45钢在应变循环与棘轮变形下的随动硬化演化实验研究

对调质处理的45碳钢进行了不同应变幅值的对称应变和具有平均应变控制下的屈服面半径和背应力演化分析,及具有不同平均应力的低应力、较高应力幅值控制下的屈服面半径和背应力演化分析．研究表明：屈服面半径在给定应变幅值下随循环周次的增加而变小,且随循环塑性应变幅值增大而减小,在单调加载时增大;循环背应力幅值随单调应变而减小,随循环塑性应变幅值增大而增大．     

THE STRAIN CYCLIC CHARACTERISTICS AND RATCHETING OF U71Mn RAIL STEEL UNDER UNIAXIAL LOADING

1.IntroductionTherailoftenenduresacomplicatecyclingload,whichismainlyasymmetricalstresscyding.Undertheasymmetricalstresscycling,acycliccreepwilltakeplaceinthema-terial.Theaccumulatedplasticstrainproducedduringthecyclicdeformationiscalledratcheingstrain.Inrecentyear8,thestudieshaverevealedtheratchetingbehaviorsofsomestructuremetalsI1-3j.Thestraincyclicbehaviorisalsoanimportanbehaviorofmaterialsubjectedtocyclingload.Somestraincyclicbehaviors,suchascyclichardeningandcyclicsofteulnghavesignifica…     

A viscoplastic constitutive model for cyclic hardening in the dynamic strain aging regime

Negative strain rate sensitivity in certain temperature regimes is considered to be a manifestation of dynamic strain aging. Within the framework of unified state variable theory, viscoplastic constitutive equations incorporating the Mises yield criterion are presented to model negative rate sensitivity of the flow stress and its gradual variation with cyclic loading. The modeling capabilities are essentially accomplished by introducing a rate sensitivity parameter into the evolution law of the back stress. The influence of the drag stress and the isotropic stress as well as the rate sensitivity parameter both on the cyclic hardening and the variation of strain rate sensitivity is systemically investigated. Excellent agreement between experimental results reported in the literature and model simulations is obtained, confirming the efficacy of the proposed constitutive model.     

304不锈钢的高温单轴应变循环与棘轮行为

对３０４不锈钢分别进行了高温单轴奕变控制和应力控制下的系统循环试验,揭示和分析了循环应变幅值,应变幅值历史以及温度历史对循环特性的影响以及应力幅值,平均应力及其历史对循环蠕变（棘轮效应）的影响,研究表明,无论单轴应变循环特性还是非对称单侧应力下的棘轮效应不但依赖于当前温度和加载状态,而且依赖于其加载历史,得到３０４不锈钢高温单轴循环行为的一些有意义的结果。     

CRACK GROWTH BEHAVIOR AND FRACTURE FEATURE OF GH2132 UNDER CREEP-FATIGUE CONDITIONS

1.IntroductionSuperalloysareirreplaceableinengines[llandenginesundergostaticandcyclicloading[2].Thereforeitisimportanttostudycrackgrowthbehaviorofsuperalloysundercreepfatigueconditions.Itisnoticedthattheeffectofcyclicloadingoncrackgrowthisconsiderablel3'4],butdifferentatalowstresslevelfromthatatahighoneIS'6].Itisfoundthattheyieldstrengthofmaterialsisanimportantparameterinstudyingfatiguepropertiesl7,8]andcrackgrowthic,lo].Soitisofmuchimportancetocorrelatecrackgrowthbehaviorandfracturefeaturew…     

Residual Stress Relaxation of Shot-Peened Deformation Surface Layer on Duplex Stainless Steel Under Applied Loading

The relaxation of residual stress in shot-peened surface layer of duplex stainless steel S32205 under static and cyclic loading was investigated. The results reveal that the compressive residual stress is relaxed under applied tensile stress. The relaxation of residual stress in longitudinal direction is more obvious than that in transverse direction in both austenite and ferrite. When the applied stress is beyond the yield strength of the materials, the relaxation of the residual stress becomes drastic. Under cyclic loading, the residual stress relaxation occurs fast in the first few cycles, it then becomes stable gradually. A model was used to quantitatively calculate the residual stress under cyclic loading with different applied tensile stresses. The relaxation behavior is determined by the applied loading, the number of cycles, dislocation density, and the residual stress gradient. The relaxation behavior difference under cyclic loading between ferrite and austenite is discussed.     

6061-T6铝合金单轴时间相关循环变形行为

对6061-T6铝合金进行系统的单轴应变循环和应力循环实验，揭示该材料在室温和高温下的循环变形行为，讨论环境温度、加载速率、峰，谷值保持对其应变循环特性及棘轮行为的影响。结果表明，6061-T6铝合金表现出弱的循环软化特性，其棘轮行为不仅依赖于平均应力和应力幅值的大小，还依赖于加载历史。尽管该合金的单拉行为对应变率的变化不敏感，但其循环变形行为却体现出明显的时间相关特性，即；应变循环下，在峰／谷值有保持时的响应应力幅值明显小于没有保持时的值，且随着保持时间的增加，响应应力幅值将进一步减小；应力循环下，在峰值有保持时产生的棘轮应变比没有保持时的值大，且随着峰值保持时间的增加及应力率的降低，棘轮应变明显增大。     

交变载荷作用下平面应力状态焊接接头焊趾处应力应变理论解的推导

基于弹塑性力学理论计算对接接头交变载荷作用下焊趾处应力和应变,以准确预测交变载荷作用下平面应力状态对接接头焊趾处的应力和应变值为目标,考察了不同加载情况下平面应力状态对接接头焊趾处应力和应变的理论计算方法,并给出了交变载荷作用下平面应力状态对接接头焊趾处应力和应变的理论解.结果表明,在材料参数、载荷大小以及焊趾应力集中系数已知的前提下,根据提出的理论解的计算式可获得相应载荷时接头焊趾处的应力应变,为焊接接头在交变载荷下的疲劳失效预测提供理论指导.     

304不锈钢非比例循环棘轮行为的实验研究

在室温下对３０４不锈钢在非比例循环加载下的棘轮行为进行了实验研究。在圆形、菱形和直线形状的非比例双轴循环加载路径下,研究不同轴向平均应力、等效应力幅值大小及基历史下材料的棘轮行为特征,为了比较,还进行了单轴加载下该材料的循环棘轮行为实验,实验表明,３０４不锈钢的室温非比例循环棘轮行为明显地依赖于加载路径的形状;与单轴循环棘轮行为一样,非比例循环棘轮行为也明显地依赖于轴向平均应力和等效应幅值的大小及     

SS304不锈钢在室温单轴循环加载下的时相关棘轮行为

对SS304不锈钢在室温单轴循环加载下的时相关棘轮行为和时相关应变循环特性进行了实验研究．讨论了材料在不同加载速率、不同保持时间和不同加载波形下的循环软／硬化行为和棘轮行为特征．结果表明：即使在室温下,SS304不锈钢也表现出明显的时相关效应;材料的循环变形行为,特别是棘轮行为明显地依赖于加载速率和保持时间以及加载波形的形状．研究得到了一些有助于时相关循环变形行为本构描述的结论．     

紫铜的非比例循环塑性变形行为实验研究

对紫铜在非比例循环加载下的循环行为进行了实验研究,开展了圆形和菱形应变路径的双轴非比例循环加载试验,并进行了４种具有平均应力排比例循环路径的循环棘轮试验,实验研究表明,紫铜的塑性应变率方向和塑性模量的演化强烈地依赖于应变路径形状和等效应等幅值;紫铜的非比例循环棘轮行为也明显依赖于加载路径形状、幅值,平均庆力及其历史。     

板料成形中材料模型的研究进展

介绍了板料成形数值模拟中材料模型的研究进展。将材料模型的理论研究分为屈服准则、强化模型、流动法则、加卸载历史4个方面,并进行简要综述;对材料在循环加载条件下应力应变曲线的实验获取方法进行了探讨,重点介绍了板料压缩、三点弯曲实验确定材料反向加载应力应变曲线的原理和方法;对当前屈服准则、强化模型的研究热点和发展方向进行了分析。     

循环载荷对USSP处理TC4表层残余应力场影响研究

采用超声喷丸技术(USSP)处理TC4试样,再进行四点弯曲疲劳加载。采用X射线应力仪测试疲劳加载前后残余应力分布;利用TEM设备,对疲劳加载前后TC4组织结构进行表征。结果表明,经表面纳米化处理后,TC4合金的最大残余应力已超过自身的屈服强度;当外加载荷高于疲劳极限时,残余应力场明显减弱;当外加载荷接近或小于疲劳极限时,循环周次与载荷的增减不再明显改变残余应力场;位错及位错胞在疲劳过程中发生了组态和数量的变化,位错密度的降低导致了残余压应力的松弛。     

不锈钢振动时效过程的循环蠕变机理

采用疲劳试验机研究了不锈钢焊接试件在循环载荷下应力应变的变化规律。采用拉-压疲劳模拟实际焊接构件的振动时效过程。结果显示动态应变呈循环蠕变状态，蠕变的大小、速度与循环应力幅值有关。循环应力越大，最终蠕变也越大，应变达到稳定所需要的循环周次越多，应用X射线衍射法测量了不同循环载荷作用下，焊趾位置的纵向残余应力的变化，根据试验结果提出了振动时效过程的循环蠕变机理。     

The effects of low frequency load cycles on crack initiation in low alloy steels

The stress corrosion behaviour of low alloy steels was investigated with low frequency cyclic loading in 2 M (NH₄)₂CO₃ at 70°C. The tests were carried out using triangular and sawtooth load cycles with a constant critical strain-rate prevailing within periods of increasing stress irrespective of frequency. Since frequency changes with stress amplitude when using equal ?-values, the effects of frequency were also determined. The experiments were concentrated on crack initiation and initial crack propagation. The results show that the threshold stress which causes stress corrosion cracking (SCC) in ammonium carbonate solution is considerably reduced for all steels tested during low-frequency load cycles. In contrast to tests with constant load, propagating stress corrosion cracks can be initiated below the lower yield strength. The effects of stress/strain amplitude and of the magnitude of deformation on crack propagation were determined.     

Behaviour of mild steel under very low frequency loading in sea water

It is widely believed that the deterioration promoted by sea water is small in high stress, low cycle fatigue. Crack growth is faster than the penetration rate of the corrosive medium. This is probably true when the cyclic frequency is moderate (ca. 0.1 Hz). In ships large changes of the still water bending moment may occur when the loading condition goes from ballast to fully loaded and back. Wave bending is superimposed on the still water stresses. The absolute maxima and minima of the combined bending moments may occur only about once a week. Other very low frequency changes of stresses are connected to temperature changes (day/night) and, for offshore structures, changes of wind and wave directions. Experiments have been carried out with Fe 410 and Fe 510 at 0.05–0.0003 Hz. The crack growth curves for air and sea water remained practically parallel in a log da/dn-log ΔK plot. For simple programmed loading in sea water (one peak among 200 low stress cycles) the difference with results obtained in air was for Fe 410 about 1 : 20 in terms of da/dn; for Fe 510 it was 1 : 10. In air the peaks were very beneficial. In sea water there was no advantage.     

Plastic relaxation of the internal stress induced by twinning

The back stress induced in a twin by the surrounding matrix is analysed using a simple two-dimensional finite element model of an elastic elliptical inclusion in a plastic medium. The influence of the ellipse aspect ratio, the matrix yield stress and the external loading on the internal shear stress are investigated. Comparison is made between isotropic and anisotropic yield criteria. It is seen that von Mises plasticity in the matrix (due to glide) leads to complete relaxation of the twin back stress. The closer the applied stress is to the stress required for macroscopic yielding of the matrix, the greater the relaxation of the back stress. To simulate the role of soft shear directions in the matrix, as seen for the case of easy basal slip in hexagonal close-packed metals, Hill’s anisotropic yield surface was employed. These simulations show relaxation of the back stress that is independent of the level of the applied stress. Comparison between simulations and experiments performed on a magnesium alloy reveals higher levels of relaxation in practice. This is possibly due to (i) further twinning as a relaxation mechanism and (ii) the occurrence of neighbouring grains ideally oriented for relaxation. Analytical expressions are developed to capture the trends seen in the simulation. A correction to bring these into line with experiment is proposed as a tentative means of applying the present findings in an analytical framework.     

On the evolution and modelling of lattice strains during the cyclic loading of TWIP steel

The evolution of lattice strains in fully annealed Fe–24Mn–3Al–2Si–1Ni–0.06C twinning-induced plasticity (TWIP) steel is investigated via in situ neutron diffraction during cyclic (tension–compression) loading between strain limits of ±1%. The pronounced Bauschinger effect observed upon load reversal is accounted for by a combination of the intergranular residual stresses and the intragranular sources of back stress, such as dislocation pile-ups at the intersection of stacking faults. The recently modified elasto-plastic self-consistent (EPSC) model which empirically accounts for both intergranular and intragranular back stresses has been successfully used to simulate the macroscopic stress–strain response and the evolution of the lattice strains. The EPSC model captures the experimentally observed tension–compression asymmetry as it accounts for the directionality of twinning as well as Schmid factor considerations. For the strain limits used in this study, the EPSC model also predicts that the lower flow stress on reverse shear loading reported in earlier Bauschinger-type experiments on TWIP steel is a geometrical or loading path effect.     

热障涂层系统的热梯度机械疲劳应力分析

基于IN738高温合金基体上涂覆的热障涂层系统(Thermal barrier coating system,TBCs),分析热循环和热梯度机械疲劳加载条件下涂层的应力分布及演变。通过有限元分析研究了热生长氧化层(Thermally growth oxidation,TGO)的应力分布,以预测不同载荷作用下TBCs的失效行为。结果可知,在热循环的基础上施加应变载荷会造成TGO应力性质及大小的改变。只施加温度载荷,在加热过程中TGO/粘结层(Bond coat,BC)界面波峰位置会承受轴向较大的拉伸应力,裂纹多会在此处萌生,且以层间开裂的方式失效。而在温度与机械载荷的共同作用下,冷却过程中会承受较大的拉伸应力,显著增大的轴向应力与径向应力共同作用,使垂直于TGO/BC界面的裂纹沿着界面方向扩展,从而造成陶瓷层(Top coat,TC)剥落。进一步对比分析了同相和反相加载时的应力分布,结果表明反相加载时一次循环周期内会产生拉伸平均应力,更易发生TBCs的失效。     

EXPERMENTAL STUDY ON TRANSFORMATION RATCHETING OF SUPERELASTIC NITI ALLOY

超弹性NiTi合金在应力控制循环载荷下会产生峰值应变和谷值应变的循环 累积-相变棘轮行为. 室温实验表明, 峰值应变和谷值应变的演化规 律与加载应力水平和加载波形密切相关; 同时, 在循环加载过程中, 随着 循环周次的增加, 奥氏体名义弹性模量减小, 马氏体名义弹性模量增大, 奥 氏体向马氏体转变的名义开始应力下降, 耗散能降低, 并在一定的循环次数 下趋于一个稳定值. 研究得到了一些有助于后续建立超弹性循环本构模型 的结论.