蜂窝纸板低周疲劳及蠕变复合作用下的损伤研究

对蜂窝纸板在低周疲劳和蠕变作用下的损伤进行了研究,并采用梁柔量作为损伤参量.研究结果表明:在80%应力水平下,梁柔量在疲劳和蠕变作用下演化规律可分为:快速增加、稳定增加和梁加速断裂3个阶段.随着蠕变保载时间的增加,蜂窝纸板的蠕变寿命分数和疲劳寿命分数均下降很快.蜂窝纸板蠕变寿命分数与蠕变保载时间几乎成线性关系下降,且蠕变寿命分数远远大于疲劳寿命分数.随着蠕变作用时间的增加,疲劳/蠕变损伤的负交互作用加剧.     

蠕变/疲劳共同作用下寿命估算方法

通过载荷谱转换，将带保载时间的蠕变／疲劳循环用不带保载时间的纯疲劳循环代替，提出蠕变／疲劳共同作用时的寿命估算方法。对１２Ｃｒ１ＭｏＶ钢母材和焊材的蠕变／疲劳交互作用试验数据的分析结果表明，本方法方便、实用。提出一个表征蠕变／疲劳连续加载时交互作用行为的参数，蠕变／疲劳寿命比。分析认为，材料的蠕变疲劳交互作用行为与该比值的大小腾。材料在蠕变／疲劳共同作用下呈这是呈负交经作用，并非材料固有的特性，还取决于载荷条件。     

托盘用竹木复合层合板在疲劳/蠕变交互作用下断裂损伤研究

研究了在疲劳/蠕变交互作用下竹木复合层合板的断裂损伤行为.结果表明,在交变载荷最大值为75%的应力水平下,其疲劳/蠕变断裂曲线为3段式曲线;在55%的应力水平下,15小时内其疲劳/蠕变曲线为两段式曲线.随着最大载荷保持时间的增加,层合板的断裂寿命降低.在疲劳/蠕变交互作用下层合板的破坏形式主要为纤维撕裂、界面剪切破坏和层间开裂3种形式的综合表现.影响疲劳/蠕变断裂寿命分散性的因素主要有层合板的铺设结构、木材缺陷和竹材结构等的影响.     

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。     

镍基单晶合金紧凑拉伸试样高温断裂特性的实验研究

采用紧凑拉伸（CT）试样对三种晶体取向[001]，[011]，[111]的镍基单晶合金DD3在950，850℃和760℃下的拉伸以及950℃时的蠕变和疲劳性能进行了实验研究。应用光学显微镜（OM）和扫描电子显微镜（SEM）分别对单晶体裂纹扩展路径和断口表面进行了观察和分析。拉伸实验结果表明：单晶体裂纹扩展路径沿着特定的晶体学矢量方向扩展而在试样自由表明呈现Z字型外观，其扩展方向及形状取决于晶体取向。温度对试样的断裂形式影响较为显著，760℃时的断裂特征为剪切型脆性断裂，断裂平面为沿滑移面的光滑斜断口；950℃下试样断裂逐渐转变为微孔聚集型韧性断裂，断口由粗糙的剪切唇和纤维区构成。蠕变和疲劳实验结果表明：镍基单晶具有明显的蠕变和疲劳性能各向异性，蠕变寿命以[011]，[111]，[001]顺序依次减小，疲劳寿命以[111]，[011]，[001]顺序依次减小；蠕变及疲劳裂纹扩展路径均与加载方向垂直，断裂表面均为光滑的平断口。试样断口细观分析显示，蠕变试样断口由杯锥状韧窝组成，而疲劳试样断口则由疲劳裂纹组成。疲劳损伤比蠕变损伤更有利于裂纹的扩展。     

316L不锈钢的高温疲劳蠕变行为和寿命预测

进行316L不锈钢在单级和两级载荷作用下的高温疲劳蠕变试验,研究了载荷历程效应对材料行为的影响.在已有统一的疲劳蠕变损伤演化模型基础上,得到了316L高温单级载荷作用下非线性损伤演化曲线.同时,建立了一种耦合载荷历程效应的多级疲劳蠕变载荷作用下的材料破坏准则.基于该破坏准则,结合材料的非线性损伤模型对316L不锈钢高温两级载荷作用下的疲劳蠕变寿命进行了预测,预测结果与试验数据符合得比较好.     

Ti_3Al基合金的高温蠕变－疲劳行为

本文测试了两种热处理状态下的Ｔｉ_３Ａｌ（ＮｂＭｏｖ）合金在７００℃时保持最大应力恒定、改变最小应力１Ｈｚ梯形波加载条件下的疲劳断裂寿命。结果表明，随平均应力增高将导致蠕变损伤加重，致使Ｓ－Ｎ曲线反向。在相同应力幅时，于β相区固溶的样品比在两相区固溶的样品具有较高的蠕变－疲劳抗力。最后发现，这些宏观性能的变化与稳态应变速率和微观断裂特征是密切相关的。     

Ti_(3)Al基合金的高温蠕变－疲劳行为EI

本文测试了两种热处理状态下的Ｔｉ_３Ａｌ（ＮｂＭｏｖ）合金在７００℃时保持最大应力恒定、改变最小应力１Ｈｚ梯形波加载条件下的疲劳断裂寿命。结果表明，随平均应力增高将导致蠕变损伤加重，致使Ｓ－Ｎ曲线反向。在相同应力幅时，于β相区固溶的样品比在两相区固溶的样品具有较高的蠕变－疲劳抗力。最后发现，这些宏观性能的变化与稳态应变速率和微观断裂特征是密切相关的。     

变形高温合金缺口试样循环蠕变断裂寿命研究

本文就GH33、GH37和GH49三种变形高温合金在不同试验应力下的光滑试样的静蠕变、循环蠕变和缺口试样的静蠕变、循环蠕变断裂寿命进行了研究。分析了缺口、循环和应力因索对寿命的作用和相对重要性。探讨了缺口循环蠕变断裂寿命增减的实质。研究表明,在缺口循环蠕变条件下,其宏观力学现象比较复杂,既不能简单地把其断裂寿命的增减看作是循环强化或软化,也不能以材料在静蠕变条件下的缺口敏感性指标来评估在循环蠕变条件下无缺口敏感性。     

GH37合金振动叠加低循环载荷的高温复合疲劳行为研究

一、前言 材料在接近使用条件下的疲劳断裂行为研究当前处在发展阶段。本文对GH37合金振动叠加低循环载荷800℃高温复合疲劳行为进行了探索,旨在提出800℃GH37合金复合疲劳寿命曲线,供GH37涡轮叶片寿命预测时参考;探索振动叠加低循环载荷对缺口试样低循环疲劳寿命的影响,以及两者断裂机制的差异。为开展金属材料在振动与低循环载荷复合作用下的疲劳与断裂研究及工程应用打下基础。     

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.     

About the creep-fatigue interaction on the fatigue behaviour of off-axis woven-ply thermoplastic laminates at temperatures higher than Tg

The influence of plasticity and viscous effects on the fatigue behaviour of off-axis C/PPS laminates was investigated at temperatures higher than glass transition temperature. The obtained results clearly show that creep and fatigue are mutually influencing phenomena. Compared to the reference fatigue behaviour (with no prior loading), the fatigue life can be significantly extended with prior creep depending on loading conditions. Indeed, the strain accumulation seems to slow down after a long time creep preload, as if the time-dependent mechanisms were “evacuated” during this preload. The same conclusion can be drawn for the damage accumulation when the prior creep stresses are higher than the damage threshold or when the hold time is long enough, inducing significant plastic deformations. In angle-ply laminates, such deformations are associated with the reorientation of fibres. They contribute to the reduction of stress intensities, which results in increasing both fatigue life and maximum strain ɛ_max at failure during fatigue loadings.     

LIFE PREDICTION OF 316 STAINLESS STEEL UNDER CREEP-FATIGUE LOADING

Life prediction for creep-fatigue loading conditions should be related to creep damage mechanisms. In order to examine the effect of the creep damage mode on rupture life under creep-fatigue loading, a “combined creep-fatigue loading test” was carried out on 316 stainless steel. In this method, creep loading and fatigue loading are repeated alternately. The fracture criteria under combined loading closely depend on the creep fracture modes of the static creep test. A new life prediction method which uses this new fracture criterion is proposed. The criteria are changed when the creep damage mode varies. In order to verify the adequacy of this method, fatigue tests with a tensile strain-hold wave form were carried out. It is clear that rupture life in such fatigue tests is dependent on the chosen fracture criteria.     

Development of a unified creep‐fatigue equation including heat treatment

Background

Creep and fatigue damages in metals are known to interact and then lead to aggregated damage. While models exist for fatigue, creep and creep‐fatigue, no models cover all 3 load regimes. Also, a heat treatment–related parameter is not well included in most creep‐fatigue models.

Need

There is a need to develop a creep‐fatigue equation, which covers the full loading regime from pure fatigue to pure creep, and creep‐fatigue. Also needed is inclusion of a heat treatment–related parameter.

Approach

The unified creep‐fatigue equation was started from the Coffin‐Manson equation and integrated with the Manson‐Haferd parameter. This equation was validated on Inconel 718.

Outcomes

The method of deriving the coefficients and the formula of the creep function are demonstrated, and the resulting equation shows a good ability to describe the grain‐size effect and the fully integrated characteristics.

Originality

Original contributions of this work are the development of a new formulation to represent creep, fatigue and creep‐fatigue in metals. Also the inclusion of grain size—which is a proxy for heat treatment—in the formulation of this equation and in a proposed modified Manson‐Haferd parameter.     

High temperature low cycle fatigue

Fatigue at high temperature is a complex phenomenon as it is influenced by a number of time-dependent processes which become important at elevated temperatures. These processes include creep, oxidation, phase instabilities and dynamic strain ageing (DSA), acting either independently or synergistically influence fatigue behaviour, often lowering the fatigue life. Current design approaches employ linear summation of fatigue and creep damage with suitable factors on permissible damage to take care of uncertainties in interaction between cyclic and time-dependent processes. It is, therefore, important to develop a deeper understanding of the processes that occur during high temperature fatigue so that realistic life predictions could be made. Results on the high temperature fatigue behaviour of austenitic stainless steels, ferritic steels and nickel base alloys are presented here. The important mechanisms of interaction of high temperature time-dependent processes with fatigue under various conditions are discussed in detail. Emphasis is placed on cyclic stress response, fatigue life, deformation substructure and fracture behaviour. This is followed by a review of important life prediction techniques under combined creep-fatigue loading conditions. Life prediction techniques considered here include linear damage summation, strain range partitioning, ductility exhaustion approach, frequency modified and frequency separation methods, techniques based on hysteresis energy and damage rate models, and methods based on crack-cavitation interation models.     

Improvement of life prediction in AISI H11 tool steel by integration of thermo-mechanical fatigue and creep damage models

In hot forging operations, the die surfaces and the nearest surface layers of the die undergo mechanical and thermal cycles which significantly influence their service life. The real thermal and mechanical cycles have been previously investigated in forging plants by measurements and numerical simulation, and a reasonable variation window of process parameters has been determined. A new simulative test applied to AISI H11 hot working die steel has been used to generate failure data in conditions similar to those of the forging dies, but under a more controlled and economical method. Fracture surfaces of specimens for different tests observed by scanning electron microscopy (SEM) indicate that both thermo-mechanical fatigue (TMF) and creep phenomena can be considered to be main damage mechanisms and their contribution to the failure differs as testing conditions vary. As a result of the experiments, the failure is affected by both thermo-mechanical cycle and resting time at high temperature. Therefore, the authors developed a new lifetime prediction model obtained by combining the damage evolution laws, at each cycle, for pure creep and pure TMF. This combination was based on the linear accumulation rule. The damage evolution law for pure creep is obtained by modifying Rabotnov's law in order to suit the case of actual hot forging cycles, where temperature and stress vary widely. The damage evolution law for pure TMF is based on a generalization of the Wöhler–Miner law. This law is modified in order to take into account the presence of thermal cycle and thermal gradient. Comparison between the experimental cycles to failure and the predicted ones was performed using tests excluded in the determination of the coefficients. The conclusion was that the accuracy of prediction appears to be quite good and that the linear accumulation and interaction of TMF and creep is confirmed.     

Fatigue behaviour and lifing of two single crystal superalloys

A model has been developed to predict the high temperature cyclic life of single crystal superalloys RR2000 and CMSX-4 under conditions of creep and fatigue. A combined creep–fatigue model is used, although it is found that failure always occurs by creep or fatigue separately, and that creep–fatigue interaction has a minor influence. Microstructural investigation of a series of interrupted high- and low-frequency tests are presented, these are combined with the results of a series of interrupted creep tests to identify the separate and interactive mechanisms of creep and fatigue. When creep damage is present the material behaves homogeneously. Under these conditions crack growth is initiation controlled, the mechanism of failure is surface or casting pore-initiated planar crack growth followed by shear on crystallographic planes. As the temperature is lowered or the cyclic frequency increased, the material behaves less homogeneously and shear bands are formed during cycling. Crack growth under these conditions is again initiation controlled and failure is by rapid crystallographic crack growth along shear bands. Such a failure is a distinct fatigue failure and occurs when little creep damage is present. Under certain cyclic conditions, mainly those where the crystallographic failure mechanism is dominant, the material shows an anomalous increase in fatigue resistance with temperature up to approximately 950 °C. This behaviour has been quantified by relating it to the effect of strain rate and temperature on the yield strength of the material.     

[±20°]钢丝/橡胶复合材料的疲劳损伤累积

对 钢丝帘线增强的橡胶复合材料在拉伸循环载荷下的疲劳损伤累积进行了研究。结果表明:在载荷控制的疲劳过程中,材料的周期最大应变发展曲线呈现明显的三阶段规律。帘线端头处基体裂纹的出现是宏观疲劳损伤的初始,损伤的累积表现为裂纹数量增加、帘线/基体脱粘和层间裂纹的扩展。以动蠕变为参量建立了线性疲劳损伤累积模型,该模型能够较好地预报两级加载条件下材料的第二级疲劳寿命。     

THERMAL FATIGUE OF MAR-M509 SUPERALLOY—II. EVALUATION OF LIFE PREDICTION MODELS

Abstract— Thermal fatigue data on MAR-M509 reported in a companion paper were used to evaluate four life prediction models. The temperature-stress-strain history of the critical element at the thin edge of wedge specimens was computed for this purpose. The analysis method uses a finite element computation of the temperature field and a uniaxial calculation of the stress-strain cycle using a cyclic viscoplastic constitutive equation. The influence of specimen geometry and of maximum temperature on the thermal fatigue life to initiate a macroscopic crack was accounted for by variations in stress and mechanical strain ranges at the thin edge. The accumulated and cyclic creep damage models were found to overestimate thermal fatigue life in all the cases. Models which describe oxidation fatigue crack growth interactions, and which are borne out by metallographic observations on wedge specimens, were found to give reliable predictions of thermal fatigue life.