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.

Cyclic behavior of reinforced sand under principal stress rotation

Although the cyclic rotation of the principal stress direction is important,its effect on the deformation behavior and dynamic properties of the reinforced soil has not been reported to date.Tests carried out on large-scale hollow cylinder samples reveal that the cyclic rotation of the principal stress direction results in significant variations of strain components(ε,ε,εand γ) with periodic characteristics despite the deviatoric stress being constant during tests.This oscillation can be related to the corresponding variations in the stress components and the anisotropic fabric that rotate continuously along the principal stress direction.Sand under rotation appears to develop a plastic strain.Similar trends are observed for reinforced sand,but the shear interaction,the interlocking between particles and reinforcement layer,and the confinement result in significant reductions in the induced strains and associated irrecoverable plastic strains.Most of the strains occur in the first cycle,and as the number of cycles increases,the presence of strains becomes very small,which is almost insignificant.This indicates that the soil has reached anisotropic critical state(ACS),where a stable structure is formed after continuous orientation,realignment and rearrangement of the particles accompanied with increasing cyclic rotation.Rotation in the range of 60°-135° produces more induced strains even in the presence of the reinforcement,when compared with other ranges.This relates to the extension mode of the test in this range in which σ>σand to the relative approach between the mobilized plane and the weakest horizontal plane.Reinforcement results in an increase in shear modulus while it appears to have no effect on the damping ratio.Continuous cycles of rotation result in an increase in shear modulus and lower damping ratio due to the densification that causes a decrease in shear strain and less dissipation of energy.

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.

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。

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。

Influence of principal stress rotation of unequal tensile and compressive stress amplitudes on characteristics of soft clay

Soil behavior can reflect the characteristics of principal stress rotation under dynamic wave and traffic loads. Unequal amplitudes of tensile and compressive stresses applied to soils have complex effects on foundation soils in comparison with the pure principal stress rotation path. A series of undrained cyclic hollow torsional shear tests were performed on typical remolded soft clay from the Hexi area of Nanjing, China. The main control parameters were the tensile and compressive stress amplitude ratio(α) and the cyclic dynamic stress ratio(η). It was found that the critical η tended to remain constant at 0.13, when the value of the compressive stress amplitude was higher than the tensile stress amplitude. However, the influence of the tensile stress was limited by the dynamic stress level when α= 1.For obvious structural change in the soil, the corresponding numbers of cyclic vibration cycles were found to be independent of α at low stress levels and were only related to η. Finally, a new method for evaluating the failure of remolded soft clay was presented. It considers the influence of the tensile and compressive stresses which caused by complex stress paths of the principal stress rotation. This criterion can distinguish stable, critical, and destructive states based on the pore-water-pressure-strain coupling curve while also providing a range of failure strain and vibration cycles. These results provide the theoretical support for systematic studies of principal stress rotation using constitutive models.

Study on one—dimensional consolidation of saturated soil with semi—pervious boundaries and under cyclic loading

The variation opf effective stress ratio of stratfied soil with semi-pervious boundaries and under cylic loading was analyzed on the basis of Terzaghi's one-dimensional consolidation assumptions.A solution by Laplace Transform was obtained for the case whe the soil was under time-varied loading.With numerical in version of Laplace Tranform.Some useful results were obtained for several kinds of commonly encountered loadings.The results can be meaningful in engineering preactice.

不同循环应力路径下饱和珊瑚砂体应变的发展特征

地震、波浪引起排水或部分排水状态的饱和砂类土体应变的累积,会导致地表、海床和附近结构物的显著破坏,且饱和砂土排水循环加载的体应变增长与不排水循环加载的孔压增长存在定量的对应关系。通过对饱和珊瑚砂开展系列均等固结条件下排水循环剪切试验,探讨了初始相对密度Dr、循环应力路径及应力水平(CSR)对饱和珊瑚砂体应变发展特征的影响。试验结果表明,同一Dr和CSR下,残余体应变εvd,ir随短长轴比和在0°~90°时的椭圆倾角存在单调增长关系。不同Dr、循环应力路径和CSR下饱和珊瑚砂εvd,ir随循环次数N的发展曲线具有统一性,提出了两者的显式关系式。饱和珊瑚砂残余体应变极值(εvd,ir)u与εvd,ir-N关系曲线的收敛速度与Dr、循环应力路径和应力水平密切相关。引入单元体循环应力比USR来表征不同循环应力路径和CSR下饱和珊瑚砂的εvd,ir,发现同一Dr下,(εvd,ir)u与USR存在正相关线性关系,εvd,ir-N曲线的收敛参数CN1与USR存在正相关线性关系,CN2与USR存在负幂函数关系,即USR越大,εvd,ir-N曲线收敛越慢。相同USR下,(εvd,ir)u随Dr的增加而减小,εvd,ir-N曲线收敛速度随Dr的增加而变快。该研究提出的体应变模型为排水循环加载条件下残余体应变的发展机理提供了新的认识。

温升作用对深海沉积物静力特性及临界循环应力比的影响研究

针对取自中国南海西部海槽的深海黏土质沉积物,在不排水升温条件下开展了重塑试样的静动力学试验。探讨了温升作用对土体的不排水抗剪强度、孔压发展规律及临界循环应力比的影响。基于Yao的理论框架给出了不排水升温导致超孔压的表达式,讨论了不同温度条件下土体的不排水抗剪强度与临界循环应力比的关系。试验结果表明,不排水升温产生的超孔压导致土体的有效应力降低,削弱了土体的不排水抗剪强度,使土体展现出了拟似超固结土的力学特性;对于分级加载的循环剪切工况,累积应变的发展速度随温度的提高显著增加,但在剪切初期,温升作用抑制了累积孔压的发展,甚至导致负累积孔压的产生。以归一化累积孔压产生的分化标准作为判定门槛循环应力比的方法不能很好地适用于具有超固结特性的土体。建议综合考虑累积变形及孔压的发展规律讨论土体的界限循环应力比。整体来看,随着温度的升高土体的抗剪强度及临界循环应力比显著降低,在工程中应当引起充分重视。

青藏铁路沿线负温填土长期动力力学特性试验研究

为了评价冻土区路基的稳定性,本文对土体长期振动荷载下的动力特性进行研究,通过室内试验和理论分析相结合的手段对青藏铁路沿线填土的动力力学特性(动剪切模量和阻尼比)进行了分析。首先开展了室内固结不排水长期循环三轴试验,分析了不同负温、循环应力比对路基粉质黏土和地基砂土的动剪切模量和阻尼比等动力特性的影响。结果表明,路基粉质黏土的动剪切模量与温度呈现明显的负相关性,且随着循环应力比增大而呈现出较明显的增长趋势,阻尼比随着动剪切模量的增长而减小;地基砂土的动剪切模量和阻尼比受温度的影响与粉质黏土的响应规律呈现相同趋势,在温度为–10℃与循环应力比为0.2的相同条件下动剪切模量增至1.22倍,而阻尼比则减少到0.66倍。值得注意的是,砂土动剪切模量在循环应力比约为1时响应达到峰值,即动应力幅值达到围压时循环应力比增大,阻尼比增大而动剪切模量则呈现减小的趋势。随后在Wichtmann提出的高周循环模型基础上,建立了动剪切模量预测模型,能良好地反映动剪切模量在温度和循环应力比影响下,随振动次数变化的响应规律,通过拟合参数w0反映动剪切模量随循环应力比先增后减的规律,拟合数学关系显示长期振动次数约在13 000次或振动时长8 min以上时,土体的动力响应规律发生了明显变化。最后,基于Hardin-Drnevich双曲线模型对动剪切模量和阻尼比的试验结果进行拟合,得到了动剪切模量与阻尼比的函数关系式,从而能大致预测阻尼比随动剪切模量变化的趋势及范围,随着归一化剪切模量的增大阻尼比呈现出减小的趋势。

橡胶砂液化特性的动扭剪试验研究

为研究橡胶砂的液化特性,在不同的橡胶质量掺量、循环应力比、相对密实度、频率和有效围压工况条件下对橡胶砂进行动扭剪试验,分析橡胶砂的动力剪切应力-应变关系、动弹性模量和阻尼比的发展趋势.结果表明:橡胶颗粒质量掺量越高,橡胶砂越不易液化,广义剪切应变越小,橡胶砂越不易破坏;在橡胶颗粒掺量一定的情况下,试样破坏周次随循环应力比增大而减少;相对密实度和有效围压增大时,试样的剪切应变降低、抗液化能力增强,而剪切频率增大时,抗液化能力减弱;随着剪切应变和橡胶颗粒掺量的增加,试样弹性模量逐渐降低,阻尼比逐渐增大.

循环荷载下粉质黏土的变形与耗能特性

循环荷载下土的累计变形与耗能特性是岩土工程设计的重要依据。对粉质黏土原位样和重塑试样开展了不排水循环三轴试验,研究了结构性对粉质黏土动力变形与滞回耗能特性的影响规律。结果表明:粉质黏土动力变形规律由循环应力比控制,原位样的临界循环应力比相比重塑样更高;双幅应变随振次的增长模式可以分为稳定型、过渡型和破坏型;累积能量耗散随双幅应变的增长模式也表现出相似的3种类型;相同双幅应变下,重塑样的滞回耗能能力比原位样的更强;相同条件下,原位样需要耗散更多的能量和更多振次才能达到与重塑样相同的双幅应变;结构性对粉质黏土动力荷载下的变形与耗能特性有明显影响。

颗粒破碎对于珊瑚砂液化特性的影响

利用GDS动三轴试验系统,对南海珊瑚砂进行了不同破碎度和不同动剪循环应力比下的等幅循环荷载试验,研究了破碎度,动剪循环应力比对南海珊瑚砂的液化特性的影响。试验结果表明:在一维固结试验条件下,随着固结围压的增大,珊瑚砂破碎度逐渐增大;对不同破碎度的珊瑚砂试样进行GDS动三轴试验,随着振动周次增加,在相同的破碎度下,随着动剪循环应力比的增加,试验孔压的发展速度逐渐加快,在相同的动剪循环应力比下,随着破碎度的增加,试样的孔压发展速度逐渐加快;饱和珊瑚砂动孔压的发展在动剪循环应力比为0.2的条件下的运用改进Booker模型拟合有较好的效果。

结构性软黏土的循环累积应变特性研究

针对动力作用下软黏土结构破坏而造成的变形问题,对杭州地区的原状软黏土进行了循环加载的动力特性试验。结果表明相同试验条件下,轴向应变在低有效固结围压、较低振动频率和较高动应力比作用下发展速率更快,且这三者存在耦合关系。综合考虑有效固结围压、振动频率以及动应力比的影响,建立了能够描述土样轴向累积变形的表达关系式。

风积沙的压实特性与循环荷载下变形性状的试验研究

在对风积沙压实特性进行分析的基础上 ,探讨了含水量对风积沙压实影响的机理。通过动三轴试验 ,得到了影响循环荷载下风积沙变形特性的主要因素为压实含水量。

交通荷载作用下低路堤动力特性试验研究

结合连盐高速低路堤,对路堤进行了动应力、振动响应等现场测试,分析了路基中附加动应力及振动位移的变化规律.并且通过应力控制的室内动三轴试验,采用一定的动应力频率、不同的循环应力比来模拟交通荷载,研究了在循环荷载作用下,连盐高速公路饱和软粘土的轴向累计应变、孔隙水压力和轴向周期应变软化的变化规律.实验结果表明:路堤中动应力和振动位移随行车速度和车重的增加而增加,随深度的增加而降低,有效影响深度达2.5m;原状土的临界循环应力比远大于重塑土,承受循环应力能力高于重塑土.因此,要加强浅层路基强度,同时要减少对底部下卧层软粘土的扰动.其结果可为在软土地基上设计和施工低路堤高速公路提供有益的参考.

基于静力触探测试的国内外砂土液化判别方法

在砂土或粉土分布广泛的地区,地震液化是导致地基失稳和上部结构受损的直接原因之一。因此,判定地基土液化可能性是抗震勘察设计中的一个重大课题。由于获得高质量"未扰动"砂土样的困难和试验成本的限制,基于静力触探(CPT)的原位测试方法通常用于砂土液化势的评价。孔压静力触探(CPTU)作为一种新型的原位测试技术,采用其测试指标进行砂土液化势的评价日益受到人们的重视。目前,基于CPT测试资料已经提出了许多砂土液化势的评价方法,但国内外采用CPT判别砂土液化的方法是不同的。在对我国《建筑抗震设计规范》(GB50011–2001)中砂土液化判别方法与国外修正的Seed简化法的原理、方法及参数进行分析的基础上,将其2种方法进行比较,进而讨论了2种方法的差异。由于这些差异的存在,采用不同方法判别砂土液化可能性时会得出不同的、甚至是相反的结果。结合济宁—徐州高速公路砂土场地的CPT和CPTU测试资料,采用我国规范判别法、Seed简化法进行了液化判别分析,并进行了对比验算。其中Robertson法与Olsen法所得到的结论基本一致,而且Robertson法似乎比Olsen法稍微精确一些,而采用规范法所判定的结果偏于保守,这与锥尖阻力基准值的取值大小有关。

循环荷载作用下水泥复合土变形性状试验研究

通过复合土体的动三轴试验 ,研究了循环荷载作用下循环应力比、加荷周数和围压对复合土体轴向应变的影响。

基于能量法的尾矿土动孔压模型研究

通过应力控制式动三轴不排水循环剪切试验,对饱和尾矿土在循环应力比、轴–径向固结应力比等因素下孔隙水压力与累积能量耗散关系进行研究。研究结果表明孔隙水压力增长与塑性应变累积能量耗散和土体黏滞累积能量耗散密切相关。当循环应力比cR介于0.205～0.238、轴–径向固结应力比cK介于1.0～1.3时,孔隙水压力–能量之间存在临界状态。通过非线性回归分析初步建立了循环应力比和轴–径向固结应力比条件下饱和尾矿土的孔隙水压力能量模型,从变化机理上阐述孔隙水压力的变化规律并消除了周次的不确定性对于孔隙水压力模型的影响,对预测循环荷载作用下饱和土的孔隙水压力具有借鉴意义。