FATIGUE UNDER OUT-OF-PHASE BENDING AND TORSION

Test data on fatigue under out-of-phase bending and torsion has been reanalysed. For the materials tested failure has been found to be consistent with the plane of maximum range of maximum range of maximum principal stress, as for the in-phase case. The critical shear planes are assumed to be those associated with this principal stress plane. A multiaxial fatigue damage parameter based on the maximum range of shear stress on the critical shear plane allowing for the effect of the range of normal stress occurring on this plane has been used to develop a proposed set of design curves for in-phase bending and torsion which are conservative for the long life out-of-phase case.     

SEMI-ELLIPTICAL FATIGUE CRACK GROWTH UNDER ROTATING OR REVERSED BENDING COMBINED WITH STEADY TORSION

Abstract— An analysis of the influence of steady torsion loading on fatigue crack growth rates under rotating or reversed bending is presented. Mixed-mode (I + III) tests were carried out on cylindrical specimens in DIN Ck45k steel and results are compared for two different testing machines: rotary bending and reversed bending obtained by cyclic Mode I (Δ K ₁) with or without superimposed static Mode III ( K _III) loading, simulating the real conditions on power rotor shafts where many failures occur. The growth and shape evolution of semi-elliptical surface cracks, starting from a chordal notch on the cylindrical specimen surface, was measured for several Mode III/ Mode I ratios. Results have shown that the steady Mode III loading superimposed on the cyclic mode I leads to a significant reduction in the crack growth rates. It is suggested that this retardation is related to an increase of plastic zone size near the cylindrical surface in association with the interlocking of rough fracture surfaces, friction and fretting debris, leading to a decrease of the ΔK effective at the crack tip profile due to the "crack closure effect". This work provides a contribution to a better understanding of crack growth rates under mixed-mode load conditions thereby allowing one to predict remaining lifetimes and to estimate the risks of pre-cracked rotor shafts.     

低碳钢在拉扭复合动应力作用下的实验研究

本文介绍了我们研制的拉—扭复合动应力材料试验机以及实验方法。在该机上对低碳钢的实验结果表明：在拉—扭复合动应力状态下，等效应变率在３５／ｓ～６５／ｓ范围内，等效应力σ和等效应变ε曲线在塑性变形阶段几乎是平行的，符合Ｍｉｓｅｓ屈服条件，动态初始屈服面几乎是相似的椭圆。验证了过应力模型的正确性，发现在给定等效应变率下，ω～ε间的曲线具有单一性的特征。     

FATIGUE LIFE AND PENETRATION BEHAVIOUR OF A SURFACE-CRACKED PLATE UNDER COMBINED TENSION AND BENDING

Abstract— The present study is to investigate the fatigue crack propagation behaviour of surface cracks subjected to combined tension and bending stress. An estimation of fatigue crack growth behaviour employed the Newman-Raju formula before penetration, and a K solution, proposed by the authors, after penetration. Crack length a_L at penetration increases with an increase in bending stress. The calculated fatigue crack shape was in good agreement with the experimental shape. It was also found that the crack growth behaviour and the crack shape after penetration can be satisfactorily evaluated using the K solution proposed by the authors.     

FATIGUE STRENGTH OF 7075-T6 ALUMINIUM ALLOY UNDER COMBINED AXIAL LOADING AND TORSION

The fatigue strength of 7075-T6 aluminium alloy under combined axial loading and torsion was examined. The S-N relations were correlated with the von Mises criterion for the high cycle region ( N _f≥ 10⁴ cycles) and with the Tresca criterion for the low cycle region ( N _f < 10⁴ cycles), where N _f is the cycles to failure. This transition at N _r= 10⁴ cycles was related to the occurrence of macroscopic plastic straining and a change in fracture modes. The results are discussed in comparison with those for a high strength steel (SNCM8) in a previous study. Particular attention is given to differences in cyclic deformation behaviour, fracture modes and fatigue crack growth rates between the two materials.     

THE EFFECTS OF MEAN STRESS AND STRESS CONCENTRATION ON FATIGUE UNDER COMBINED BENDING AND TWISTING

The Gough test data on fatigue under combined bending and twisting with superimposed mean bending and torsion stresses with and without stress raisers has been re-investigated in terms of the stresses acting on the plane of maximum range of shear stress. It has been shown that the allowable amplitude of shear stress on this plane can be predicted from an equation of the form τ_a= [t - c₁ (K_t×σ_a)^1.5?c₂σ²_m]/K_t where σ_a and σ_m are the normal stress amplitude and mean normal stress respectively on the plane of maximum range of shear stress, c₁ and c₂ are defined material constants and K_t is the theoretical stress concentration factor.     

FATIGUE OF POLYCRYSTALLINE COPPER AT CONSTANT AND VARIABLE PLASTIC STRAIN AMPLITUDES

Fatigue behavior of polycrystalline copper of two different grain sizes was studied using constant and randomly varying plastic strain amplitude tests. The different regimes in the ess-curves of large-grained samples (d? 2.0 mm) were found to be reflected also in the measured fatigue lives in both test modes. The variable amplitude fatigue lives of the small-grained samples (d? 0.03 mm) seemed to follow the Coffin–Manson linear law. The damage accumulation rate in variable amplitude tests was found to be lower than in the corresponding constant amplitude tests, e.g. when expressed as cumulative plastic strains to failure. The fatigue life predictions based on the Palmgren–Miner rule, rain-flow analysis and the constant amplitude life data measured in this work were also conservative.     

CFL增强RC梁抗弯疲劳强度的实验研究

疲劳强度是进行结构抗疲劳设计的重要力学参量。通过对5组24条碳纤维薄板(CFL)增强RC梁进行三点弯曲常幅疲劳试验,得到了增强梁的容许疲劳寿命和极限疲劳寿命S-N曲线及其相应的表达式,并推定其容许疲劳强度和极限疲劳强度分别为其极限承载力的62%和68%。还给出了新的疲劳寿命曲线的表达形式:载荷做功的功率H与增强梁的疲劳寿命N的关系曲线(H-N曲线),并由此推定了增强梁的容许疲劳强度和极限疲劳强度所对应的功率值。     

A NEW CRITERION OF FATIGUE STRENGTH OF A ROUND BAR SUBJECTED TO COMBINED STATIC AND REPEATED BENDING AND TORSION

Abstract Criteria of fatigue strength of a round bar subjected to combined static and repeated bending and torsion have already been published by Findley and Sines together with empirical formulas by Gough. However, it is only Sines'criterion, by which the fatigue limit under combined bending and torsion is calculated, when completely reversed and pulsating fatigue limits and mean stresses are known. The authors show that Sines'criterion does not conform strictly with experimental results and they correct Sines'criterion so that fatigue failure occurs when the octahedral shearing stress amplitude attains a material constant value which decreases not only in proportion to the octahedral normal mean stress, but also in proportion to the octahedral normal stress amplitude. Applying the authors'criterion to combined repeated bending and torsion, plus coexistent static bending and torsion, a design formula was derived. The formula was compared with experimental results of Gough.     

钢纤维混凝土等幅弯曲疲劳加载下的疲劳应变和疲劳模量以及损伤研究

利用已有试验数据,对最大疲劳应变和疲劳残余应变进行拟合,得到疲劳应变演化方程,其相关系数均在0.97以上。考虑到等幅弯曲疲劳加载条件下,疲劳模量与疲劳应变成反比关系,利用对称性,由疲劳应变演化方程得到疲劳模量演化方程,通过拟合试验结果,发现由疲劳模量演化方程表达的拟合曲线与试验曲线吻合很好,其相关系数均在0.99以上。最后,利用疲劳应变与疲劳模量定义损伤变量,得到损伤变量演化曲线,经对比发现,由最大疲劳应变和疲劳残余应变定义的损伤变量演化曲线基本一致,且相差很小;而由疲劳模量定义的损伤变量演化曲线明显大于由疲劳应变定义的损伤变量演化曲线。     

调质42CrMo钢的弯曲微动疲劳实验研究

针对循环软化材料调质42CrMo钢进行了常规弯曲疲劳实验和弯曲微动疲劳实验,分析了常规弯曲疲劳和弯曲微动疲劳之间的差异,并讨论了循环弯曲载荷对疲劳寿命的影响。通过分析不同弯曲载荷下弯曲微动疲劳试样断口的形貌和不同循环次数下微动损伤的情况,揭示调质42CrMo钢弯曲微动疲劳过程中的损伤特性。研究结果表明：同一循环载荷作用下,弯曲微动疲劳的寿命明显低于常规弯曲疲劳的寿命;随着循环弯曲载荷的增大,弯曲微动疲劳的寿命降低更明显;微动引起的局部接触疲劳和局部塑形变形促进了弯曲微动疲劳裂纹的萌生和进一步扩展。     

弯扭组合振动应变导纳的试验研究

探讨了弯扭组合振动分解为弯曲和扭转振动的应变导纳测定方法。应用第1阶固有频率的横向和纵向应变导纳的线性谱幅值测定了杆材的泊松比，并与静态测量结果作了比较。通过应变导纳位相，分析了弯扭杆件材料的扭振阻尼是大于弯曲振动的阻尼。作为本例，弯扭组合时的动应力约是静应力的5倍。     

圆管翼缘钢-混凝土新型组合梁负弯矩区力学性能试验研究

为研究圆管翼缘组合梁负弯矩区的力学性能,进行了5根简支组合梁负弯矩作用下的静力加载试验,分析了试验梁的变形、应变发展规律。依据试验梁承载能力极限状态下的受力特性,采用简化塑性理论推导了圆管翼缘组合梁负弯矩区纯弯、纯剪承载力的计算公式,比较了公式对试验梁极限承载力的计算精度。对比EC4、GB 50917-2013、ASCE及Liang等公式的弯剪相关关系及对试验梁极限承载力的计算结果,提出了圆管翼缘组合梁负弯矩区弯剪相关承载力的适用计算公式。研究结果表明：试验梁在试验过程中表现出良好的稳定性和延性性能,最终破坏时伴随发生了局部剪切屈曲、下翼缘侧向屈曲、梁端腹板压屈及钢梁整体弯扭屈曲等4种典型破坏形态;当混凝土翼板受拉开裂后,不计混凝土抗剪作用的计算弹性剪应变分布较为符合实测剪应变曲线;受高剪力、高弯矩相关效应影响,计算负弯矩区圆管翼缘组合梁承载力时应考虑弯剪相关作用;GB 50917-2013给出的弯剪相关关系对试验梁极限抗弯承载力的计算平均值为96%,是综合计算精度与结果安全性的较适用公式。建议在应用所提纯弯、纯剪承载力公式的基础上,采用GB 50917-2013的弯剪相关公式计算负弯矩区圆管翼缘组合梁的弯剪相关承载力。     

钢管混凝土哑铃型梁受弯试验研究

进行了钢管混凝土哑铃型梁的试验与分析.哑铃型截面包含腹腔内填有混凝土的传统形式和腹腔内不填混凝土的新型截面.试验结果表明,钢管混凝土哑铃型梁具有良好的弹塑性和延性,构件挠度达跨径的1/20时仍能继续承载.腹腔内的混凝土对腹板的受力性能有一定的影响,但对梁的总体受力性能影响不大.     

波形钢腹板PC组合箱梁纯扭作用下抗扭承载力试验研究

为了更好的了解波形钢腹板PC组合箱梁扭转力学机理及其抗扭承载力影响因素,该文通过模型试验的方法,对4根波形钢腹板PC组合箱梁模型进行纯扭作用下的承载力研究。通过偏心加载的方式对模型梁施加单调纯扭矩。试验结果表明：波形钢腹板PC组合箱梁在纯扭作用下,混凝土顶、底板裂缝方向与梁纵轴线基本成45°角;截面中心纵向钢筋对组合箱梁抗扭贡献不大;抗扭刚度与波形参数η成正比;钢腹板屈服扭矩与组合箱梁极限扭矩随着钢腹板厚度的增加而增大。     

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.     

对钢筋混凝土弯扭构件承载力计算公式的修正

GB50010-2002《混凝土结构设计规范》(以下简称“规范”)分别按纯弯和纯扭进行弯扭构件承载力计算。基于国内外大量试验数据的分析表明,该设计计算方法与试验结论相矛盾,未能正确反映构件的受力状态,使得计算值与试验数据拟合较差且偏于不安全,可靠指标偏低,不能满足GB50068-2001《建筑结构可靠度设计统一标准》(以下简称《统一标准》)的要求,配筋需要较大幅度地提高。通过对现有试验数据的分析,给出了基于1/4圆弧模型的建议修正公式和其相应的计算方法,该公式考虑了构件的整体受力相关性,具有明确的物理意义,配筋能返回公式验算。分析结果表明:该文建议的修正公式比规范公式的计算模式准确,可靠指标满足《统一标准》的要求。     

THE CREEP/FATIGUE BEHAVIOUR OF TYPE 316 STEEL UNDER REVERSE BENDING CONDITIONS

Abstract— Reverse bending creep/fatigue tests have been performed on Type 316 steel at temperatures of 550 and 600°C for test durations of up to 12,000 h. It is shown that endurances obtained are comparable to those observed under push-pull conditions and that the introduction of a hold period can significantly reduce the endurance. Detailed fractography indicates that creep/fatigue in tension is more damaging than creep/fatigue in compression. The crack initiation behaviour is shown to be temperature dependent and may result from the combination of tensile and shear stresses, the relative importance of which will depend on the precise conditions of temperature, stress level and possibly microstructure.     

钢-混凝土组合梁复合弯扭作用下非线性分析

根据广义坐标法原理,以拉伸弹簧和滑移弹簧来模拟混凝土翼板和工字钢梁之间交界面的脱离和滑移效应,通过给定其广义坐标,推出了组合梁约束扭转分析的微分方程组。编制了有关主程序,借助于常微分方程(ODEs)求解器COLSYS 可方便地进行求解。最后给出了计算实例, 予以说明。