficient of intermediate principal stress and stress mode of cyclic shear on the critical cyclic stress ratio wereinvestigated. It is found that the critical cyclic stress ratio decreases significantly with increasing initial orientation angle of major principal stress and initial ratio of deviatoric stress. Compared with the effects of the initial orientationangle of major principal stress and initial ratio of deviatoric stress, the effect of initial coefficient of intermediate prin-cipal stress is less evident. Under the same consolidation condition, the critical cyclic stress ratio from the cyclic cou-pling shear test is lower than that from the cyclic torsional shear test, indicating that the stress mode of cyclic shear has an obvious effect on the critical cyclic stress ratio. The main reason is that the continuous rotation in principal stressdirections during cyclic coupling shear damages the original structure of soil more than the cyclic torsional shear does.

A New Method for the Dynamic Reserves of Gas Condensate Reservoir Using Cyclic Gas Injection Based on the Effects of Reinjection Ratio and Water Influx

As we all know, cyclic gas injection is one of the most effective development methods to improve condensate oil recovery. When dealing with the calculation of the reserves, the injection-production differences and water influx create great influence on the accuracy. Based on the existing research, we proposed a new material balance equation which considered the differences of composition between produced and injected fluids and the effect of water influx, and a solution was provided in this paper. The results of the method are closer to the actual situation because they are built on the law of conservation of mass, and the using of curve fitting method can not only avoid the use of water influx coefficient but also obtain the water influx rate and reserves at the same time. The YH-23 gas condensate reservoir is taking as a typical subject to do the research, which has been exploited by cycle gas injection for 14 years. Three different methods are used to calculate the reserves, and the results show that the method proposed in this paper has minimum error of 2.96%.

重载铁路基床级配碎石动弹性模量及颗粒破碎试验研究

针对级配碎石的动力特性以及颗粒破碎性质,采用室内动三轴试验,研究其在循环荷载作用下循环应力比(cyclic stress ratio,CSR)、围压以及加载次数对累积动应变、动弹性模量、颗粒形状系数和颗粒破碎的影响。研究结果表明,不同CSR条件下,加载次数达到10000次时,累积动应变基本完成。随着加载次数的增加,不同CSR下的动弹性模量逐渐减小,滞回圈面积逐渐增加后保持稳定。低围压下,CSR对动弹性模量影响不显著,动弹性模量增加幅度较少;高围压下,动弹性模量随着CSR的增加而增长较大。循环荷载作用下,颗粒破碎方式主要为研磨;颗粒粒径越大,试验后的颗粒形状系数球度越大,颗粒越靠近球形,其轮廓线越圆滑。当CSR小于0.3时,影响颗粒破碎的主要因素为动应力大小,而围压影响不显著;当CSR大于0.3时,颗粒破碎主要因素受动应力和围压共同影响。

不均匀波浪荷载作用下海积软黏土的动力特性

为研究饱和海积黏土在均匀与不均匀波浪荷载作用下的循环特性,利用GDS空心圆柱扭剪仪进行了一系列不排水试验,在相同的初始固结状态下,对试样施加不同最大偏应力q_(max)、最小偏应力q_(min)的圆形及螺旋形应力路径的轴—扭联合循环载荷。分析了在考虑主应力轴旋转时,这两种不同应力路径的循环应力比R_(CS)与动应力比δ=q_(mim)/q_(max)对循环加载期间土体动力特性的影响。试验结果表明:R_(CS)与δ的增大均能提升黏土在波浪荷载下的孔压、轴向应变、双幅剪切应变及其速率,同时也能使土体轴向回弹模量与动剪切模量的衰减更严重,且δ的作用效果会随R_(CS)的增大而增大。当R_(CS)≤0.05时,土体在循环期间保持为“弹性安定或塑性安定”状态;而当R_(CS)≥0.06时,土体会处于“疲劳破坏”或“塑性蠕变”状态,其具体形式由R_(CS)与δ共同决定。土体在循环过程中的轴向回弹模量与动剪切模量近似呈一定比例。

风机荷载作用下频率比对砂土动力响应的影响

海上风机在运行过程中风荷载与波浪荷载频率耦合作用对土体动力特性的影响不容忽略,为了探究这种影响,采用多向循环动单剪系统(VDDCSS)对砂土进行了一系列试验研究,分析了双向剪切频率比f_(r)对砂土动应变、动孔压比以及动强度的影响。试验结果表明:剪应变发展受双向频率耦合及应力组合效应的影响。循环应力比(C_(SR))为0.15时,双向频率耦合效应大于应力组合作用;随着应力水平的增加(C_(SR)=0.20或0.25),频率耦合效应和应力组合作用均得到了加强。在低应力水平时,破坏试样的孔压曲线呈“上凹型”且频率比对试样孔压发展影响不大。而随着应力水平增加(C_(SR)=0.20或0.25),孔压最终值u_(max)随之增大。除f_(r)=1.00与10.00对应试样外,土体动强度随f_(r)与C_(SR)值增大而减少。f_(r)=5.00对应试样应变大、孔压值高、强度低的原因与此比值下土体受双向频率耦合与应力组合的叠加效果增强有关。

水平往复荷载作用下钢箱提篮拱弹塑性力学性能研究

目的 探究倾角、矢跨比及宽跨比三个参数对钢箱提篮拱在水平往复荷载作用下力学性能的影响,为钢提篮拱桥抗震设计提供理论基础。方法 通过改变参数,建立54个钢箱提篮拱有限元模型,在水平往复荷载作用下进行滞回分析,研究钢箱提篮拱水平力-位移滞回性能、极限承载力和拱顶横撑内力、变形在不同参数条件下的变化规律。结果 随着拱肋倾斜角度、宽跨比的增加或矢跨比的减小,结构初期刚度、极限承载能力均有较大提高,但横撑内力也随之增加,并会产生局部失稳破坏,影响结构的承载能力。结论 增加拱肋倾斜角度对钢箱提篮拱横向力学性能有较大改善,但在设计时应注意横撑内力增加带来的不利影响。

循环荷载作用下含水合物粉细砂的残余变形

本文针对不同饱和度和不同围压水平的含水合物粉细砂进行了一系列不排水循环三轴剪切试验,研究了轴向应变和孔隙水压力的发展,分析了影响含水合物粉细砂残余应变的主要因素:循环应力比、有效围压和水合物饱和度。在Monismith公式的基础上,提出了一种新的经验公式,能够预测循环荷载作用下含水合物粉细砂的残余变形,预测结果与实验结果的对比表明该经验公式具有良好的适用性。

基于多带加权包络谱的轴箱轴承故障诊断

为增强复杂噪声干扰下轴箱轴承故障检测的鲁棒性,基于循环谱分析并考虑轴承故障信息分布差异和阈值降噪,对轴箱轴承故障诊断的包络谱构造方法进行了研究.首先,提出频域信噪比作为轴承故障信息量化的新测度,用于评估谱相干中不同谱频带内的轴承故障相关信息;其次,构造以谱频率为变量的故障特征信息分布函数,并自适应确定信息阈值来辨识谱相干中故障信息丰富和干扰噪声主导的谱频率分量,进一步基于故障特征信息分布函数和信息阈值设计权重函数;最后,由谱相干和权重函数生成融合多带信息的多带加权包络谱,通过分析谱中的轴承故障特征频率来检测轴箱轴承的不同故障.铁路轴箱轴承实验数据的分析结果表明:相比于基于谱相干的典型包络谱方法,多带加权包络谱能够在复杂噪声干扰下准确识别轴箱轴承的外圈、滚动体和内圈故障,并能取得更高的性能量化指标(频域信噪比和负熵).

高轴压比下小剪跨比剪力墙的抗震性能试验研究

通过5片不同轴压比下小剪跨比剪力墙构件的低周反复荷载试验,研究了高轴压比对小剪跨比剪力墙的破坏模式、受剪承载力、延性、刚度特征及耗能能力的影响,并对加载全过程中剪力墙腹板双向钢筋的受力情况及传力机制进行分析。研究结果表明:所有构件均发生了剪切破坏,并在高轴压力作用下发生了平面外的错动;高轴压比阻碍了裂缝的产生及发展,提高了构件的受剪承载力,但在构件达到峰值荷载后,强度和刚度退化剧烈,极限位移小,提高边缘约束构件配箍率可以有限地提高构件的变形能力、延性及耗能能力,但对破坏模式、传力机制的影响不大。轴压比对构件的传力机制有着显著的影响,当轴压比较小时,桁架作用明显,水平钢筋与竖向钢筋对受剪承载力的贡献相当,均能充分发挥作用;当轴压比较高时,拱作用明显,水平钢筋未能充分发挥作用,对受剪承载力的贡献有限,竖向钢筋主要对拱作用有贡献。

循环剪切作用下土工袋组合体动力特性试验研究

土工袋具有承载力高、阻尼大、造价低等诸多优点,是一种适用于村镇地区中、低层房屋的减/隔震材料。通过开展室内循环剪切试验对土工袋组合体的剪切应力⁃剪切应变关系、动力特性参数以及抗剪强度进行分析讨论,探究不同竖向应力与剪切应变幅值条件下土工袋组合体动剪切模量与等效阻尼比的变化规律,同时研究了循环次数对土工袋组合体动力特性参数以及抗剪强度参数的影响。试验结果表明:在小变形情况下,土工袋组合体的峰值剪切应力和动剪切模量随着循环次数的增加而逐渐增大,等效阻尼比则随着循环次数的增加而逐渐减小;在大变形情况下,土工袋组合体发生层间滑移并出现界面软化现象,此时的峰值剪切应力(抗剪强度)随着循环次数的增加而逐渐减小,动力特性参数随循环次数的变化与小变形情况下完全相反。随着循环次数的增加,土工袋组合体的抗剪强度参数似摩擦角逐渐减小,似黏聚力逐渐增大并趋于稳定。在剪切应变幅值较大的情况下,土工袋组合体通过袋内土体的剪切变形以及土工袋层间滑移产生摩擦耗能,具有较好的减振消能效果。

T形圆钢管节点受弯抗震性能试验研究

为了研究T形圆钢管节点受弯抗震性能,进行了支主管直径比不同的2个节点受循环往复弯矩作用下的试验研究。试验结果表明:节点主要耗能方式为主管管壁的塑性变形和裂纹扩散,在试验后期还有一部分耗能来自支管根部管壁的塑性变形;最终破坏模式为节点域主管管壁延性撕裂破坏;节点的荷载—相对位移滞回曲线饱满,开裂后节点承载力逐渐下降,裂纹延性扩散,表现出良好的抗震耗能能力。试验所得的极限承载力比规范计算值高了20%~33%。对比2个节点的试验数据及结果,发现支主管直径比较大节点的延性和耗能能力均比支主管直径比中等节点的更好。

Dynamic behaviour of weathered red mudstone in Sichuan(China)under triaxial cyclic loading

The construction of a high-speed railway(HSR) in Southwest China is being hindered by a severe shortage of high-quality subgrade materials. However, red mudstone is widely distributed in the Sichuan Basin of China. The ability to use weathered red mudstone(WRM) to fill subgrade beds by controlling its critical stress and cumulative strain would enable substantial savings in project investments and mitigate damage to the ecological environment. To better understand the dynamic behaviour of WRM, both monotonic and cyclic triaxial tests were performed. The evolution of the cumulative strain vs. increased loading cycles was measured. The influences of confining pressure and loading cycles on the dynamic modulus, damping ratio, critical cyclic stress ratio(CSR), and dynamic stress level(DSL) were investigated. The relationship between the CSR and loading cycles under different failure strain criteria(0.1%-1.0%) was analysed. The prediction model of cumulative strain was also evaluated. The results indicated that the shear strength of WRM sufficiently meets the static strength requirements of subgrade. The critical dynamic stress of WRM can thus satisfy the dynamic stress-bearing requirement of the HSR subgrade. The critical CSR decreases and displays a power function with increasing confining pressure. As the confining pressure increases, the DSL remains relatively stable, ranging between 0.153 and 0.163. Furthermore, the relationship between the dynamic strength and loading cycles required to cause failure was established. Finally, a newly developed model for determining cumulative strain was established. A prediction exercise showed that the model is in good agreement with the experimental data.

橡胶-粉土混合土动力特性试验研究

【目的】为了解橡胶颗粒掺入粉土中对其动力特性的影响,对橡胶粉土混合土进行动力特性研究。【方法】将不同体积分数(0%、5%、10%、20%)的橡胶颗粒掺入粉土试样中,在不同竖向应力作用下,采用GDS动态循环单剪试验系统,对混合土的动剪切模量和阻尼比进行研究。【结果】混合土的动剪切模量随竖向应力的增大而增大,随橡胶颗粒体积分数和动剪应变的增大而减小;阻尼比随竖向应力的增大而减小,随动剪应变的增大而增大;橡胶颗粒体积分数对阻尼比的影响分为两个阶段,在动剪应变小于0.1%时,阻尼比随着橡胶颗粒体积分数的增大而增大,而在动剪应变大于0.1%时,阻尼比随着橡胶颗粒体积分数的增大而减小。【结论】本研究结果可为废旧轮胎橡胶颗粒在实际工程中的应用提供参考。

循环荷载下粉煤灰复合地基竖向承载特性模型试验研究

为研究粉煤灰单桩复合地基竖向循环承载特性,开展在循环荷载作用下的室内模型试验。通过监测桩身及土体变化情况,分别从桩顶累计沉降、土压力、桩身内力等方面进行分析。试验结果表明:循环桩顶累计沉降前期呈现快速增长,而循环荷载比临界值为0.6,将后期沉降变化划分为稳定型和增长型两种趋势;桩侧土压力主要影响桩身上半部分土体,并沿竖向和径向均出现衰减现象,且竖向衰减更为明显;对桩端阻力强化系数Z和桩侧阻力弱化系数D分别进行拟合,可得到对实际工程中不同循环周期下桩端、桩侧阻力强弱变化情况分析,具有参考依据的函数方程;循环荷载下的桩端/桩侧荷载分担比呈现动态变化,循环周期的增加,将会存在桩端所占承载比例超越桩侧所占比例的占比反超点,循环幅值越大,出现反超点所需的循环次数也越少。

Constitutive Model of Saturated Soft Clay with Cyclic Loads under Unconsolidated Undrained Condition

An equivalent visco-elastic model of saturated soft clay was studied under unconsolidated undrained (UU) condition, which can be used to evaluate the stability of ocean foundation. Cyclic triaxial compression and extension tests were conducted to study the parameters of the model. Results showed that the relationships of the damping ratio and the octahedral shear modulus with the octahedral cyclic shear strain were nearly unique, when the initial octahedral shear stress ratios of specimens were equal to 0.3, 0.5 and 0.7. The relationships of the damping ratio and the octahedral shear modulus with the octahedral cyclic shear strain determined from the cyclic triaxial compression tests were basically the same as those determined from the cyclic triaxial extension tests. Furthermore, the relationships were not related to the initial stress condition, the test stress state and the octahedral cyclic shear stress ratio. The relationships determined from the cyclic triaxial tests under no deviatoric stress were basically the same as those determined from the cyclic triaxial tests under deviatoric stress. The change of the octahedral cyclic accumulative strain with the number of cycles was unique under different tests stress states. An equivalent visco-elastic constitutive model of saturated soft clay under UU condition was initially established.

Dynamic performance of angle-steel concrete columns under low cyclic loading-II:parametric study

Tests of nine angle-steel concrete column （ASCC） specimens under low cyclic loading are described in a companion paper （Zheng and Ji, 2008）. In this paper, the skeleton curves from the numerical simulation are presented, and show good agreement with the test results. Furthermore, parametric studies are conducted to explore the influence of factors such as the axial compression ratio, shear steel plate ratio, steel ratio, prismatic concrete compression strength, yield strength of angle steel and shear span ratio, etc., on the monotonic load-displacement curves of the ASCCs. Based on a statistical analysis of the calculated results, hysteretic models for load-displacement and moment-curvature are proposed, which agree well with the test results. Finally, some suggestions concerning the conformation of ASCCs are proposed, which could be useful in engineering practice.

Three-Dimensional Models for Analyzing the Cyclic Variations in a Lean Burn CNG Engine

Three-dimensional models, consisting of the flame kernel formation model, flame kernel development model and natural gas single step reaction model, are used to analyze the contribution of cyclic equivalence ratio variations to cyclic variations in the compressed natural gas （CNG） lean burn spark ignition engine. Computational results including the contributions of equivalence ratio cyclic variations to each combustion stage and effects of engine speed to the extent of combustion variations are discussed. It is concluded that the equivalence ratio variations affect mostly the main stage of combustion and hardly influence initial kernel development stage.

Cyclic stress-controlled tests on offshore clay

One of the main concerns in cyclic behavior of soft clay is gradual degradation with the progression of loading cycle.A series of cyclic constant-volume direct simple shear(CDSS) loading tests was performed on Malaysia offshore clay to study its undrained degradation.The testing program consists of stresscontrolled tests with cyclic shear ratio ranging from 0.34 to 0.83 at different overconsolidation ratios(OCRs).For a given cyclic stress ratio in stress-controlled tests,the accumulated cyclic strain and pore water pressure increase with elevated number of cycles.In heavily overconsolidated clay specimens,the negative cyclic pore water pressure is generated followed by positive cyclic pore water pressure as cyclic tests progress.The post-cyclic strength of offshore clay specimens is reduced by undrained cyclic stresscontrolled loading.

高速开关阀控液压马达系统性能对比研究

采用一款高响应电液伺服阀并使其处于阀口开、关两种极限工况下工作的高速开关控制方式,构建一种阀控液压马达系统,实现对液压马达输出转速及扭矩的控制,以探究液压马达高速开关控制方法的基本特性。完成该高速开关阀控液压马达系统的设计及AMESim仿真模型的搭建。利用PWM信号控制高响应电液伺服阀实现对液压马达的高速开关控制,并通过仿真获得转速等参数随占空比和频率的变化规律。开发基于高响应电液伺服阀的高速开关阀控液压马达系统实验样机,进行实验与仿真结果的对比研究。结果表明:实验与仿真结果较为一致,液压马达转速随着占空比的增大而增大,随着外负载的增大逐渐降低;而仿真结果中负载的增加会轻微加快液压马达转速的稳定时间的结论,在实验中无法得到印证。