基于残余应变的多侧压下混凝土受拉疲劳损伤研究

在侧压分别为0.25fc和0.50fc及其组合条件下,进行了混凝土在多侧压下的受拉等幅与变幅疲劳试验,得到了疲劳累积极限应变和残余极限应变,建立了残余应变与疲劳次数关系方程,并给出了残余应变与循环次数关系曲线。根据残余应变的稳定性,也即不受加载历程的影响,主要与侧应力水平和相对疲劳次数有关,以残余应变与相应应力水平下的残余极限应变之比作为损伤变量,建立了损伤变量与疲劳次数关系的统一方程,并给出了损伤变量与疲劳次数关系曲线。同时,对单轴、单、双侧压下的混凝土受拉疲劳的累积极限应变和残余极限应变与相应应力条件下的静态峰值应变进行了比较,其结果是随着侧应力水平的增加,相应的极限应变增加,再次说明了侧应力水平的影响。静态峰值应变与相应条件下的疲劳累积极限应变接近,残余极限应变与静态峰值应变及疲劳累积极限应变的比值较为稳定,其变化区间在0.22~0.28之间,且两种比值之间相差0.0~0.2。最后,对Miner准则进行了改进,并根据所建立的损伤模型与改进的Miner准则,对变幅疲劳寿命进行了预测,其结果是较为合理的。     

混凝土三轴变幅拉-压疲劳性能试验研究

进行了最小应力水平为0.20fc,最大应力水平为0.20ft~0.55ft,定侧压为0.30fc的变截面混凝土试件三轴拉-压等幅和变幅疲劳试验,分析了混凝土三轴拉-压疲劳最大和最小纵向总应变的三阶段演变规律和级间相似性,给出了疲劳损伤演化规律。进一步验证了Miner线性损伤累积理论的不适用性,提出了非线性损伤累积模型,并进行了疲劳剩余寿命预测,通过与试验结果的比较表明该模型具有较高的精度和适用性。     

混凝土拉-压疲劳损伤模型及其验证

基于连续损伤力学理论,提出了混凝土单轴拉-压疲劳损伤模型。模型中采用了拉和压两个边界面。加载面、边界面方程均以损伤能量释放率表示。在能量释放率空间内,由加载面与初始损伤面、边界面之间的位置描述损伤状态。通过建立累积损伤与相应循环损伤能量释放率阈值之间的关系,确定了疲劳加载中极限断裂面尺寸的变化规律,由此模拟混凝土在循环荷载作用下的刚度退化过程。结合作者完成的疲劳试验结果,确定了理论模型中的计算参数。经比较,理论模型预测的应力-应变数值、疲劳寿命和试验结果吻合较好。     

高强高性能混凝土三轴拉压压力学性能试验研究

采用大型动静真三轴伺服液压试验系统,对单轴压强度为90.6 MPa的高强高性能混凝土进行三轴拉压压等比例试验研究,获得了各应力比下试件的破坏模式、多轴强度及应力-应变曲线。试验结果表明:高强高性能混凝土三轴拉压压应力状态下的破坏为典型的受拉破坏;最大主应力方向的极限强度远低于其单轴压强度,中间主应力效应不明显;拉应力的存在对最大主应力方向应力-应变曲线影响十分明显,呈现出明显的线性特征。基于试验结果提出了适用于高强高性能混凝土的强度准则,为高强高性能混凝土本构关系的建立提供了试验和理论依据。     

混凝土受拉疲劳试件的有限元分析

以国家自然科学基金资助项目《混凝土多轴疲劳破坏准则》(50078010)的试验研究为背景,运用有限元分析软件ANSYS对混凝土多轴疲劳试验中采用的试件进行单轴静态受拉分析,指出了受拉疲劳试件在设计及试验中所存在的问题并给出合理建议。     

双侧压下混凝土静态受拉与受拉疲劳性能研究

进行了双向侧压作用下混凝土静态受拉与受拉疲劳试验。对静态受拉试验的极限抗拉强度、轴向拉伸应变和侧向压缩应变进行了比较分析,轴向拉伸应变因侧压的存在而快速增长,侧向压缩应变也因轴拉作用而增大,而极限抗拉强度则因侧压影响而降低;对受拉疲劳试验,则得到了S-N方程,应力应变曲线,应变、疲劳变形模量比与疲劳次数关系曲线,其特征表现为三阶段规律;并通过定义的对数线性变换把上述三阶段曲线转换为直线;反之,利用这种线性关系可以求得三阶段曲线关系。同时对静态受拉与受拉疲劳试验的相关物理量进行了比较分析,疲劳寿命和疲劳变形模量也因侧压的影响而有较大幅度的降低。     

一种基于残余应变的混凝土疲劳损伤模型

介绍了混凝土在承受疲劳荷载时内部微裂纹的扩展规律,根据疲劳损伤变形的3阶段特征,可知混凝土内部微裂纹的扩展是导致其疲劳损伤的本质原因,提出了残余应变可作为建立疲劳损伤模型的基础。研究并介绍了混凝土疲劳损伤破坏时峰值应变εunstab的确定、等幅或变幅荷载作用下残余应变ε残和总应变ε总的计算理论。根据疲劳等效累积原则和混凝土疲劳残余应变的计算理论,推导出混凝土在疲劳荷载作用下基于残余应变的疲劳损伤模型,并提出了建立混凝土疲劳损伤模型需要进一步研究的重点和难点。     

不同温度下混凝土抗拉疲劳性能试验研究

进行了不同温度下混凝土在等幅重复荷载作用下的抗拉疲劳性能试验研究,分析了不同温度下砼抗拉疲劳强度、刚度、变形规律,建立了相应的疲劳方程及考虑温度影响的统一疲劳方程,并将常温下的砼疲劳性能试验结果同其他研究者的结果作了比较。给出了纵向总应变、割线模量的经验公式,及其第二阶段总应变增长率、割线模量衰减率分别与疲劳寿命的关系式。研究结果,为高温环境下受重复循环荷载作用的砼结构设计和分析提供了试验依据。     

高温后高强混凝土受压疲劳性能研究

利用电液伺服疲劳试验机,进行了C60高强混凝土的单轴受压疲劳试验,研究了其经100℃、400℃和700℃高温后表观特征、残余应变、疲劳寿命等的变化规律。试验结果表明:高温后高强混凝土的色泽变浅,部分400℃恒温0.5 h、1 h的试块呈铁锈红色,700℃时试块外表呈灰白色;在单轴受压疲劳荷载作用下,高温后高强混凝土的残余应变符合三阶段发展规律,较普通混凝土有更长的第二阶段。定义相对残余应变为损伤变量,建立了高温历程与受压疲劳损伤的关系模型,为经历重复荷载作用与不同加温历程等综合工况下高强混凝土疲劳试验研究及疲劳损伤评价奠定了基础。     

高强高性能混凝土多轴拉压力学性能

利用大型静动真三轴试验机,进行了高强高性能混凝土在各种应力比下的多轴拉压试验,多轴拉压试件尺寸为100mm×100mm×100mm立方体。试件受压面的减摩垫层为三层塑料薄膜中间夹三层甘油;受拉面先用打摩机去掉表面层,然后用结构胶与加载板粘结。阐述了试件破坏形态特点以及观察了裂缝发展方向,测得了多轴强度与应变,剖析了应力状态和应力比对高强高性能混凝土多轴拉压强度与变形的影响规律性。试验结果表明:多轴拉压强度与变形变化规律取决于应力状态和应力比。多轴拉压强度σ1f、σ3f在所有应力比下分别小于其单轴拉压强度ft、fc;主拉峰值应变ε1p在单轴拉时最小,主压峰值应变ε3p在单轴压时最大。建立了高强高性能混凝土在多轴拉压应力状态下的破坏准则公式。     

Multiaxial fatigue life assessment of welded structures

Welded structures, such as welded pressure vessel components subjected to multiaxial cyclic loading, are particularly susceptible to fatigue damage. In this paper, a new path-length-based effective stress range is proposed to assess the fatigue life of weld joints under multiaxial fatigue loading. The path-length measure, a function of both normal and shear components on a critical crack plane, has a solid root in classic fracture mechanics and its application is validated by correlating nominal fatigue data including pure-bending, pure-torsion, in-phase, and out-of-phase loading. Path-Dependent Maximum Range (PDMR), a unique general-purpose fatigue life assessment package for multiaxial variable-amplitude loading, is introduced in this paper. Finally, the application of PDMR to multiaxial fatigue life assessment of complex loading cases is also discussed.     

Selection of the critical plane orientation in two-parameter multiaxial fatigue failure criterion under combined bending and torsion

The present paper is focused on engineering application of the algorithm of fatigue life calculation under multiaxial fatigue loading. For that reason, simple two-parameter multiaxial fatigue failure criterion is proposed. The criterion is based on the normal and shear stresses on the critical plane. Experimental results obtained under multiaxial proportional, non-proportional cyclic loading and variable-amplitude bending and torsion were used to verify the proposed two-parameter criterion and other well-known multiaxial fatigue criteria. Elastic–plastic behaviour of the bulk material was taken into account in calculation of the stress/strain distribution across the specimen cross-section. It is shown that the proposed two-parameter multiaxial fatigue failure criterion gives the best correlation between the experimental and calculated fatigue lives.     

Online multiaxial fatigue damage evaluation method by real‐time cycle counting and energy‐based critical plane criterion

To realize online multiaxial fatigue damage assessment for the mechanical components in service, an online multiaxial cycle counting method is proposed coupled with the segment processing technique and Wang‐Brow's relative equivalent strain concept. Meanwhile, considering all the stress and strain components, which contribute to the fatigue damage on the critical plane, a multiaxial fatigue damage model without any weight coefficients is also proposed in an equivalent form of shear strain energy. Then, an online fatigue damage evaluation method for multiaxial random loading is developed by combining with the proposed damage model and online cycle counting method. The experimental results showed that the proposed online cycle counting method can be successfully applied to the calculation of multiaxial fatigue damage under random loading. Moreover, the proposed online multiaxial fatigue damage evaluation method can provide satisfactory predictions.     

Multiaxial notch fatigue life prediction based on the dominated loading control mode under variable amplitude loading

A new calculation approach is suggested to the fatigue life evaluation of notched specimens under multiaxial variable amplitude loading. Within this suggested approach, if the computed uniaxial fatigue damage by the pure torsional loading path is larger than that by the axial tension–compression loading path, a shear strain‐based multiaxial fatigue damage parameter is assigned to calculate multiaxial fatigue damage; otherwise, an axial strain‐based multiaxial fatigue damage parameter is assigned to calculate multiaxial fatigue damage. Furthermore, the presented method employs shear strain‐based and axial strain‐based multiaxial fatigue damage parameters in substitution of equivalent strain amplitude to consider the influence of nonproportional additional hardening. The experimental data of GH4169 superalloy and 7050‐T7451 aluminium alloy notched components are used to illustrate the presented multiaxial fatigue lifetime estimation approach for notched components, and the results reveal that estimations are accurate.     

三维机织复合材料在拉压循环载荷下的疲劳性能

为研究三维机织复合材料在拉伸-压缩循环载荷下的疲劳性能,对材料进行了应力比R=-1的疲劳试验。在不同的载荷水平下,分别进行了纬向和经向两类拉压疲劳试验。试验获得了试样在疲劳载荷下的滞回曲线和全过程中剩余刚度比随寿命的变化曲线。结果表明,在拉伸-压缩循环载荷下,三维机织复合材料的疲劳损伤过程主要包含3个阶段,分别发生基体破坏、纱线横向裂纹扩展和纱线的最终断裂。基体的破碎和开胶、垂直于载荷方向排布的纱线撕裂和沿载荷方向排布的纱线断裂是试样内部的主要失效模式。试验还获得了纬向和经向拉压疲劳的拟合S-N曲线,可应用于工程中对该型材料进行疲劳寿命估算。该型材料的疲劳寿命在低应力区和高应力区均显示出较小的分散性,双对数坐标系下的拟合S-N曲线具有较好的线性度。     

Multiaxial fatigue damage parameter and life prediction for medium-carbon steel based on the critical plane approach

The tension–torsion fatigue characteristics were investigated under proportional and non-proportional loading in this paper. The fatigue cracks on the surface of multiaxial fatigue specimens were observed and analyzed by a scan electron microscope. On the basis of the investigation on the Kindil–Brown–Miller and Fatemi–Socie’s critical plane approaches, a shear strain based multiaxial fatigue damage parameter was proposed by von Mises criterion based on combining the maximum shear strain and the normal strain excursion between adjacent turning points of the maximum shear strain on the critical plane. The proposed multiaxial fatigue damage parameter does not include the weight constants. According to the proposed multiaxial fatigue damage parameter, the multiaxial fatigue life prediction model was established with the Coffin–Manson equation, which is used to predict the multiaxial fatigue life of medium-carbon steel. The results showed that the proposed multiaxial fatigue damage parameter could be used under either multiaxial proportional or non-proportional loading.     

Experimental Study on Uniaxial and Multiaxial Strain Cyclic Characteristics and Ratcheting of 316L Stainless Steel

An experimental study was carried out on the strain cyclic characteristics and ratcheting of 316L stainless steel subjected to uniaxial and multiaxial cyclic loading. The strain cyclic characteristics were researched under the strain-controlled uniaxial tension-compression and multiaxial circular paths of loading. The ratcheting tests were conducted for the stress-controlled uniaxial tension-compression and multiaxial circular, rhombic and linear paths of loading with different mean stresses, stress amplitudes and histories. The experiment results show that 316L stainless steel features the cyclic hardening, and its strain cyclic characteristics depend on the strain amplitude and its history apparently. The ratcheting of 316L stainless steel depends greatly on the Values of mean stress, stress amplitude and their histories. In the meantime, the shape of load path and its history also apparently influence the ratcheting.     

Nondestructive Testing and Prediction of Remaining Fatigue Life of Metals

A nondestructive testing method is presented for the prediction of the remaining fatigue life (RFL) of metals with prior fatigue damage subjected to tension-compression fatigue load. It is shown that the slope of temperature rise obtained from a short-time excitation fatigue test is a good candidate to assess the present state of fatigue damage in the material. Three series of uniaxial tension-compression normal fatigue tests are carried out with two different materials under different loading conditions to characterize their fatigue behavior. Eight validation tests are performed under different loading conditions to evaluate the RFL prediction capability of the proposed method. Results show that the proposed method has good potential for predicting RFL of metallic specimens.