汽轮机转子10%Cr钢的高温低周疲劳特性

在分别控制应变和温度的条件下进行超超临界汽轮机转子10%Cr钢的高温低周疲劳实验,用SEM和TEM分析了10%Cr钢实验前后的表面形貌和亚晶粒结构。根据实验数据拟合应力-应变曲线、应力-寿命曲线和应变-寿命曲线,得到了反映材料高温低周疲劳特性的Ramberg-Osgood参数和Manson-Coffin参数。对比分析了低周疲劳实验初始和结束两个阶段的滞回曲线和应力-寿命曲线,并讨论了不同温度和不同应变幅控制的塑性应变。结果表明,在高温工况下10%Cr钢的塑性应变较为明显,随着应变幅的增大钢的疲劳寿命降低,随着循环周期的变化塑性应变速率经历了下降-过渡-上升三个阶段,曲线出现拐点。最大裂纹长度随循环周期呈非线性扩展,高温低周疲劳载荷使钢中的亚晶粒尺寸增大。     

热障涂层对K417G合金高温低周疲劳行为的影响

研究了涂覆热障涂层的K417G合金800℃下由应变控制的高温低周疲劳行为,并对其循环应力-应变数据和应变-疲劳寿命数据进行了分析,给出了涂覆热障涂层K417G合金的高温应变疲劳参数。结果表明:K417G合金无涂层和涂覆热障涂层状态下的低周疲劳均属于应力疲劳,以弹性损伤为主;总应变幅较大时,涂覆热障涂层的K417G合金低周疲劳寿命稍优于无涂层的K417G合金。疲劳试样断口的微观分析表明:热障涂层对K417G合金低周疲劳裂纹萌生方式无明显影响;疲劳裂纹通常萌生于疲劳试样的表面或近表面,表现为穿晶扩展。     

AZ91D室温环境棘轮及其低周疲劳行为

通过AZ91D室温环境应力控制下的低周疲劳试验,对铸造镁合金棘轮及其低周疲劳行为进行了研究,讨论了室温环境下材料的应力循环特性、棘轮行为、塑性应变范围、全应变范围等疲劳参量随载荷水平和加载历史的变化规律,同时基于平均应力修正对材料的应力-寿命曲线进行了讨论。研究结果表明:AZ91D在室温环境下的应力循环呈循环硬化,材料的棘轮行为和塑性应变范围、全应变范围等疲劳参量依赖于载荷水平和加载历史,另外考虑平均应力修正后的应力-寿命曲线预测效果有明显改观。     

考虑循环塑性修正的薄片材料低周疲劳试验方法

对N18锆合金和SAPH440板材采用薄片漏斗试样进行了室温下的等幅低周疲劳试验,基于局部应力、应变相等的疲劳损伤等效原理,建立了用于估算N18和SAPH440材料疲劳寿命的Manson-Coffin模型和循环应力-应变方程。对于符合Masing律的材料,提出了一种获得循环本构关系的新方法,用转子材料B65A-S在300℃高温下的低周疲劳试验进行了验证。通过基于标定循环本构关系的有限元分析,建立了两种材料薄片漏斗试样平均应力到漏斗根部轴向应力、测试应变到漏斗根部轴向应变的转换模型。完成了SAPH440材料等直圆棒试样的低周疲劳试验,证明了采用薄片漏斗试样的低周疲劳测试新方法具有较高的精度。     

温度对GH34钢低周疲劳性能的影响

本文用应变控制的低周疲劳试验方法,研究了室温、350、450和550℃下,燃气轮机叶轮用钢GH34的低周疲劳性能。试验表明,各温度下的循环应力——应变特性均为循环软化。试验温度提高,低周疲劳性能下降。由于蓝脆性的发展,450℃的低周疲劳性能迅速下降,接近550℃的低周疲劳性能。COFFIN—MANSON方程中的指数α随试验温度提高而增大。高温低周疲劳裂纹的扩展,主要为Ⅱ阶段扩展,在所采用的试验条件下,没有发现蠕变引起的治晶疲劳断裂。     

TA5钛合金大厚板低周疲劳特性研究

本文给出了ＴＡ５钛合金的低周疲劳性能试验数据。研究了主冷凝器在循环载荷下，材料循环硬化或软化特性、应力与应变曲线以及应变与寿命的关系，并预示了低周疲劳寿命。此外，对疲劳断口进行了分析对比。     

镍基单晶合金多轴非比例加载低周疲劳单胞模型

在680和850℃下对DD3镍基单晶合金进行多轴非比例加载低周疲劳试验,结果表明等效应变范围△ε_e、试验温度、等效应力范围△σ_e对单晶合金的低周疲劳寿命有显著影响。基于能量耗散理论,引入参量k表征多轴非比例加载对疲劳寿命的影响,构造循环塑性应变能作为损伤参量,建立镍基单晶合金低周疲劳寿命预测模型。参量k与循环寿命之...     

不同环境对2D70铝合金低周疲劳性能的影响研究

循环应力和低周疲劳寿命都是材料疲劳损伤的关键指标.循环应力表征的是材料在循环载荷下抵抗变形的能力.大多数飞机构件由于拐角、圆孔、沟槽等的存在都会存在应力集中现象,当构件承受循环应力载荷时,虽然构件总体处于弹性范围内,但局部已进入弹塑性状态,即已处于循环应变的疲劳过程中.这类服役条件下的构件疲劳寿命一般小于105周次,称为低周疲劳,又由于低周疲劳断裂时应力水平往往较高,低周疲劳也称高应力疲劳或者应变疲劳.衡量材料抗低周疲劳特性的指标主要有低周疲劳极限和低周疲劳寿命,二者统称材料的低周疲劳性能.低周疲劳极限,是指材料所承受应力低于该应力极限值时其理论循环次数可为无限次;低周疲劳寿命,即构件破坏前能承受应力循环的次数.     

车轮应变疲劳性能及损伤演变规律研究

试验分析了Φ９１５喂线车轮的低周疲劳性能，测定了循环稳定和单调拉伸应力－应变曲线，给出了应变－寿命及应力－寿命关系，讨论了循环软化循环硬化特性，导出应变疲劳损伤演变方程和寿命估算方程，还列表给出车轮钢的全部应变疲劳参数和与之对应的静态参数，可供设计选材、寿命估算时使用。     

4.5Ni钢表面裂纹的低周疲劳扩展行为研究

采用悬臂弯曲加载方式，以总应变范围作为受检和控制参数，分析了高强度４．５Ｎｉ钢表面裂纹的低周疲劳扩展行为，给出了裂纹扩展速率ｄ（２ａ）／ｄＮ与总应变范围ΔεＴ的关系式及关系曲线。同时对弯曲加载条件下低周疲劳损伤断口微观形貌进行了观察分析。指出４．５Ｎｉ钢的低周疲劳裂纹扩展方式主要是穿晶，疲劳辉纹为晶体学延性辉纹，疲劳裂纹扩展属于塑性钝化模型机制。     

A continuum damage mechanics model with the strain-based approach to biaxial low cycle fatigue failure

A continuum damage mechanics model for low cycle fatigue failure of initially isotropic materials under biaxial loading conditions is presented. The expression for the equivalent strain in the fatigue damage evolution equation contains the three material parameters, and the strain intensity as well as the maximum principal strain and the volume strain for amplitudes. It is shown how these material parameters can be determined from a series of basic experiments using a cruciform specimen. Particular expressions for the equivalent strain with a smaller number of material parameters and invariants are obtained. Model predictions are found to be in satisfactory agreement with the experimental low cycle fatigue data under full ranged biaxial loadings obtained in the test using a cruciform specimen.     

Ultra‐low cycle torsion fatigue of annealed copper

Torsion experiments show that pure annealed copper is able to withstand very high plastic strain amplitudes when it is loaded cyclically with less than 30 cycles to failure. Under these ultra‐low cycle fatigue conditions, the performance of copper is significantly better than that of the annealed steels A36 and AISI 304, which were also tested in this study for comparison. The dependence of fatigue life on strain range can be described by a power law. In the case of an initial overloading, fatigue life can be estimated using the Palmgren–Miner rule. The long low cycle fatigue life of copper is explained by a thermally activated softening mechanism which takes place while the material heats up as a result of the cyclically repeated plastic deformation. The softening is accompanied by a change in microstructure. The low cycle fatigue properties of copper can be utilized for designing hysteretic dampers for seismic protection.     

EVALUATION OF LOW CYCLE FATIGUE UNDER NON-PROPORTIONAL LOADING

Abstract— A series of low cycle fatigue experiments have been conducted on a 42CrMo steel under tension-torsion loading. Thin-walled tube specimens were used. Low cycle fatigue under various loading paths, including circular and square paths, have been investigated.
The plastic work criterion for low cycle fatigue failure has previously been generally accepted, but it is difficult to calculate stress and strain for complex loading paths, especially for non-proportional loading. This present study suggests a simple method for the calculation of the stable cyclic stress and strain values based on a Modified Endochronic Constitutive Theory (MECT) that redefines an intrinsic time scale. The loading path effect under non-proportional loading is also considered when evaluating fatigue life.
The results show that the plastic work approach using the MECT method in multiaxial fatigue calculations correlates reasonably well the data and is a reflection of loading path dependence.     

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。     

Fatigue strength assessment of load-carrying cruciform joints with material mismatching in low- and high-cycle fatigue regions based on the effective notch concept

This study examined the applicability of the effective notch concept to the low- and high-cycle fatigue regions for load-carrying cruciform joints. Low- and high-cycle fatigue tests were performed on specimens with various strength matching conditions and incomplete penetration ratio. Fatigue test results revealed that the effect of strength matching becomes obvious in low cycle fatigue region, while, it was negligible in high cycle fatigue region. Different failure patterns can be observed for specimen with different strength mismatching condition in the low cycle fatigue tests. Elasto-plastic finite element analyses were performed under the same conditions as the experiments. The effective notch strains were determined at the crack initiation points. A unique relationship between effective notch strain and fatigue life was obtained regardless of matching condition and incomplete penetration ratio.     

A design procedure for assessing low cycle fatigue life under proportional and non-proportional loading

This paper discusses the design assessment for structural components subjected to proportional and non-proportional loading. Multiaxial low cycle fatigue lives are influenced by stress and strain multiaxiality, their non-proportionality and a material property that relates to the degree of additional hardening. Many low cycle fatigue studies under proportional and non-proportional loading were carried out in laboratories, but a little study discussed the application of the results obtained in laboratories to an actual design for structural components. This paper proposes a fatigue life assessment for structural components subjected to proportional and non-proportional low cycle fatigue loading. The assessment provides a simple method for evaluating principal strain range, strain multiaxiality and strain non-proportionality. This paper also discusses low cycle fatigue parameters suitable for the life assessment of structural components subjected to multiaxial loading.     

HIGH TEMPERATURE FATIGUE BEHAVIOUR OF A CAST COBALT-BASE SUPERALLOY

Abstract The initiation and the propagation of fatigue cracks and the low cycle fatigue life of a cast cobalt base superalloy was studied at 293K, 973K and 1173K by optical and scanning electron microscopy. A substantial decrease in fatigue life occurred at 973 and 1173K when compared to room temperature life. A time-dependent bulk damage was evidenced at 1173K which was determined by quantitative microscopy for two plastic strain levels. High strain fatigue crack propagation experiments were carried out at room temperature and at 1173K. From these experiments the decrease of the overall fatigue life at high temperature was shown to result from a considerable reduction of the initiation period due to oxidation and also from a significant acceleration of the crack propagation rate in the presence of oxidation and bulk damage.