GH34钢的热—机械疲劳性能和等温低周疲劳性能

本文研究了珠光体耐热钢GH34的热—机械疲劳性能。热—机械疲劳试验在最大应变与最高温度同相位和逆相位两种情况下进行。控制波形为三角波,频率为0.008Hz,温度范围为200(?)550℃。GH34在淬火+高温回火状态下,经受热—机械循环,表现出循环软化的特性。热—机械疲劳时的循环应变硬化指数,大于上限温度的等温循环应变硬化指数。同相位和逆相位热—机械疲劳抗力相差不大,但都稍高于550℃等温循环时的疲劳抗力。用550℃等温低周疲劳寿命代替200(?)550℃的热—机械疲劳寿命是可行的,且结果偏于安全。     

循环温度范围对Ti-6-22-22合金热机械疲劳行为的影响

采用拉压对称的机械应变控制,研究了Ti-6-22-22合金在200～400℃和200～520℃两个温度范围的热机械疲劳(TMF)行为.结果表明,在200～400℃内,同相和反相热机械疲劳寿命均高于400℃等温疲劳寿命;在200～520℃范围,反相热机械疲劳寿命明显低于520℃等温疲劳寿命.在两个温度范围内,热机械疲劳的循环应力都与相应等温疲劳的循环应力响应有关.纵向剖面金相观察表明,520℃时等温疲劳表面的裂纹更长.循环温度范围扩大导致环境破坏作用增强是热机械疲劳具有明显破坏作用的原因.     

热作模具钢在高温热机械应力循环下的疲劳断裂行为

研究了热作模具钢在应力控制下的等温疲劳和同相热机械疲劳寿命，发现在相同的应力幅下，同相热机械疲劳寿命低于上限温度的等温疲劳寿命。通过研究疲劳过程中的循环应变响应和疲劳断口特征时发现，等温疲劳条件下，滞后环朝压缩方向发展，疲劳裂纹主要为穿晶萌生与扩展；在热机械疲劳条件下，滞后环朝拉伸方向发展，疲劳裂纹主要沿晶萌生与扩展。这是导致同相热机械疲劳寿命低于等温疲劳的主要原因。     

纤维增强金属基复合材料层板热/机械疲劳性能试验研究

进行了B／Al层板250～350℃温度循环范围内的同相位、反相位的热／机械疲劳寿命试验以及250℃和350℃下的等温疲劳试验与宏微观分析研究。结果表明：同相位与反相位的热／机械疲劳S—N曲线出现相交，以交点做应力水平线FPF，在FPF以上，同相位的热／机械疲劳(TMF)比反相位的要短；而在FPF以下，同相位的TMF寿命比反相位的要长；无论是同相位，还是反相位的TMF寿命，均低于250℃和350℃下的等温疲劳寿命；疲劳裂纹起源于纤维与基体界面，并随着基体的横向开裂而扩展，但最终的疲劳损伤机理不仅取决于应力水平，还取决于试验环境条件；纤维与基体之间界面反应区在TMF的损伤扩展方面起了主要作用。     

镍基高温合金K417热机械疲劳行为

主要研究了Ｋ４１７高温合金在不同实验条件下的热机械疲劳（ＴＭＦ）行为。在计算机辅助控制伺服液压疲劳实验机上进行了同相和反相热机械疲劳实验。实验结果表明，与等温低周疲劳性能相比较，无论是同相和反相的热机械疲劳均使寿命大大降低，不同相位对ＴＭＦ性能产生不同的影响，此外，对于热机械疲劳过程中的塑性应变分量中的时间相关相（蠕变）和非时间相关相（纯塑性变形）应当分别进行考虑，这样才能对该合金的热机械疲劳行为有更加准确的认识。最后用ＳＥＭ对不同试验条件下的断口进行了详细观察，并对其微观断裂机制进行了分析和探讨。     

竹木复合层合板疲劳与循环蠕变的累积损伤研究

为了对疲劳与循环蠕变交互作用下的损伤进行定量描述,采用一种考虑应变比率和弹性模量2个参数共同描述损伤的变量,并建立了损伤力学模型,该模型能够综合表现疲劳循环过程中塑性应变变化和弹性模量变化规律。通过对竹木复合层合板在80%、75%和70%3种应力水平下的纯疲劳弯曲试验,得出其损伤变量变化规律的实验曲线。通过对力学模型进行分段求解的方法,得到了竹木复合层合板在疲劳与循环蠕变交互作用下的累积损伤的拟合曲线。结果表明,该损伤参量及损伤模型可以较准确的描述在疲劳与循环蠕变交互作用下竹木复合层合板的损伤累积变化规律。     

镍基高温合金GH4169的热机械疲劳行为

在不同温度区间、不同条件下进行GH4169合金的热机械疲劳实验测试其热机械疲劳数据,研究了这种合金的热机械疲劳行为。结果表明：GH4169合金在热机械条件下的迟滞回线具有明显的拉压不对称性;同相位时材料承受压应力,反相位时承受拉应力。拉应力,是影响疲劳寿命的主要因素。应变幅较高时GH4169合金出现平均应力松弛,在高温半周为先循环软化后循环稳定,在低温半周始终趋于循环稳定。     

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...     

考虑平均应变的低周疲劳寿命本征损伤耗散预测方法

金属构件疲劳破坏是工业中常见的破坏形式。为了提高构件疲劳寿命的预测精度，针对低周疲劳载荷下平均应变对疲劳寿命的影响，基于连续介质损伤力学及其不可逆热力学框架，并引入Ramberg-Osgood循环本构模型，以等本征损伤耗散功作为等寿命条件，建立了一种考虑平均应变的低周疲劳寿命预测模型。为对比验证新建模型的有效性和先进性，采用新建模型、修正的Ohji模型、Sandor模型和Wei-Wong模型分别对叠加平均应变的45钢和2124-T851铝合金的低周疲劳寿命进行了预测，并与对应的试验结果进行对比。结果表明：新建模型的预测结果与试验结果吻合较好，其预测效果优于现有模型。基于本征损伤耗散理论的疲劳寿命预测方法为金属材料疲劳寿命的预测提供了新思路。     

应力控制下HTPB推进剂的疲劳损伤及寿命研究

为研究HTPB推进剂应力控制下的疲劳特性,在动态热机械分析仪上开展了不同应力幅值的疲劳试验。结果表明,应力幅值增大会使疲劳寿命缩短,应力幅值越大,相同加载次数下的疲劳峰值应变越大,疲劳谷点应变越小,最终断裂应变值基本一样。以疲劳峰值应变作为损伤因子,基于损伤力学理论建立了不同应力幅值下的疲劳损伤演化模型,克服了原有的Chaboche模型不能解释循环应变松弛的缺点,可以很好地表征材料损伤演化规律。     

A continuum damage model applied to high-temperature fatigue lifetime prediction of a martensitic tool steel

High‐temperature operational conditions of hot work tool steels induce several thermomechanical loads. Depending on the processes, (i.e. forging, die casting or extrusion), stress, strain, strain rate and temperature levels applied on the material are nevertheless very different. Thus, lifetime prediction models need to be able to take into account a broad range of working conditions. In this paper, a non‐isothermal continuum damage model is identified for a widely used hot work tool steel AISI H11 (X38CrMoV5) with a nominal hardness of 47 HRc. This investigation is based on an extensive high‐temperature, low‐cycle fatigue database performed under strain rate controlled conditions with and without dwell times in the temperature range 300–600°C . As analysis of experimental results does not reveal significant time‐dependent damage mechanisms, only a fatigue damage component was activated in the model formulation. After normalization, all fatigue results are defined on a master Woehler curve defined by a nonlinear damage model, which allows the parameter identification. Last, a validation stage of the model is performed from thermomechanical fatigue tests.     

Fatigue design of structures under thermomechanical loadings

This paper presents a global approach to the design of structures that experience thermomechanical fatigue loading, which has been applied successfully in the case of cast‐iron exhaust manifolds. After a presentation of the design context in the automotive industry, the important hypotheses and choices of this approach, based on a thermal 3D computation, an elastoviscoplastic constitutive law and the dissipated energy per cycle as a damage indicator associated with a failure criterion, are first pointed out. Two particular aspects are described in more detail: the viscoplastic constitutive models, which permit a finite element analysis of complex structures and the fatigue criterion based on the dissipated energy per cycle. The FEM results associated with this damage indicator permit the construction of a design curve independent of temperature; an agreement is observed between the predicted durability and the results of isothermal as well as non isothermal tests on specimens and thermomechanical fatigue tests on real components on an engine bench. These results show that thermomechanical fatigue design of complex structures can be performed in an industrial context.     

Thermomechanical fatigue behaviour of nickel base superalloy IN738LC Part 2 – Lifetime prediction

Abstract

An experimental programme was carried out to study the thermomechanical fatigue life of the nickel base superalloy IN738LC used in gas turbines. First, out of phase and in phase thermomechanical fatigue experiments were performed on uncoated and air plasma spray coated materials. In the temperature range investigated, it was observed that deposition of a NiCrAlY coating did not affect the thermomechanical fatigue resistance. A physically based life prediction model that takes into account the contribution of different damage mechanisms was then applied. This model successfully reflected the temperature and strain rate dependences of isothermal cycling fatigue lifetimes, and the strain–temperature history effect on thermomechanical fatigue lifetimes.     

An overview of the damage approach of durability modelling at elevated temperature

Lifetime prediction techniques for components working at elevated temperature are revisited. Two damage approaches in which time effects at high temperature are introduced in different ways are discussed in greater detail. First, a creep–fatigue damage model considers the interaction of the two processes during the whole life before macrocrack initiation; and second, a creep–fatigue–oxidation model separates the fatigue life into two periods: during initiation the environment-assisted processes interact with fatigue, although bulk creep damage only interacts during the micropropagation period. The second model is illustrated by its application to a coated single-crystal superalloy used in aerojet turbine blades. Its capabilities are illustrated in a number of isothermal and thermomechanical fatigue tests. Anisotropy effects are also briefly discussed and a special test, introducing cyclic thermal gradients through the wall thickness of a tubular component, demonstrates the predictive capabilities for actual engine conditions.     

Cyclic stress-strain response during isothermal and thermomechanical fatigue

Isothermal high-temperature low-cycle fatigue and in-phase and out-of-phase thermomechanical fatigue tests were carried out on 316L austenitic stainless steel specimens controlled by computer. A non-linear kinematic hardening model with internal variables was used to simulate the cyclic stress-strain behaviour of isothermal fatigue. This model was modified by considering thermal cyclic effects in order to describe the cyclic stress-strain behaviour of thermomechanical fatigue (TMF) using only isothermal fatigue data and the material performance data. A very good approximation of the hysteresis loops was obtained by comparing with experiments of both in-phase and out-of-phase cases. The thermomechanical fatigue behaviour described by isothermal fatigue data gives the possibility of developing the TMF lifetime prediction technique.     

Modeling of cyclic stress–strain behavior and damage mechanisms under thermomechanical fatigue conditions

A multi-component model was applied to predict the cyclic stress–strain response of different alloys under thermomechanical fatigue conditions based upon isothermal hysteresis loops. A ductile AISI 304 L-type stainless steel and two high strength alloys, the near-α titanium alloy IMI 834 and the nickel-base superalloy IN 100, were chosen as test materials. These represent alloys with rather different dislocation slip modes, stress–strain characteristics and damage mechanisms. Model predictions are compared with experiments and the differences in cyclic stress–strain response and damage mechanisms under isothermal and thermomechanical fatigue conditions, respectively, are discussed based upon microstructural observations.     

Thermomechanical Fatigue Behaviour of a Nickel Base Superalloy

The isothermal low cycle fatigue (LCF)and thermomechanical fatigue (TMF) behaviourof a Ni-base superalloy was investigated. Theresults show that temperature plays an importantrole in both LCF and TMF. The alloy shows thelowest LCF fatigue resistance in the intermediatetemperature range (～760℃). For strain-controlledTMF, in-phase (IP) cycling is more damagingthan out-phase (OP) cycling. The high tempera-ture exposure in the TMF cycling influencesthe deformation behaviour at the low temperature.LCF lives at different temperatures, and IPand OP TMF lives are successfully correlatedby using the hysteresis parameter Δσ·Δε_p.     

Cumulative damage model based on equivalent fatigue under multiaxial thermomechanical random loading

A new creep–fatigue damage cumulative model is proposed under multiaxial thermomechanical random loading, in which the damage at high temperature can be divided into the pure fatigue damage and the equivalent fatigue damage from creep. During the damage accumulation process, the elementary percentage of the equivalent fatigue damage increment is proportional to that of the creep damage increment, and the creep damage is converted to the equivalent fatigue damage. Moreover, combined with a multiaxial cyclic counting method, a life prediction method is developed based on the proposed creep–fatigue damage cumulative model. In the developed life prediction method, the effects of nonproportional hardening on the fatigue and creep damages are considered, and the influence of mean stress on damage is also taken into account. The thermomechanical fatigue experimental data for thin‐walled tubular specimen of superalloy GH4169 under multiaxial constant amplitude and variable amplitude loadings were used to verify the proposed model. The results showed that the proposed method can obtain satisfactory life prediction results.     

LIFE PREDICTION OF CROSS-PLY METAL MATRIX COMPOSITES UNDERGOING THERMOMECHANICAL FATIGUE

A life prediction model that was originally developed for the axial loading of unidirectional metal matrix composites (MMCs) undergoing combined thermal and mechanical loading is extended to the axial loading of cross-ply MMCs by adding an internally initiated matrix fatigue damage term. This new term accounts for the growth of cracks that initiate at the location where fibre–matrix separation occurs in the transversely-oriented plies. A comparison of the model predictions to experimental data on SCS-6/Timetal 21S shows that the model reasonably accounts for the dependence of applied stress, temperature and environment, as well as cyclic frequency. The dominant damage accumulation process for cross-ply MMCs with weak fibre–matrix bonds is described by this internally initiated matrix fatigue damage process for most stress–temperature cycle combinations. However, the fibre-dominated damage accumulation process operates under in-phase TMF when both stress and temperature are high. Environment-enhanced matrix fatigue is the dominant damage accumulation process under isothermal fatigue when stress is low and temperature is high.     

Bithermal low-cycle fatigue evaluation of automotive exhaust system alloy SS409

This investigation provides thermomechanical fatigue data for the ferritic stainless steel alloy SS409, used extensively in automotive exhaust system components. The data were generated to assess the total strain version of the strain range partitioning (TS-SRP) method for the design and durability assessment of automotive exhaust systems. The cyclic lifetime and the cyclic stress–strain–temperature–time behaviour for alloy SS409 were measured using bithermal tests with extreme temperatures of 400 and 800 °C. Fatigue lives ranged up to 10 000 cycles with hold-times of 0.33–2.0 min. The bithermal fatigue behaviour was compared to isothermal, strain-controlled fatigue behaviour at both 400 and 800 °C. Thermomechanical cycling was found to have a profound detrimental influence on the fatigue resistance of SS409 compared to isothermal cycling. Supplementary bithermal tests with hold-times ranging from 40 s to 1.5 h were conducted to calibrate the TS-SRP equation for extrapolation to longer lifetimes. The observed thermomechanical (bithermal) fatigue lives correlated well with estimated lives using the TS-SRP equations: 70% of the bithermal fatigue data fall within a factor of 1.2 of calculated life; 85% within a factor of 1.4; and 100% within a factor of 1.8.