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．     

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。     

FRP约束混凝土快速荷载下的强度计算

通过42个不同包裹条件下的FRP约束混凝土圆柱体试件静载和在不同应变速率下的试验,研究FRP约束混凝土快速荷载下的强度．结果表明,FRP约束混凝土在快速荷载下的强度随着应变速率的增加而提高,与对数应变速率呈线性关系,其斜率α与约束比ξ有关,α 随ξ的增加而增大．拟合了FRP约束混凝土快速荷载下的强度计算公式,计算结果与试验值吻合较好,为FRP约束混凝土合理应用于混凝土结构抗震加固提供了参考依据．     

混凝土动态拉伸试验及其数据分析方法研究

在MTS电液饲服万能试验机上对64个哑铃形试件在应变速率10-5~10-0.3/s内进行单轴直接拉伸试验,研究了混凝土强度、含水量、应变速率对混凝土直接拉伸特性的影响.结果表明:随着应变速率的提高,混凝土动态拉伸强度明显增加;含水量较高的混凝土其拉伸强度提高更显著.针对混凝土动态特性影响因素众多的特点,提出利用人工神经网络方法来综合反映多种因素对混凝土动态特性的影响作用.与传统动态试验数据分析方法比较表明:利用人工神经网络来分析研究多种因素影响的混凝土动态特性,是一种简便、适用的新方法.     

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.     

基于模糊理论的混凝土疲劳强度计算模型

分析了混凝土常规疲劳试验结果具有很大离散性的原因；分析得到疲劳寿命概率分析方法和初始强度概率分析方法计算疲劳强度标准值具有一致性；从这一结论出发利用模糊数学和统计理论对常规混凝土疲劳试验结果进行模糊可靠性分析，材料强度采用正态分布，模糊隶属函数采用工程上常用的降半梯形分布，进而得到计算疲劳强度标准值的方法。     

Residual fatigue strength of 48MnV crankshaft based on safety factor

Residual fatigue strength of 48MnV crankshaft was studied and analyzed based on safety factor. Three different status crankshafts were used to the hop-up tests, which maintain new after 500 h hop-up tests and after 1 000 h hop-up tests. Then, crankshafts were cut into unit cranks. The unit cranks were used to do endurance bending tests to get the residual fatigue strength. Finally, the results were analyzed based on safety factor. The results show that safety factor of crankshaft descends a little with the increase of the running time, and the residual safety factor is still much bigger than the endurable safety factor. Furthermore, after the crankshaft accomplishes a full life cycle, the residual fatigue strength of the crankshaft is enough to remanufacture and fulfill the next life cycle.     

Residual fatigue strength of 48MnV crankshaft based on safety factor

Residual fatigue strength of 48MnV crankshaft was studied and analyzed based on safety factor. Three different status crankshafts were used to the hop-up tests, which maintain new after 500 h hop-up tests and after 1 000 h hop-up tests. Then, crankshafts were cut into unit cranks. The unit cranks were used to do endurance bending tests to get the residual fatigue strength. Finally, the results were analyzed based on safety factor. The results show that safety factor of crankshaft descends a little with the increase of the running time, and the residual safety factor is still much bigger than the endurable safety factor. Furthermore, after the crankshaft accomplishes a full life cycle, the residual fatigue strength of the crankshaft is enough to remanufacture and fulfill the next life cycle. Foundation item: Key Project (50235030) supported by the National Natural Science Foundation of China     

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.     

疲劳荷载作用下混凝土的边界面模型研究

边界面模型是少数可以描述混凝土在复杂的多轴循环荷载作用下性能的本构模型之一,它具有相对简单、应用方便的优点．首先总结了混凝土边界面模型研究的历史沿革,分析了它的构建方法;为了研究以损伤参数控制收缩的一类边界面模型对混凝土的疲劳寿命预测、非线性力学性能描述的能力以及对疲劳加载参数的敏感性,编制了计算程序,将计算结果与试验值进行了比较;分析了这类边界面模型的主要缺陷,提出了进一步完善的思路．     

单轴反向加载对管线钢强度性能的影响规律研究

本文根据钢板在大直径管线钢管UOE成形工艺过程的变形情况,针对X70和X80管线钢材料进行了单轴反向加载试验,通过单向拉伸试验测试了在经历不同程度预压缩应变之后其屈服强度和抗拉强度的变化情况,并对其强度性能指标随预压缩应变的变化规律进行了分析与归纳。结果表明,在单轴反向加载条件下,两种材料的屈服强度均因包辛格效应而降低,其下降趋势均可拟合为二次函数;预压缩变形对试验材料的抗拉强度基本没有影响。     

Improved parameter selection method for mesoscopic numerical simulation test of direct tensile failure of rock and concrete

In order to numerically simulate the failure process of rock and concrete under uniaxial tension, an improved method of selecting the mechanical properties of materials was presented for the random mechanic parameter model based on the mesoscopic damage mechanics. The product of strength and elastic modulus of mesoscale representative volume element was considered to be one of the mechanical property parameters of materials and assumed to conform to specified probability distributions to reflect the heterogeneity of mechanical property in materials. With the improved property parameter selection method, a numerical program was developed and the simulation of the failure process of the rock and concrete specimens under static tensile loading condition was carried out. The failure process and complete stress-strain curves of a class of rock and concrete in stable fracture propagation manner under uniaxial tension were obtained. The simulated macroscopic mechanical behavior was compared with the available laboratory experimental observation, and a reasonable agreement was obtained. Verification shows that the improved parameter selection method is suitable for mesoscopic numerical simulation in the failure process of rock and concrete.     

溶蚀混凝土力学性能退化的试验研究

通过实验室试验研究了不同配合比混凝土力学性能随溶蚀时间的演化规律.试验设计变量主要为水灰比和骨料体积率,采用硝酸铵溶液加速混凝土溶蚀过程.试验结果表明：酚酞指示剂法能够较准确地表征混凝土的溶蚀深度,溶蚀深度近似与溶蚀时间的平方根成线性关系.混凝土弹性模量、抗压强度和劈拉强度均随着溶蚀时间的增长而减小,水灰比和骨料体积率对它们的减小速率有较大影响.与弹性模量相比,溶蚀对抗压强度和劈拉强度影响更大.当溶蚀损伤变量小于0.6时,溶蚀混凝土抗压强度模型与试验结果良好吻合.     

双向等比例加载下的混凝土动态抗压性能

为了研究混凝土试件在多轴受力、不同应力途径下的动态力学性能,采用大型静动态电液伺服试验系统,对混凝土试件进行双轴应力状态下等比例加载的动态抗压试验.双轴加载比例为1∶0、1∶05、1∶1,应变速率分别为10^-5/s、10^-4/s、10^-3/s、10^-2/s 4个量级.通过试验,研究了在不同应变速率、不同应力比情况下,混凝土抗压强度、弹性模量、峰值应变、应力应变曲线的性能.试验结果表明,随着应变速率的提高,各种应力比下混凝土的抗压强度、弹性模量均有提高.在相同应变速率下,与单轴动态受压相比,1∶0.5的应力比其抗压强度提高幅度大.     

高温后粗砂岩单轴抗拉强度实验研究

对不同温度（25℃,100℃,200℃,300℃,400℃,500℃,600℃,700℃,800℃,900℃等10个等级）作用后的砂岩试样进行了巴西劈裂实验,分析了粗砂岩单轴抗拉强度经历了不同温度作用后的变化趋势.研究结果表明：在25-150℃时,单轴抗拉强度变化很小;150-500℃时,随着温度升高,单轴抗拉强度单调增加;500-750℃时,单轴抗拉强度单调减小;750-900℃时,抗拉强度基本保持一个定值.总体来讲,粗砂岩的单轴抗拉强度随着温度的增加先增大,后减小.     

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.     

钢纤维混凝土动载疲劳特性的试验研究

运用TAW-2000型微机伺服多功能材料试验机,对不同强度混凝土掺入不同含量的钢纤维进行疲劳破坏试验研究。结果表明:在钢纤维体积含量一定范围内,随钢纤维体积含量增加,钢纤维混凝土的疲劳极限抗压强度增长十分明显;随钢纤维体积含量提高,混凝土的抗疲劳能力显著增强;随时间增长,应力曲线的上升段更加陡峭,下降段更缓慢,即钢纤维混凝土的疲劳强度、韧度均得到显著增长。     

Property of corroded concrete under compressive uniaxial loads

In order to study the compressive property of corroded concrete, accelerated corrosion test were performed on concrete C30.6 corrosive solutions, including hydraulic acid solution （pH=2）, hydraulic acid solution （pH=3） were applied as the corrosive medium. 6 series of corrosion tests, including 111 specimens, were carried out. Mechanical properties of all the corroded specimens were tested respectively. Compressive properties of the corroded specimens （e.g. compressive strength, stress-strain relation, elastic modulus etc.） were achieved. Taking the strength degradation ratio and strain energy loss as damage index, effects of the corrosion solution on the compressive property of corroded concrete were discussed in detail. Relationship between the damage index and corrosion state of specimens were achieved.     

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.