Experimental and theoretical research on residual strength of plain concrete under compressive fatigue loading

To investigate the residual strength of concrete under fatigue loading, experiments were conducted to determine the functional relation between residual strength and the number of cycles. 80 100mm × 100mm × 100mm specimens of plain concrete were tested under uniaxial compressive fatigue loading. Based on probabili- ty distribution of the residual strength of concrete under fatigue loading, the empirical expressions of the residual strength corresponding to the number of cycles were obtained. There is a good correlation between residual strength and residual secant elastic modulus. Thus the relationship between residual secant elastic modulus and the number of cycles is established. A damage variable based on the longitudinal maximum strain is defined, and a good linearity relationship between residual strength and damage is found out.     

混凝土抗压疲劳剩余强度损伤模型

目的建立混凝土抗压疲劳剩余强度随疲劳次数衰减的模型,揭示混凝土疲劳剩余强度衰减规律．方法基于连续体损伤力学理论,采用应变能量释放率空间的边界面模型,通过极限断裂面的不断移动来模拟疲劳过程中损伤阂值的变化情况．结果建立了累积损伤与相应循环损伤能量释放率之间的函数关系,确定了疲劳加载中极限断裂面尺寸的变化规律,在高周疲劳过程里损伤模量表达式中D是变量,且与剩余强度有关．利用模型编制计算机程序,绘出了不同最大应力水平下混凝土疲劳剩余抗压强度的衰减曲线．结论变量D反映出混凝土疲劳剩余衰减的特性,模型的计算结果能较好反映试验结果的变化规律．     

定侧压混凝土双轴变幅拉-压疲劳累积损伤研究

为了调查混凝土在双轴拉-压交变荷载作用下的疲劳性能和累积损伤规律,利用改造的MTS疲劳试验机对变截面棱柱体混凝土试件进行了定侧压下轴心拉-压单级等幅和多级高-低、低-高变幅疲劳试验．总结了实测等幅与变幅疲劳应变与变形模量的衰减规律,基于疲劳变形模量定义了损伤变量,拟合试验结果得到了两阶段的损伤演化方程,依据损伤演变与损伤状态、加载条件间的相关性,建立了相应的疲劳累积损伤模型,并给出了利用本文模型进行疲劳损伤分析和剩余寿命预测的方法．模型预测的剩余寿命与实际试验结果的对比验证了模型的精度与有效性．     

混凝土单轴动态抗拉特性的试验研究

在应用过程中,混凝土结构肯定要受到动态荷载的作用,而混凝土材料的动态力学特性与静态情况下有非常大的区别.为了考察动态荷载作用下混凝土的抗拉特性,本文应用MTS试验机,对C40混凝土在应变速率为10-5/s～10-2/s范围内进行单轴动态拉伸试验,系统研究了不同应变速率下混凝土的抗拉强度、弹性模量、峰值应变等抗拉力学特性,并分析了应变速率对混凝土抗拉强度、弹性模量等的影响规律.试验结果表明:混凝土抗拉强度、弹性模量会随应变速率的增加而增加,泊松比离散性比较大,规律不明显,动态受拉应力应变曲线与静态相似.这些成果可以为混凝土的结构设计提供技术参数.     

反分析法确定钢纤维水泥砂浆的拉伸残余强度

根据三点弯曲缺口梁的荷载与裂缝张开位移曲线,用反分析法确定了钢纤维水泥砂浆的拉伸残余强度。试验制作了5种不同体积含量的钢纤维水泥砂浆试件。采用虚拟裂缝模型用有限元法数值模拟了三点弯曲缺口梁的荷载与裂缝张开位移,用反分析法确定了钢纤维水泥砂浆的弹性模量和拉伸残余强度。研究结果表明,数值模拟得到的钢纤维水泥砂浆弹性模量以及三点弯曲缺口梁荷载与裂缝张开位移曲线均与试验结果吻合较好,且在常用的钢纤维含量范围内,钢纤维水泥砂浆的残余强度与钢纤维含量基本上成正比。     

添加不同纤维的高性能混凝土力学性能试验

通过试验研究了弹性模量具有明显差异的3种纤维对于混凝土的力学性能改善所起的作用,以及钢纤维、碳纤维和聚丙烯纤维单掺或复掺对于混凝土的抗压强度、劈裂抗拉强度和弹性模量的影响。结果表明：添加0．5％高弹性模量的钢纤维对于混凝土的强度和弹性模量均有提高作用,复掺0．3％钢纤维和0．2％碳纤维的混凝土抗拉强度的提高大于抗压强度;添加0．5％钢纤维的混凝土HPC-2的弹性模量最大,比基准混凝土提高6．5％;添加0．2％聚丙烯纤维的混凝土HPC-3的弹性模量最小,且小于基准混凝土;此外,混凝土抗压强度的影响程度与纤维的弹性模量的关系更为直接,混凝土劈裂抗拉强度的改善与纤维的抗拉强度的关系更为直接,纤维的弹性模量与基体弹性模量的比值,对复合材料的弹性模量有直接的影响。     

Simulation of Fatigue Stiffness Degradation in Prestressed Concrete Beams under Cyclic Loading

In order to investigate and research the fatigue cracking of prestressed concrete fatigue properties and loading and stiffness degeneration process,cyclic loading tests were carried out on six prestressed concrete beams and the stiffness degradation under fatigue was investigated. A simulation model of stiffness degradation is proposed based on the stiffness analysis of the fatigue-damaged section. The elastic modulus of damaged concrete and the effective residual area of steel were introduced as well as an adjusted three-stage concrete fatigue damage evolution model. The strip method was used to analyze concrete damage due to changing stress along the depth of the beam section. The simulation and test results were compared and a method of predicting fatigue deflection was presented based on the simulation model. The predicted results were compared with that of the neural network method. It is in good agreement for the simulation results with the test results. It is only less than 5% error for the simulation model which can reveal the two-stage degradation of prestressed concrete beams under cyclic loading. It is more precise for the simulation prediction method under proper conditions.     

水-温循环作用下千枚岩的动态拉伸特性

为研究水-温耦合作用下0°层理倾角千枚岩的动态拉伸特性变化规律,分别对3组试样进行0、1、3、5、7、8、11次温度循环自然降温、温度循环冷水降温、干湿循环后,采用霍普金森杆试验装置对0°层理倾角千枚岩试样开展动态巴西劈裂试验,从动态拉伸应变曲线、动态峰值抗拉强度、动态弹性模量、能量分析与宏观破坏5个角度研究水、温劣化条件下千枚岩的动态拉伸特性。结果表明：千枚岩应力应变曲线包括极速弹性变形阶段、屈服变形阶段、破坏变形阶段;随着水-温循环次数的增加,千枚岩应力-应变曲线极速弹性变形阶段逐渐缩短,屈服变形阶段的应变增长率不断增大;千枚岩动态峰值抗拉强度呈负指数函数关系变化,耗散能比不断减小;水-温耦合条件下,千枚岩峰值抗拉强度、耗散能比普遍小于温度循环自然降温时;动态冲击下,千枚岩发生贯穿层理的张拉破坏,主要破碎为2块;随着水-温循环次数的增加,千枚岩主碎块发生沿层理面的张拉与穿层理面的剪切复合破坏,千枚岩碎块的平均尺寸不断减小;温度循环冷水降温条件下,千枚岩碎块的平均尺寸更小,且降幅最为显著。     

超高性能轻质混凝土的循环拉伸力学性能

超高性能轻质混凝土(ultra-high performance lightweight concrete,UHPLC)是一种具有高拉压比和拉伸应变强化特性的轻质水泥基材料.为了阐述UHPLC与钢筋在钢筋屈服点之前的协同受拉机制,针对一种密度为1789 kg/m3、抗压强度为63.1 MPa、极限拉伸应变约为2.4×1...     

Deterioration of dynamic mechanical property of concrete with mineral admixtures under fatigue loading

The dynamic mechanical property of concrete is one of the key parameters, which greatly influences durability of infrastructures subjected to continuous heavy loading, such as girder and track slab of high-speed railway foundation structure. This paper reports serials of experiments designed to investigate the deterioration of dynamic mechanical properties of different concretes under fatigue loading condition. Four parameters including relative dynamic elastic modulus (RDEM), relative dynamic shear modulus (RDSM), relative compressive strength (RCS) and water absorption (WA) of concrete were evaluated to assess the dynamic properties and microstructures of concretes. Results show that the fatigue stress levels and fatigue cycle durations significantly influence the dynamic mechanical properties of concrete including dynamic elastic modulus and dynamic shear modulus. Addition of proper mineral admixture can improve the dynamic mechanical characteristics of concrete and increase its resistance against the fatigue loading effect. Keeping the amount of mineral admixture in concrete constant, its dynamic mechanical property with fly ash is lower than that with fly ash and silica fume. The water absorption in concrete, which is an indirect parameter reflecting capillary porosity, increases evidently after bearing fatigue-loading. There is a close correlation between the deterioration of dynamic mechanical property and the increasing of water absorption of concrete. This indicates that the damage of microstructure of concrete subjected to fatigue loading is the indispensable reason for the decay of its dynamic mechanical performance.     

金属材料割线模量峰值与其弹性极限的相关性

为了快速确定金属材料单轴应力状态下的弹性极限或屈服应力,对高碳钢与低碳钢进行了拉伸和压缩试验,绘制了割线模量-应变曲线,分析了割线模量峰值与弹塑性变形的相关性.结果表明,在应力-应变全过程中割线模量存在明显峰值,且达到峰值后呈现衰减特征.峰值所在的位置与其弹性极限具有较为稳定的对应关系,因而通过确定该峰值所在位置可以间接确定弹性极限.割线模量峰值与弹性极限均可能受到加载速度、边界条件或应力状态等因素的影响,还可能存在尺寸或形状效应.     

混凝土三轴受压等幅疲劳力学性能试验研究

为研究混凝土在多轴疲劳应力作用下的性能,进行了0.0fc、0．1fc、0．25fc、0．4fc4个不同初始定侧压级别下的混凝土三轴压静载试验和0．1c、0．25fc两个不同初始定侧压级别下的混凝土三轴压等幅疲劳试验,疲劳试验中最小应力水平均为0．1fc,最大应力水平为1．6fc-2．5fc．根据初始定侧压下混凝土三轴压本构特征,试验中将侧压变化的拐点作为其多轴疲劳破坏的判据．试验分析表明,三轴压疲劳荷载作用下混凝土纵向最大、最小应变表现出同单轴和双轴疲劳一样的三阶段规律,而变形模量则衰减不大．由各侧压级别下的S-N曲线得到混凝土在相应工况下的疲劳强度折减系数,其研究成果可为混凝土结构疲劳设计提供参考．     

钢纤维超高强混凝土的力学性能试验

为了配制出强度高、韧性好、抗冲击性能良好的超高强混凝土,对钢纤维掺入体积率(Vf)为0~3%、基体强度为110 MPa以上的钢纤维超高强混凝土(SFRVHSC)进行立方体抗压、轴向抗压、劈裂抗拉、抗折强度和弹性模量等性能的测试,并对钢纤维超高强混凝土弯曲韧性进行了试验研究.结果表明,SFRVHSC抗压强度随Vf(0~3%)的增加有一定的增长,弹性模量随着材料抗压强度的提高略有增加;钢纤维对SFRVHSC的劈裂抗拉、抗折强度有显著的增强作用.SFRVHSC表现出优异的韧性,弯曲韧性指数I5、I10、I20分别达基体混凝土的4.71~5.15、9.47~11.23、19.02~24.06倍.SFRVHSC梁的荷载-位移曲线与坐标轴包含的面积也明显增加.     

混凝土劈裂抗拉强度与轴心抗拉强度关系研究

国内外学者对混凝土劈拉与轴拉强度的强度关系的研究结论并不一致,为更好地研究两者强度关系,基于黏聚裂纹模型理论,进行了的圆柱体试件尺寸为150 mm×300 mm,垫条宽度变化范围为5~20 mm共8个级别的劈拉试验和尺寸为100 mm×100 mm×550 mm的棱柱体轴拉试验。建立了劈拉强度和直拉强度关系比fts/ft与圆柱体试件直径和特征长度比D/Lch的回归方程,并给出了公式中参数的取值,对大体积混凝土结构的设计及检测有一定的参考价值。     

基于破坏类型的本溪灰岩本构关系研究

根据单轴和三轴条件下本溪灰岩的压缩试验和峰后循环加载试验,总结本溪灰岩的强度、变形随围压的变化规律,研究不同围压下本溪灰岩的破坏过程和重复加载过程,分析不同应力条件下本溪灰岩破坏的方式.结果表明:本溪灰岩在应力刚过峰值且未完全进入残余强度阶段,其弹性模量与峰前相同,此阶段进行循环加载时,新的峰值应力低于卸载点应力;在残余强度阶段,残余强度不再随重复加载发生明显变化;采用比较峰值时的环向弹性应变值与环向应变值的方法来判别本溪灰岩的破坏类型是可行的;不同围压下,本溪灰岩的破坏方式有张性破坏和剪切破坏2种类型,这2种破坏方式下本构模型的控制参数是不同的.分别选取了环向应变和剪切应变作为控制参数建立了本构模型,该模型很好地描述了本溪灰岩峰后阶段的应力脆性跌落现象及应力与应变的关系.     

伸张疲劳对未填充顺丁橡胶/反式-1,4-聚异戊二烯并用硫化胶结构和性能的影响

研究伸张疲劳对顺丁橡胶(BR)及不同配比BR/反式-1,4-聚异戊二烯(TPI)并用硫化胶的拉伸性能、交联结构及形变能密度的影响。结果表明,BR并用TPI以后,硫化胶力学性能明显改善,但交联密度降低;在伸张疲劳初期,随着疲劳次数的增加,BR的拉伸强度、交联密度及形变能密度变化不明显,而BR/TPI并用硫化胶的拉伸强度、交联密度及形变能密度呈现上升趋势,当TPI并用量少于30份时,BR/TPI并用硫化胶在疲劳60万次时,拉伸强度、交联密度和形变能密度均达到最大值。     

早龄期混凝土力学性能试验及其单轴本构模型

为了研究早龄期混凝土的立方体抗压强度、轴心抗压强度、劈裂抗拉强度、静力受压弹性模量以及单轴受压应力应变曲线的规律,进行了系统的加载试验和试验数据分析.总结了早龄期阶段力学指标随龄期的变化规律以及与立方体抗压强度之间的换算关系,通过拟合相关参数,提出了分段式的受压应力应变全曲线拟合公式,拟合结果与试验数据吻合较好.试验结果表明,早龄期C40混凝土的强度和弹性模量随着龄期非线性增长,7 d是增长变化的分界时点;随着龄期的增长,峰值应变减少,脆性变大;早龄期C40混凝土的单轴受压应力应变曲线与充分养护龄期混凝土形状相似.     

Residual strength model for composite laminates

To research the approach of predicting composites fatigue life,the cumulative fatigue damage of fiber-reinforced plastic laminates(FRP) was investigated,and based on the complex exponential function,the residual strength model was obtained. This model can accurately describe the propagation of cumulative fatigue damage of FRP in three stages,especially in the initial stage and the ceasing stage. Applying this model in the experiment with two loading cycles,it can be found that the prediction result has good coincidence with experimental data. So a reliable residual strength model can be provided for studying the cumulative fatigue damage of FRP.     

轻集料取代碎石对混凝土强度和弹性模量影响的研究

利用轻集料与碎石集料混合配制混凝土,可使混凝土的性能得到优化,该类混凝土已显示出较好的应用前景.通过研究轻集料取代碎石对混凝土强度和弹性模量影响规律,确定了轻集料取代率与混凝土密度、弹性模量之间的定量关系,探讨了轻集料取代碎石对混凝土极限应变的影响,提出通过集料混合调控混凝土弹性模量、提高混凝土极限应变和抗拉能力的技术思路,为新型轻质高性能混凝土的研究提供了有益的参考.     

FRC路面配合比设计

从普通水泥混凝土路面的基本要求可知：混凝土路面要有较高的弯拉强度,较低的弹性模量和良好的耐疲劳和抗冲击性能,而纤维混凝土（简称FRC）比普通混凝土具有更优良的动、静力学性能.为了发挥FRC的力学性能,必须有合理的设计配合比.文章以常见的剪切钢纤维和杜拉纤维（Durafibre）为例,设计路面的材料配合比.