混凝土拱坝地震非线性响应中的应变率效应研究

通过引入多轴有效硬化函数和S型率相关强度准则,改进了混凝土损伤塑性(concrete damage plasticity, CDP)模型,称为S-CDP模型,能够更加合理地反映混凝土材料的应变率效应和三维塑性变形行为。基于S-CDP模型建立了大岗山拱坝的率相关数值模型。利用所建的率相关数值模型,分析了地震作用下坝体混凝土的应变率分布规律、动态增长因子的变化规律以及混凝土应变率效应对坝体动力响应的影响。结果表明,S-CDP模型能够合理反映受拉区混凝土材料的率效应,在地面峰值加速度为0.557g,0.663g,0.836g的地震动荷载作用下,拱坝坝体混凝土的动强度增长最多能达到23%,然而并不能保证坝体混凝土的动强度始终提高20%及以上。在水工建筑物抗震设计时,若将混凝土的动强度提高固定的20%,可能会造成计算的损伤结果与实际的损伤有所偏差,动强度提高固定的20%计算得到的损伤结果相较于该文中采用S-CDP模型计算得到的损伤结果偏大。因此在水工建筑物抗震设计时,推荐选用合适的混凝土率相关本构模型进行计算,以尽可能的保证计算的精确度。     

岩石-混凝土界面拉伸断裂性能的率相关性研究EI北大核心CSCD

该文弯曲断裂试验获得了不同应变率下界面的抗拉强度、荷载-加载点位移曲线、荷载-裂缝口张开位移曲线、起裂荷载和峰值荷载,通过夹式引伸计法和DIC法获得了临界裂缝扩展长度。并计算了界面断裂能及双K断裂参数,分析了不同应变率下界面断裂过程区演化规律及特征长度的变化。结果表明:随应变率的增大,断裂能和起裂韧度增大,临界裂缝长度和失稳韧度先增加后减小,断裂过程区长度及特征长度随应变率的提高而减小。该文从裂缝发展路径、自由水粘性、惯性效应三方面探讨了岩石-混凝土界面断裂参数的率效应。     

数值模拟中混凝土类材料应变率效应曲线的惯性效应修正

数值模拟是研究混凝土类材料在动载下响应的有效方法,准确输入材料真实的应变率效应曲线对预测结构在动载下的响应有重要作用。收集了1990年以来针对混凝土类材料的动态抗压实验数据,并将惯性效应对材料动态强度的影响进行剥离,得到真实的材料应变率效应引起的抗压动态强度放大因子（DIF_ε）与应变率对数的关系曲线。分析数据发现：实验得到的抗压动态强度放大因子（DIF_s）和惯性效应引起的抗压动态强度放大因子（DIF_i）都随试件尺寸的增大而增大;随着材料准静态强度增大,混凝土类材料DIF_i随应变率增长的增长幅度减小。对比拟合曲线与其他曲线可知,在高应变率下,新的应变率效应拟合曲线比已有半经验公式能更好地反映实验数据的DIF_ε;模拟时分别输入新的应变率效应曲线和CEB推荐公式,将输出结果与DIF_s对比,验证了新的应变率效应曲线的优越性。     

应变率突增对混凝土动态拉伸破坏影响的细观模拟

实际工程中混凝土结构往往遭遇多次动态荷载作用或在承受一定初始损伤荷载基础上再承受不同应变率的动态荷载.建立了哑铃型混凝土三维细观数值模型,模拟不同名义应变率单独作用下混凝土材料的单轴动态拉伸破坏行为,又分别对混凝土单轴拉伸应力应变曲线上升段和软化段的应变率突增行为开展了细观模拟,初步分析了应变率突增行为对动态拉伸破坏强...     

柔度法测量三点弯曲试样弹性模量的影响因素

根据柔度法测量金属材料裂纹长度的原理,讨论了测量三点弯曲SE(B)试样弹性模量的影响因素,并分析了主要因素的影响。在此基础上,用柔度法测量了裂纹尖端张开位移(CTOD)试验用大尺寸SE(B)试样预制疲劳裂纹的长度。经断口测量发现,用柔度法测量的两组母材与焊缝试样的原始裂纹长度具有很好的同一性,为进一步进行CTOD试验提供了保证。     

温度、应变率对地质聚合物混凝土抗压强度的影响

采用自主研制的高温SHPB试验系统,对高温条件下地质聚合物混凝土的动态抗压强度进行了试验研究。结果表明:200℃时地质聚合物混凝土的动态抗压强度较常温时有所增长,800℃时强度则急剧下降;应变率随弹速近似线性增长;同一弹速水平下,200~600℃时的应变率与常温接近,800℃时较常温提高明显;高温条件下,由侧向约束引起的附加应力可以忽略不计,试验所测得的动态强度增长因子(DIF)的应变率增强效应反应了材料的本质属性;在30~130 s-1应变率范围内,高温下地质聚合物混凝土的DIF与应变率的对数呈线性关系,且温度越高,应变率效应越明显。     

低应变率下混凝土动态拉伸破坏尺寸效应细观模拟

在混凝土静态破坏尺寸效应方面已取得了较完善的成果,而在动态破坏尺寸效应方面,包括其产生机制及对应的尺寸效应律的研究则非常匮乏。为探讨动态荷载作用下混凝土尺寸效应行为,从细观角度出发,结合混凝土细观结构特征,考虑动态加载下细观组分应变率效应的影响,建立了混凝土破坏行为研究的细观力学分析模型与方法。以双边缺口混凝土试件为例,对其在低应变率（10^-5 s^-1~1 s^-1）下混凝土动态拉伸破坏行为及尺寸效应进行细观数值模拟,并分析了应变率效应对动态破坏尺寸效应的影响。最后,结合应变率效应对强度及尺寸效应的影响规律—“强度增强效应”与“尺寸效应削弱效应”,在静态破坏尺寸效应律的基础上,建立了混凝土拉伸强度的“静动态统一”尺寸效应理论公式,并验证了理论公式的准确性和合理性。     

考虑应变率效应的混凝土单轴压缩统计损伤本构模型

基于统计损伤理论,建立考虑应变率效应的混凝土单轴压缩统计损伤本构模型。考虑细观断裂和屈服两类损伤模式,将临界状态作为均匀损伤阶段向局部破坏阶段过渡的转折点,且滞后于峰值应力状态。在动态荷载作用下,混凝土内部细观结构的力学性能发生变化,同时微裂纹的扩展形态、路径和和数量较准静态发生显著改变,进而改变了两类细观损伤模式的演化过程,可由5个特征参数来表征。开展混凝土单轴压缩动态力学性能试验,获得了10-5～10-2/s应变率范围内的应力-应变曲线。利用6组试验数据对模型进行验证,结果表明：模型预测曲线与试验曲线吻合良好,表征细观损伤机制的特征参数随着应变率的提高显示出明显的规律性。该模型可以较好地描述混凝土的动态力学行为,在应变率效应机理、细观损伤机制、宏观非线性本构行为之间建立起有效的联系。     

考虑不均匀界面时混凝土弹性模量预测

提出了考虑不均匀界面时混凝土弹性模量预测的解析法。根据界面层上水泥颗粒的分布特性, 给出了界面层上任一点处的局部水灰比和孔隙率。将不均匀界面层划分成一系列同心球壳单元, 通过反演方法确定了每个球壳单元和水泥石基体的弹性模量。将三相混凝土分解成一系列两相复合子结构, 应用两相复合球模型的正确解导出混凝土弹性模量。通过与文献中的两组实验结果比较验证了本文方法的有效性。数值结果表明, 对于给定的骨料体积分数, 混凝土弹性模量随着最大水泥颗粒直径和水灰比的增大而减小, 但随着最大骨料直径的增大而增大, 骨料级配对混凝土弹性模量也有一定的影响。      

The effect of indentation-induced microcracks on the elastic modulus of hydroxyapatite

The presence of microcracks in materials affects a wide range of mechanical properties including elastic modulus, Poisson’s ratio, fracture strength, and fracture toughness. The microcrack-induced reductions of the Young’s modulus, E, and Poisson’s ratio, υ, are functions of the size, geometry, and number density of microcracks. In this study, an array of Vickers indentation-induced microcracks was placed on the surfaces of two hydroxyapatite (HA) specimens with totals of 391 and 513 indentations per specimen. This study tests the validity of theoretical studies of microcrack damage-induced changes in E and υ, where the changes are expressed either by (i) the volumetric crack number density, N and (ii) the crack damage parameter, ε. All elasticity measurements were done via resonant ultrasound spectroscopy. For both the HA specimens included in the study and alumina specimens indented in an earlier study [J Mater Sci 38:1910. doi: , 1], E and υ decreased approximately linearly with increasing microcrack damage. The slopes of the E and υ versus N and ε are also computed and compared to the available theoretical models.     

The elastic modulus of aluminium-lithium alloys

Young's modulus measurements have been made on Al-Li alloys containing up to 32 at % lithium, in an attempt to determine the cause of the high modulus that characterizes this potentially important alloy system. In alloys of commercial interest (7–11 at %, 2–3 wt % lithium) the modulus is in the range 79 to 83 GPa, the actual value depending on heat-treatment conditions. The major contribution to this increased modulus arises from lithium in solid solution. The Young's moduli of the Al₃ Li and AlLi intermetallic phases are estimated to be 96 GPa and 105 GPa respectively. Additions of magnesium to the Al-Li system produce a small decrease of the modulus, e.g. 4.5 at % (4 wt %) magnesium reduces the modulus by approximately 2 GPa.     

Designing concrete mixtures for strength,elastic modulus and fracture energy

There are many methods for determining a concrete mix proportion when the compressive strength is the design criterion; however, there is much less information available when other criteria, such as the fracture energy or the elastic modulus, are specified. For these cases, a new mix design nomogram has been developed from well-established concrete relationships. The application of this method is demonstrated by an experimental programme which shows the influence of cement content, water-to-cement ratio and aggregate-to-cement ratio on the compressive strength, modulus of elasticity, spliting tensile strength, fracture energy, and characteristic length of concrete. Six concrete mixtures with different water-to-cement ratios and workabilities were studied. The mix design nomogram, besides being a tool for the practitioner, can also help the researcher in selecting the most adequate mix parameters for experimental and scientific purposes. It is noted that when studying the effect of mix parameters on the properties of concrete certain constraints should be used: for instance when varying the water-to-cement ratio, the workability of fresh concrete should be kept constant and vice versa.

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Resume Il existe plusieurs méthodes de détermination du dosage d'un béton quand la résistance à la compression est prise comme critère de calcul; toutefois, on dispose de nettement moins de données quand il s'agit d'autres critères, tels l'énergie de rupture ou le module d'élasticité. Pour ces derniers cas, on a développé un nouveau nomogramme de formulation à partir des relations connues du béton. L'application de cette méthode est démontrée par un programme expérimental qui montre l'influence de la teneur en ciment, des rapports eau/ciment et granulat/ciment sur la résistance à la compression, le module d'élasticité et la résistance à la traction par fendage, l'énergie de rupture et la longueur caractéristique du béton. On a étudié six mélanges de béton présentant des rapports eau/ciment et une ouvrabilité différents. Outre qu'il est un outil pour le praticien, le nomogramme de formulation peut aussi aider le chercheur à sélectionner les paramètres de dosage les mieux appropriés à des buts expérimentaux et scientifiques. On remarque que certaines contraintes devraient être observées quand on étudie l'effect des paramètres de dosage sur les propriétés du béton.

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Effect of nonlinear response of concrete on its elastic modulus and strength

The purpose of this paper is to investigate the effect of nonlinear response of concrete on the relationship between modulus of elasticity at static and dynamic loading as well as on its strength. The obtained relationships are based on the thermofluctuation strength theory coupled with a nonlinear stress–strain material model. From the corresponding equations it was found that the ratio of the static to the dynamic modulus of elasticity depends on the strength of concrete, its temperature, rate of loading. Also it was confirmed that the dynamic modulus is greater than static modulus of elasticity. These equations explain the influence of the value of applied stress on the value of the static or dynamic modulus of elasticity. Comparative study shows substantial agreement with existing experimental results and the general equations given in standard BS 8110, Part 2:1985, and ACI documents. Based on the obtained relationships new methods for evaluating the static modulus of concrete and its strength from the results of dynamic tests are described subsequently.     

Effects of strain and electropulse duration on elastic modulus of TiNi alloy

The elastic modulus of TiNi alloy was tailored by electroplastic rolling deformation and the effects of rolling strain and electropulse duration on the elastic modulus of electroplastic rolled TiNi alloy were systematically investigated. With rolling strain increasing from 0 to 1.70, the elastic modulus decreases from 61 to 30?GPa, which can be attributed to the increase in dislocation density and deformation-induced low modulus B19^′ martensite phase. With electropulse duration increasing from 80 to 120?µs, the elastic modulus first decreases due to the volume fraction increase in low modulus B19^′ martensite phase and then slightly increases on account of the dynamic recovery of dislocation and reverse martensite transformation resulted from electroplastic effect induced by high-energy electropulse.