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.     

STATIC AND CYCLIC FRETTING FATIGUE BEHAVIOUR OF SILICON NITRIDE

A cyclic fretting fatigue test machine was constructed. The piezoelectric bimorphs were used as actuators for cyclic loading and fretting motion at the resonance frequency of the system. Fretting fatigue tests under static and cyclic loading conditions were carried out using HIP-sintered silicon nitride. From the experimental results, it was found that fretting fatigue strengths under the two test conditions were identical and hence the effect of cyclic loading on fretting fatigue strength of silicon nitride was almost negligible. A fretting crack initiated in a very early stage of the fatigue life at the position of the maximum frictional stress in the contact area. Fretting fatigue life prediction based on fracture mechanics was also carried out. The predicted lives were in good agreement with the experimental results.     

EFFECT OF BIAXIAL MEAN STRESS ON CYCLIC STRESS-STRAIN RESPONSE AND BEHAVIOUR OF SHORT FATIGUE CRACKS IN A HIGH STRENGTH SPRING STEEL

Abstract— Biaxial fatigue tests were conducted on a high strength spring steel using hour-glass shaped smooth specimens. Four types of loading system were employed, i.e. (a) fully reversed cyclic torsion, (b) uniaxial push—pull, (c) fully reversed torsion with a superimposed axial static tension or compression stress, and (d) uniaxial push—pull with a superimposed static torque, to evaluate the effects of mean stress on the cyclic stress—strain response and short fatigue crack growth behaviour. Experimental results indicate that a biaxial mean stress has no apparent influence on the stress—strain response in torsion, however a superimposed tensile mean stress was detrimental to torsional fatigue strength. Similarly a superimposed static shear stress reduced the push—pull fatigue lifetime. A compressive mean stress was seen to be beneficial to torsion fatigue life. The role of mean stress on fatigue lifetime, under mixed mode loading, was investigated through experimental observations and theoretical analyses of short crack initiation and propagation. Using a plastic replication technique the effects of biaxial mean stress on both Stage I (mode II) and Stage II (mode I) short cracks were evaluated and analysed in detail. A two stage biaxial short fatigue crack growth model incorporating the influence of mean stress was subsequently developed and applied to correlate data of crack growth rate and fatigue life.     

预应力钢绞线网加固混凝土圆柱的轴压性能

开发了一种预应力钢绞线(钢丝绳)网加固混凝土柱新技术, 对24根预应力加固混凝土圆柱和2根未加固对比柱进行了试验, 研究在单调轴向荷载作用下, 钢绞线间距、预应力水平对混凝土柱加固效果的影响。试验结果表明：较对比柱, 预应力加固柱的轴向峰值应力和应变最大提高幅度为83%和95%, 加固效果随预应力水平的提高而提高;随钢绞线间距的减小, 加固效果大幅度提高;钢绞线间距对加固效果的影响大于预应力水平的影响。建立了预应力钢绞线加固混凝土柱峰值应力和应变计算式, 提出了两种应力-应变全曲线关系模型, 计算结果与试验吻合良好, 可为相关研究提供理论支持。预应力钢绞线网加固混凝土柱技术是一种主动、高效、无损加固新技术, 具有推广和应用价值。     

OBSERVATIONS OF TENSION/COMPRESSION ASYMMETRY IN THE CYCLIC DEFORMATION OF ALUMINUM ALLOY 7075

Abstract— The cyclic deformation of AA7075 in the T6 and T651 tempers has been investigated at room temperature under plastic strain control at amplitudes from 0.1% to 1.0%. Although both tempers represented peak aged conditions, their cyclic deformation behavior differed. The T6 material exhibited a slight amount of cyclic hardening to saturation, with softening evident only at the highest amplitudes. At low amplitudes, the peak compression stresses were higher than the tension peaks throughout the test. The extent of this asymmetry in peak stresses diminished at higher amplitudes. In contrast, the peak compression stresses for the T651 temper were initially lower than the tension peaks. The tension peaks remained relatively stable throughout each test, but the compression peaks exhibited hardening such that they became greater in magnitude than the tension peaks. A nonlinear elastic strain contribution is shown to account for asymmetries in hysteresis loop shape, but not the asymmetric peak stresses. We hypothesize that the tension/compression asymmetry results from strain localization caused by the inhomogeneous microstructure created during quenching and aging. In addition, stretching prior to aging gives rise to the unusual behavior of the T651 material.     

FATIGUE OF A PARTICULATE REINFORCED ALUMINIUM METAL MATRIX COMPOSITE SUBJECTED TO AXIAL, TORSIONAL AND COMBINED AXIAL/TORSIONAL LOADING CONDITIONS

Abstract— The goals in this research were to analytically and experimentally investigate the fatigue behavior of a particulate reinforced metal matrix composite subjected to axial, torsional and combined axial/torsional loadings. A series of fully-reversed uniaxial, torsional and combined axial/torsional fatigue tests were performed on a 6061/Al₂O₃/20p-T6 metal matrix composite material. This research investigated the ability of the Fatemi-Kurath and the Smith-Watson-Topper (SWT) damage parameters to correlate the experimentally obtained fatigue life data and also to represent the fatigue life using uniaxial strain-life constants. The Fatemi-Kurath damage parameter correlated the experimental fatigue data from all loading cases better than the SWT damage parameter. Using uniaxial strain-life constants, both damage parameters predicted fairly reasonable fatigue life calculations for the intermediate fatigue lives (10³ to 10⁴ cycles to failure), while producing non-conservative results for the shorter fatigue lives (< 10³ cycles to failure).     

THE APPLICABILITY OF NEURAL NETWORKS IN MODELLING THE GROWTH OF SHORT FATIGUE CRACKS

This paper examines the use of a neural network to model the chaotic behaviour of the growth of short fatigue cracks which are characterized by a decreasing crack growth rate with increasing crack length. Fatigue crack growth is modelled in terms of the Hobson short fatigue crack growth law. The neural network is exclusively trained and tested on Hobson's experimental data of short fatigue cracks propagating in a 0.4% carbon steel. The empirical constants d, α and C of Hobson's growth law are determined from the neural network predictions and are found to be within the following approximate ranges 63 < d < 400 (μm), −0.27 < α < 0.08 and 1 × 10⁻⁴ < C < 509 × 10⁻⁴ with no proportional relationship observed between the constant C and the applied cyclic stress. It is shown that neural networks are a viable computational tool for modelling the chaotic behaviour of short fatigue crack growth.     

MIXED MODE FATIGUE CRACK THRESHOLDS FROM BENDING AND CYCLIC TORSION OF A PLATE

The effects of plate thickness and crack shape on the threshold condition for crack propagation under cyclic bending or torsion, was studied using a through or a surface pre-crack in a carbon steel plate of various thicknesses. It was concluded that the magnitude of the frictional stress acting on the crack surface at the propagation threshold under cyclic torsion was significantly affected by the plate thickness. An estimation method for the threshold condition for crack propagation under mixed-mode loading for a particular plate thickness was proposed on the basis of the strain energy density criterion.     

CYCLIC FATIGUE CRACK GROWTH AND CLOSURE BEHAVIOUR IN ALUMINA CERAMICS

Cyclic fatigue crack growth behaviour in alumina ceramics is investigated and the effect of grain size discussed. Special attention is given to crack closure effects. Cyclic fatigue tests were carried out using four-point bend specimens, and the load–strain and load–differential strain curves were monitored. These curves show hysteretic behaviour probably related to frictional sliding of bridging grains, and also include non-linearities due to crack closure. The crack opening load is determined from the load–differential strain curve by using a method introduced in this study. Growth rates can be successfully described by the relationship da/dN = C[ΔK_{eff /(1 ? Kmax /KIC )]m} which is proposed in this study to account for the effects of crack closure and the maximum stress intensity factor. Irrespective of grain size, growth rates can be well represented by the above relationship, implying that the grain size exerts an influence on growth rates not only because of crack closure behaviour but also the material fracture toughness. The growth rate curve based on the proposed relationship shows a sigmoidal form for ceramic materials, which is similar to metals.     

THE SIGNIFICANCE OF CRACK TIP DEFORMATION FOR SHORT AND LONG FATIGUE CRACKS

Abstract— The behaviour of physical short mode I cracks under constant amplitude cyclic loading was investigated both numerically and experimentally. A dynamic two-dimensional elastic-plastic finite element technique was utilised to simulate cyclic crack tip plastic deformation. Different idealisations were investigated. Both stationary and artificially advanced long and short cracks were analysed. A parameter which characterises the plastically deformed crack tip zone, the strain field generated within that zone and the opening and closure of the crack tip were considered. The growth of physically short mode I cracks under constant amplitude fully reversed fatigue loading was investigated experimentally using conventional cast steel EN-9 specimens. Based on a numerical analysis, a crack tip deformation parameter was devised to correlate fatigue crack propagation rates.     

应变疲劳循环软化机制的观察,确证及定量估测

通过透射电镜观察和循环应变加时效的试验方法，对淬火回火态地低碳合金钢的循环软件机制进行了研究。     

含曲线裂纹柱体扭转问题的新边界元法

研究了带曲线裂纹柱体的扭转断裂问题,推导出了可以直接应用于任意形状截面含有任意形状曲线裂纹的柱体扭转问题的新的边界积分方程,并建立了带裂纹柱体扭转问题的边界元数值计算方法,提出了裂纹尖端的奇异元和线性元插值模型,给出了抗扭刚度和应力强度因子的边界元计算公式。该文对含有圆弧裂纹、曲线裂纹及直线裂纹的不同截面形状柱体的典型问题进行了数值计算,所得结果证明了边界元方法的正确性和有效性。     

钢箱梁斜拉桥索梁锚固区的抗疲劳性能试验研究

广东湛江海湾大桥采用锚拉板式索梁锚固结构,锚拉板式锚固结构存在严重应力集中,特别是在运营活载作用下的抗疲劳性能值得关注。介绍了锚拉板式索梁锚固结构的特点及其在大跨斜拉桥中的应用,对湛江海湾大桥锚拉板式索梁锚固结构的足尺模型疲劳试验进行介绍,结合理论分析和疲劳试验结果,研究循环荷载作用下锚拉板式索梁锚固结构的抗疲劳性能,对锚拉板式索梁锚固结构耐久性做出了评价。     

A MODEL OF FATIGUE CRACK GROWTH AFTER AN OVERLOAD IN D16T Al-ALLOY SUBJECTED TO BIAXIAL CYCLIC LOADS AT VARIOUS R-RATIOS

Abstract— Fatigue crack growth after a biaxial overload has been investigated. The crack retardation parameters, N _D, and, a _D, do not have monotonous dependencies on the biaxial stress ratio, λ, because the shear stress, τ_III, acting in the perpendicular direction of the specimen face, influenced the values of these parameters.
It has been found that the plastic zone size parameters, r _ab, and Δ, do not increase monotonously with increasing λ ratio. The plastic zone size in the crack growth direction, r _ho= a _D13, was calculated on the basis of newly proposed relations.
Crack growth after an overload was simulated on the basis of the equivalent mode I stress intensity factor, IC_C, invoking a unified kinetic diagram and calculated crack increments, a _D13 and a _D=Δ_c, where Δ_c is the maximum value of the calculated size of plastic zone. The experimental data for crack growth after an overload had good agreement with the calculated data.     

渗碳钢扭转变形时断裂行为和渗层厚度对接触疲劳的影响

本文以20CrMnMo 钢为对象比较全面地研究了不同渗碳层厚度、渗碳层碳含量以及不同心部组织对扭转流变硬化行为和断裂特征的影响。结果表明,渗碳件的屈服强度和流变硬化行为主要取决于心部组织。在心部组织相同的条件下,渗碳层的成分、组织和渗层厚度主要影响渗碳件的断裂应变,从而影响其断裂韧度。此外,在材质相同、强度水平相近的条件下,渗碳件的接触疲劳寿命随渗层厚度增加而增大的现象,从力学性能角度看是由于断裂应变随渗层厚度增加而增大,使断裂韧度增大所致,其原因与残余压力大小分布随渗层厚度变化有关。     

钢筋混凝土构件在压、拉弯剪共同作用下的强度包络图研究

试验表明,钢筋混凝土构件在压(拉)弯剪共同作用下,各种受力状态下的强度是相关的。为了反应构件在压弯剪和拉弯剪共同作用下各强度之间的相关性,该文以压(拉)弯相关性为基础,通过引入剪力项,推导出钢筋混凝土构件在压(拉)弯剪复合受力下的强度相关性公式,公式同时能够分别体现各种两两受力状态的强度相关性,并且与试验数据拟合良好,能够较真实地反映构件的受力状态。同时,以强度相关性公式为基础,绘制出了压(拉)弯剪强度包络图,并以该强度相关性公式和包络图为基础制定钢筋混凝土构件在压(拉)弯剪作用下的损伤标准,最终为结构抗倒塌研究中的构件抽离提供依据。     

CONFOCAL SCANNING LASER MICROSCOPY MEASUREMENTS OF THE GROWTH AND MORPHOLOGY OF MICROSTRUCTURALLY SHORT FATIGUE CRACKS IN A1 2024-T351 ALLOY

Abstract— A new technique for studying the growth and morphology of microstructurally short fatigue cracks is described. The technique allows short crack growth, crack depth, and crack front configuration to be measured using Confocal Scanning Laser Microscopy (CSLM). Good agreement is shown between CSLM measurements of the initial stage of crack growth (along an inclined shear plane), crack depth and aspect ratio, together with the measurements obtained using a surface layer removal technique. It is also found that non-propagating short fatigue cracks have approximately the same aspect ratios in different specimen orientations and that the aspect ratio is independent of stress amplitude.     

各种平均应力下高周疲劳极限间的定量关系

疲劳极限是材料工程应用中的一个重要特性参数,但它并不是材料常数。在不同循环载荷作用下,材料会有不同的疲劳极限值。疲劳极限值很大程度上依赖于循环载荷中的平均应力或最大应力幅值,两者之间的关系是材料自身固有的特性关系。因为在接近疲劳极限的低循环应力幅下,S-N曲线受疲劳极限控制,所以在工程应用中对其定量关系有迫切的需求,但现有的经典经验关系只能对其进行非常粗略的估算,不能满足工程应用中有关寿命设计的需要。现状是只能通过大量的实验,得出其实验关系。该文提出了一种函数形式的关系式,该关系式可以足够精确地描述各种材料的疲劳极限和平均应力之间的关系,且只包含三个材料常数。只要这三个材料疲劳特性常数被事先确定,则任意疲劳载荷下的疲劳极限值,都可由该关系式估计得到,因此可以作为材料本身固有的疲劳强度的状态关系式使用。     

FATIGUE CRACK GROWTH BEHAVIOUR OF SM45C STEEL UNDER CYCLIC MODE I WITH SUPERIMPOSED STATIC MODE II LOADINGS

Abstract— The practical applications of studies related to constant amplitude mode I loading are somewhat limited, since mode I crack growth is often influenced by mode II (sliding mode) or mode III (tearing mode) in industrial situations. For these cases, criteria, rules, and laws have to be worked out and verified by experiments. However, it is very difficult to evaluate mixed-mode fatigue cracking due to crack surface interference, crack closure, crack branching, etc. This paper, which defines the length of a branched crack as an effective slant crack with a length equal to the distance between the two crack tips, explains the influences of crack surface interference by introducing concepts of adhesive wear and scrutinizes some related researches on mixed-mode crack growth behaviour. Additionally an effective stress intensity factor range is described which considers crack closure and crack surface interference and is verified with crack growth tests under mode I fatigue loading and cyclic mode I with a superimposed static mode II loading.