Dynamic shear modulus and damping ratio characteristics of undisturbed marine soils in the Bohai Sea,China

This paper presents results from a series of stress-controlled undrained cyclic triaxial tests on the undisturbed marine silty clay,silt,and fine sand soils obtained from the Bohai Sea,China.Emphasis is placed on the major factors for predominating the dynamic shear modulus(G)and damping ratio(λ)in the shear strain amplitude(γ_(a))from 10^(-5) to 10^(-2),involving depth,sedimentary facies types,and water content of marine soils.The empirical equations of the small-strain shear modulus(G_(max))and damping ratio(λ_(min))using a single-variable of depth H are established for the three marine soils.A remarkable finding is that the curves of shear modulus reduction(G/G_(max))and the damping ratio(λ)with increasing γ_(a) of the three marine soils can be simply determined through a set of explicit expressions with the two variables of depth H and water content W.This finding is validated by independent experimental data from the literature.At the similar depths,the G value of the marine soils of terrestrial facies is the largest,followed b_(y) the neritic facies,and the G value of the marine soils of abyssal facies is the smallest.The sedimentary facies types of the marine soils have slight effect on theλvalue.Another significant finding is that the shear modulus reduction curves plotted against the γ_(a) of the three marine soils at the similar depths are significantly below those of the corresponding terrigenous soils,while the damping curves plotted against γ_(a) are just the opposite.The results presented in this paper serve as a worthful reference for the evaluation of seabed seismic site effects in the Bohai Sea due to lack of experimental data.

Dynamic shear modulus of undisturbed soil under different consolidation ratios and its effects on surface ground motion

The dynamic shear modulus for three types of undisturbed soil under different consolidation ratios is presented by using the resonant column test method. Its effects on surface ground motion is illustrated by calculation. The test results indicate that the power function is a suitable form for describing the relationship between the ratio of the maximum dynamic shear modulus due to anisotropic and isotropic consolidations and the increment of the consolidation ratio. When compared to sand, the increment of the maximum dynamic shear modulus for undisturbed soil due to anisotropic consolidation is much larger. Using a one-dimensional equivalent linearization method, the earthquake influence factor and the characteristic period of the surface acceleration are calculated for two soil layers subjected to several typical earthquake waves. The calculated results show that the difference in nonlinear properties due to different consolidation ratios is generally not very notable, but the degree of its influence on the surface acceleration spectrum is remarkable for the occurrence of strong earthquakes. When compared to isotropic consolidation, the consideration of actual anisotropic consolidation causes the characteristic period to decrease and the earthquake influence factor to increase.

Shear modulus and damping ratio of sand-granulated rubber mixtures

Recycled waste tires when mixed with soil can play an important role as lightweight materials in retaining walls and embankments, machine foundations and railroad track beds in seismic zones. Having high damping characteristic, rubbers can be used as either soil alternative or mixed with soil to reduce vibration when seismic loads are of great concern. Therefore, the objective of this work was to evaluate the dynamic properties of such mixtures prior to practical applications. To this reason, torsional resonant column and dynamic triaxial experiments were carried out and the effect of the important parameters like rubber content and ratio of mean grain size of rubber solids versus soil solids(D50,r/D50,s) on dynamic response of mixtures in a range of low to high shearing strain amplitude from about 4×10-4% to 2.7% were investigated. Considering engineering applications, specimens were prepared almost at the maximum dry density and optimum moisture content to model a mixture layer above the ground water table and in low precipitation region. The results show that tire inclusion significantly reduces the shear modulus and increases the damping ratio of the mixtures. Also decrease in D50,r/D50,s causes the mixture to exhibit more rubber-like behavior. Finally, normalized shear modulus versus shearing strain amplitude curve was proposed for engineering practice.

Influence of repeated freeze-thaw on dynamic modulus and damping ratio properties of silty sand

Under repeated freezing and thawing in deep seasonal frozen regions, the stability and strength of the soil are imposed in the form of large uneven settlement, instability and strength reduction, which affect the normal operation of railway lines. This study is to obtain the influencing rules of freeze-thaw on the dynamic properties （dynamic strain, confining pressure and compactness） of silty sand. Based on an amount of inner tests, the dynamic modulus and damping ratio properties of silty soil subjected to repeated freeze-thaw cycles were deeply researched and analyzed. The results are as follows： At the same dynamic strain, the relationship of dynamic stress and freeze-thaw cycles presents negative cor- relation, and the relationship of dynamic stress, confining pressure and compactness present positive correlation. The dynamic modulus double decreases while the damping ratio double increases with incremental increase in dynamic strain. The dynamic modulus sharply decreases while the damping ratio increases with incremental increase in freeze-thaw cycles, and then the changes level off after six freeze-thaw cycles. The dynamic modulus increases while the damping ratio decreases as the confining pressure and compactness increase at the same strain level.

Effect of consolidation ratios on maximum dynamic shear modulus of sands

The dynamic shear modulus (DSM) is the most basic soil parameter in earthquake or other dynamic loading conditions and can be obtained through testing in the field or in the laboratory. The effect of consolidation ratios on the maximum DSM for two types of sand is investigated by using resonant column tests. And, an increment formula to obtain the maximum DSM for cases of consolidation ratio κc>1 is presented. The results indicate that the maximum DSM rises rapidly when κc is near 1 and then slows down, which means that the power function of the consolidation ratio increment κc-1 can be used to describe the variation of the maximum DSM due to κc>1. The results also indicate that the increase in the maximum DSM due to κc>1 is significantly larger than that predicted by Hardin and Black's formula.

Experimental Research on Dynamic Shear Modular Ratio and Damping Ratio of Sandy Gravel Soil

The dynamic shear modulus ratio and damping ratio of sandy gravel are important parameters for the seismic response analysis of valley geomorphic sites,which have an important impact on the determination of design ground motion parameters. In this paper,the dynamic triaxial test of sandy gravels has been performed based on the project of the Shangluo Seismic Microzonation. Combined with the other results of sandy gravel,the recommended results of slightly dense,medium dense and dense sandy gravel were obtained. By building the typical site model,the influence of the dynamic shear modulus ratio and the damping ratio uncertainty on the seismic response of the site is studied. The results show that the uncertainty of the average of the dynamic shear modulus ratio and the damping ratio ± 1 times the standard deviation has little effect on the peak acceleration of the sandy gravel site,and the rationality of the grouping and statistical results is explained. Under different probability levels,the change in the shear modulus ratio and damping ratio leads to a significant difference in the high frequency response spectrum.The response spectrum of 0. 04-0. 1s ranges from about 20%,but it has little effect on the long period spectrum of more than 1. 0s. The study of dynamic shear modulus ratio and damping ratio of sandy gravel has the ability to improve the reliability of the designing ground motion parameters.

The Effect of Mass Ratio and Air Damper Characteristics on the Resonant Response of an Air Damped Dynamic Vibration Absorber

In this paper, it is shown that, a road vehicle 2DOF air damped quartercar suspension system can conveniently be transformed into a 2DOF air damped vibrating system representing an air damped dynamic vibration absorber (DVA) with an appropriate change in the ratio μ of the main mass and the absorber mass i.e. when mass ratio μ >> 1. Also the effect of variation of the mass ratio, air damping ratio and air spring rate ratio, on the motion transmissibility at the resonant frequency of the main mass of the DVA has been dis- cussed. It is shown that, as the air damping ratio in the absorber system increases, there is a substantial decrease in the motion transmissibility of the main mass system where the air damper has been modeled as a Maxwell type. Optimal value of the air damping ratio for the minimum motion transmissibility of the main mass of the system has been determined. An experimental setup has been designed and developed with a control system to vary air pressure in the damper in the absorber system. The motion transmissibility characteristics of the main mass system have been obtained, and the optimal value of the air damping ratio has been determined for minimum motion transmissibility of the main mass of the

Dynamic Flexural Modulus and Low-Velocity Impact Response of Supercomposite^TMLaminates with Vertical Z-Axis Milled Carbon Fiber Reinforcement

In work reported here, the dynamic properties and low-velocity impact response of woven carbon/epoxy laminates incorporating a novel 3D interlaminar reinforcement concept with dense layers of Z-axis oriented milled carbon fiber SupercompositeTM prepregs, are presented. Impulse-frequency response vibration technique is used for non-destructive evaluation of the dynamic flexural modulus (stiffness) and loss factor (intrinsic damping) of woven carbon/epoxy control and SupercompositeTM laminates. Low-velocity punch-shear tests were performed on control and SupercompositeTM laminates according to ASTM D3763 Standard using a drop-weight impact test system. Control panels had all layers of 3K plain woven carbon/epoxy prepregs, with a dense interlaminar reinforcement of milled carbon fibers in Z- direction used in designing the SupercompositeTM laminate—both having same areal density. Impulse-frequency response vibration experiments show that with a 50% replacement of woven carbon fabric in control panel with milled carbon fibers in Z direction dynamic flexural modulus reduced 25% - 30% (loss in stiffness) and damping increased by about the same 25% - 30%. Low-velocity punch-shear tests demonstrated about 25% reduction in energy absorption for SupercompositeTM laminates with the replacement of 50% woven carbon fabric in control panel.

实时剪切波弹性成像检测甲状腺结节杨氏模量值及Ratio值鉴别良恶性的临床价值

目的探讨剪切波弹性成像（SWE）检测甲状腺结节杨氏模量值及Ratio值鉴别良恶性的价值。方法选取2014年10月~2015年1月在本院手术治疗的39例甲状腺实性结节患者作为研究对象,术前均行SWE检测,记录甲状腺组织弹性杨氏模量的平均值（E_（mean））、最小值（E_（min））及最大值（E_（max））及以颈前肌为参照的Ratio值,应用ROC曲线分析杨氏模量值（最大值）及Ratio值在甲状腺结节良恶性鉴别中的价值。结果良、恶性结节的杨氏模量值（最大值）及Ratio值比较,差异有统计学意义（P〈0.05）,ROC曲线下面积（AUC）分别为0.90、0.81。结论 SWE可定量测量甲状腺组织的杨氏模量值及Ratio值,获得客观的组织硬度信息,能够为甲状腺疾病的诊断提供更多依据。

Test and numerical investigations on static and dynamic characteristics of extra-wide concrete self-anchored suspension bridge under vehicle loads

The present work is aimed at studying the mechanic properties of the extra-wide concrete self-anchored suspension bridge under static and dynamic vehicle loads. Based on the field test using 12 heavy trucks and finite element simulations, the static deformations of different components, stress increments and distributions of the girder, as well as the vibration characteristics and damping ratio of the Hunan Road Bridge were analyzed, which is the widest self-anchored suspension bridge in China at present. The dynamic responses were calculated using the Newmark-β integration method assisted by the simulation models of bridge and vehicles, the influences on the dynamic impact coefficient(DIC) brought by the vehicle parameters, girder width, eccentricity travel and deck flatness were also researched. The spatial effect of the girder is obvious due to the extra width, which performs as the stress increments distribute unevenly along the transverse direction, and the girder deflections and stress increments of the upper plate change as a "V" and "M" shape respectively under the symmetrical vehicle loads affected by the shear lag effect, cross slope and local effect of the wheels, the maximum of stress increments are located in the junctions with the inner webs. The obvious girder torsional deformation and the apparent unevenness of the hanger forces between the two cable planes under the eccentric vehicle loads, together with the mode shapes such as the girder transverse bending and torsion which appear relatively earlier, all reflect the weakened torsional rigidity of the extra-wide girder. The transverse displacements of towers are more obvious than the longitudinal ones. As for the influences on the DIC, the static effect of the heavier vehicles plays a major role when pass through with a higher speed and the changes of vehicle suspension stiffness generate greater impacts than the suspension damp. The values of DIC in the vehicle-running side during the eccentric travel, affected by the restricts from the static effects of the eccentric moving trucks, are significantly smaller than the vehicle-free side, the increase in the road roughness is the most sensitive one among the above influential factors. The results could provide references for the design, static and dynamic response analysis of the similar extra-wide suspension bridges.

Size dependency and potential field influence on deriving mechanical properties of carbon nanotubes using molecular dynamics

A thorough understanding on the mechanical properties of carbon nanotube （CNT） is essential in extending the advanced applications of CNT based systems. However, conducting experiments to estimate mechanical properties at this scale is extremely challenging. Therefore, development of mechanistic models to estimate the mechanical properties of CNTs along with the integration of existing continuum mechanics concepts is critically important. This paper presents a comprehensive molecular dynamics simulation study on the size dependency and potential function influence of mechanical properties of CNT. Commonly used reactive bond order （REBO） and adaptive intermolecular reactive bond order （A1REBO） potential functions were considered in this regard. Young＇s modulus and shear modulus of CNTs are derived by integrating classical continuum mechanics concepts with molecular dynamics simulations. The results indicate that the potential function has a significant influence on the estimated mechanical properties of CNTs, and the influence of potential field is much higher when studying the torsional behaviour of CNTs than the tensile behaviour.

Experimental study on the dynamic modulus of compacted loess under bidirectional dynamic load

The dynamic characteristics of compacted loess are of great significance to the seismic construction of the Loess Plateau area in Northwest China,where earthquakes frequently occur.To study the change in the dynamic modulus of the foundation soil under the combined action of vertical and horizontal earthquakes,a hollow cy-lindrical torsion shear instrument capable of vibrating in four directions was used to perform two-way coupling of compression and torsion of Xi'an compacted loess under different dry density and deviator stress ratios.The results show that increasing the dry density can improve the initial dynamic compression modulus and initial dynamic shear modulus of compacted loess.With an increase in the deviator stress ratio,the initial dynamic compression modulus increases,to a certain extent,but the initial dynamic shear modulus decreases slightly.The dynamic modulus gradually decreases with the development of dynamic strain and tends to be stable,and the dynamic modulus that reaches the same strain increases with an increasing dry density.At the initial stage of dynamic loading,the attenuation of the dynamic shear modulus with the strain development is faster than that of the dynamic compression modulus.Compared with previous research results,it is determined that the dynamic modulus of loess under bidirectional dynamic loading is lower and the attenuation rate is faster than that under single-direction dynamic loading.The deviator stress ratio has a more obvious effect on the dynamic compression modulus.The increase in the deviator stress ratio can increase the dynamic compression modulus,to a certain extent.However,the deviator stress ratio has almost no effect on the dynamic shear modulus,and can therefore be ignored.

Topological Design of Microstructures of Materials Containing Multiple Phases of Distinct Poisson’s Ratios

A methodology for achieving the maximum bulk or shear modulus in an elastic composite composed of two isotropic phases with distinct Poisson’s ratios is proposed.A topology optimization algorithm is developed which is capable of finding microstructures with extreme properties very close to theoretical upper bounds.The effective mechanical properties of the designed composite are determined by a numerical homogenization technique.The sensitivities with respect to design variables are derived by simultaneously interpolating Young’smodulus and Poisson’s ratio using different parameters.The so-called solid isotropicmaterial with penalizationmethod is developed to establish the optimization formulation.Maximum bulk or shearmodulus is considered as the objective function,and the volume fraction of constituent phases is taken as constraints.Themethod ofmoving asymptotes is applied to update the design variables.Several 3D numerical examples are presented to demonstrate the effectiveness of the proposed structural optimization method.The effects of key parameters such as Poisson’s ratios and volume fractions of constituent phase on the final designs are investigated.A series of novel microstructures are obtained fromthe proposed approach.It is found that the optimized bulk and shearmoduli of all the studied composites are very close to the Hashin-Shtrikman-Walpole bounds.

单双向循环荷载作用下砂砾料动模量和阻尼比试验研究

通过大型振动三轴试验,研究了单双向循环荷载作用下砂砾料动弹性模量和阻尼比的变化规律,分析围压和径向循环动应力对砂砾料动力参数的影响。研究结果表明:在双向振动三轴试验中,砂砾料的轴向动应变受径向动应力的影响较小,动应变主要与施加的轴向动应力大小有关;单双向振动下砂砾料的动弹性模量均随着动应变的增大而逐渐降低,双向振动时砂砾料动弹性模量随动应变的衰减速率基本不变,在相同动应变下双向振动的动模量均低于单向振动;砂砾料在双向振动时的阻尼比大于单向振动,双向振动时消耗的动应变能更大。通过对两种试验条件下的最大动弹性模量、动模量比进行分析,建立了表述单双向试验条件下最大动弹性模量的换算关系式和双向振动试验中动模量比和动应变的修正模型。

不同冲击气压下煤样动态剪切强度的长径比效应

采用Φ50mm分离式霍普金森压杆(SHPB)试验系统,开展了不同冲击气压下直径75mm,长径比分别为0.20,0.27,0.33,0.40和0.47的5组煤样的动态剪切试验,划分了煤动态剪应力时程曲线的阶段,探讨了冲击气压对煤样动态剪切强度的影响,分析了煤样动态剪切强度和加载率的长径比效应,并建立了长径比效应理论模型。研究结果表明:①煤样动态剪应力时程曲线可分为应力初始上升、应力线性增长、应力缓慢上升和应力下降4个阶段;②煤样动态剪切强度与冲击气压呈正线性相关,但不同长径比下增加幅度存在差异,具体表现为:相同冲击气压增量下,煤样长径比越小,动态剪切强度的增加幅度越大;③煤样动态剪切强度和加载率均与长径比有关,在0.25,0.35 MPa较低冲击气压与0.45,0.55 MPa较高冲击气压下分别呈现出正、负长径比效应,并通过方差分析确定了长径比对其影响最小的冲击气压为0.376MPa;④建立了不同冲击气压下煤样动态剪切强度长径比效应理论模型,通过加载率效应推导出加载率长径比效应理论模型,并验证了模型的合理性和准确性。

基于超大型三轴仪的堆石料动力特性缩尺效应研究

堆石料的缩尺效应是导致高土石坝变形预测不准的重要因素之一,目前针对堆石料静力缩尺效应的影响已有部分研究,但针对堆石料动力变形特性的缩尺效应研究仍十分鲜见。本文对两河口板岩及古水玄武岩开展了超大型(试样直径800 mm,最大粒径为160 mm)和常规大型三轴(试样直径300 mm,最大粒径为60 mm)动剪切模量和阻尼比试验,研究了平行相似级配条件下最大粒径对最大动剪切模量、动剪切模量比、阻尼比以及沈珠江等效线性模型参数的影响。试验结果表明:①超大型三轴试验的最大动剪切模量大于常规大型三轴试验,随着动剪应变的增大,超大型和常规大型三轴试验的最大动剪切模量差别逐渐减小;②超大型三轴试验的动剪切模量比G_(d)/G_(dmax)小于常规大型三轴试验,即超大型三轴试验的动剪模量比衰减得更快;③超大型三轴试验的阻尼比大于常规大型三轴试验,随着动剪应变的增加,二者的差距逐渐增大;④超大型三轴试验得到的沈珠江等效线性模型参数k_(2)、k_(1)以及λ_(max)分别是常规大型三轴试验的1.14~1.32倍、1.26~1.31倍以及1.31~1.44倍,超大型和常规大型三轴试验得到的模量指数n差别不大。本文研究成果可为同类工程动力响应分析提供更加合理的试验与数值计算依据。

颗粒表面粗糙度对材料小应变动力特性的影响

土体的小应变剪切模量和阻尼比是表征土体动力学特性的重要参数,不仅受到土体密实度和应力状态的影响,还受到土体颗粒级配、形状等颗粒特征的影响。颗粒表面粗糙度是重要的土体颗粒特征之一,然而关于颗粒表面粗糙度对土体小应变动力特性影响的研究较为匮乏。利用能量注入式虚拟质量共振柱设备,系统地测试了颗粒表面粗糙度不同的玻璃珠所成试样的小应变剪切模量和阻尼比;采用三维干涉显微镜测量了玻璃珠的表面粗糙度,并量化表征粗糙度对试样小应变剪切模量和阻尼比的影响。试验结果表明,在相同孔隙比和有效应力条件下,试样的小应变剪切模量随颗粒表面粗糙度增大而减小,而小应变阻尼比受颗粒表面粗糙度影响的规律不明显。研究结果表明,当材料的颗粒形状、级配等因素相近时,颗粒表面粗糙度对材料小应变剪切模量的影响不应被忽略。

橡胶混合黏土小应变剪切模量特性试验研究

为了研究橡胶混合黏土(混合土)的动变形特性,对不同橡胶掺量、橡胶粒径和围压的混合土开展共振柱试验,分析动剪切模量G和阻尼比λ的发展规律,并基于二元介质模型,提出了表达混合土接触状态的骨架孔隙比e_(sk)计算方法,进一步依托骨架孔隙比e_(sk)对混合土的最大动剪切模量G_(max)进行评价。结果表明:掺入橡胶颗粒后,混合土的G减小,λ增大,且橡胶掺量增大时,混合土的G减小、λ增大;围压增大时,混合土的G增大、λ减小;橡胶粒径增大时,混合土的G增大、λ减小。随着橡胶掺量增大,骨架孔隙比e_(sk)增大,G_(max)降低;在相同橡胶掺量时,随着橡胶粒径的增大,e_(sk)增大,G_(max)升高。在Hardin公式的基础上,基于骨架孔隙比e_(sk)提出了考虑橡胶掺量和橡胶粒径的G_(max)表征模型,该模型具有较好的准确性,可为评价橡胶混合黏土G_(max)提供依据。

雄安新区粉质黏土的动力特性试验研究

为分析雄安新区粉质黏土的动力特性,采用GDS多功能动态循环三轴试验系统,对雄安新区粉质黏土进行14种工况(不同围压、固结比和加载频率)下的动三轴试验,深入分析了动骨干曲线、动弹性模量、动剪切模量、阻尼比等在不同工况下的变化规律及相关参数。试验结果表明:随动应力幅值的增大,动应变呈非线性增长,且存在某一临界动应力值;随动应变的增大,动模量逐渐减小,且减小速率先快后慢、最后趋于稳定;随动剪应变增长,阻尼比呈非线性增长,且阻尼比最大值不超过0.12;随围压与固结比增大,动强度和动模量均明显增大;围压与固结比对阻尼比影响较小,具有归一性特征;随频率增大,动应力幅值、动弹性模量增大,但增长幅度并不明显。综合分析三变量对雄安新区粉质黏土动力特性的影响程度得出:围压影响最大、固结比次之、加载频率最小。

盾构施工扰动对天津结构性黏土动力特性影响的试验研究

盾构隧道施工扰动会改变周围土体的应力、应变状态,破坏土的结构性,从而影响隧道后期的地震响应.通过共振柱试验和动单剪试验,分析了施工扰动对天津结构性黏土动剪切模量和阻尼比的影响.试验结果表明:施工扰动程度越大,平均应力归一化动剪切模量的初始值越小(应力释放程度较大的D4试样的平均应力归一化动剪切模量初始值为无应力释放的D1试样的74%),在相同剪应变下的归一化动剪切模量越小,动剪切模量的衰减速率越快,在相同剪应变下的阻尼比越大;土的结构性越弱,初始动剪切模量越小(无水泥掺量的B1试样的初始动剪切模量为4%水泥掺量的B4试样的59%),在相同剪应变下的动剪切模量越小,动剪切模量衰减速率越快,在相同剪应变下的阻尼比越大.盾构隧道施工扰动对结构性黏土动力特性的影响机理为:施工扰动会使结构性黏土的结构破损、结构性减弱,从而导致其归一化动剪切模量减小,动剪切模量衰减速率增大,阻尼比增大.因此,在进行盾构隧道抗震设计时应考虑前期施工扰动引起的结构性黏土的结构破损对动剪切模量的影响.