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.     

LIFE PREDICTION FOR CREEP-FATIGUE BASED ON THE GROWTH OF SHORT CRACKS

Abstract— For two types of steels (comparable to the 304-type and 347-type of steels) the nucleation and growth of short surface cracks was examined as a function of hold-time, strain rate, and temperature. In addition, for the same range of parameters, damage parameter vs cycles to failure curves were calculated from measured lifetimes. The measured rates, dα/d N , were compared with a model for the growth of creep-fatigue cracks. At large strains, life was found to be dominated by the growth of short cracks. The life data were compared with the life calculated from the crack growth data. The results showed that except for cases in which crack growth is controlled by the coalescence of cavities, the specimen life can be predicted correctly by integrating the growth rate of microcracks.     

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.     

转子裂纹检测与寿命预测技术研究

现代的抗疲劳设计法——损伤容限设计使零件在出现疲劳裂纹后通过严格的检修、剩余寿命计算和断裂控制仍能安全使用 ,使充分利用零件的剩余强度成为可能。对损伤容限设计的关键技术——初始缺陷或服役中出现的疲劳裂纹检测和疲劳剩余寿命预测技术进行了研究。成功地研制了一套性能优良的交流电位表面裂纹监测系统 ,分辨率为 0 .1 mm,测量精度达到± 2 % ,测量范围为 0～ 1 0 0 mm;开发出一套微机平台上的基于断裂力学的 (含表面裂纹 )转子剩余寿命计算程序 ,为分析裂纹扩展形态和概率断裂力学方法估算剩余寿命提供了快速计算途径。有较强的工程实用性和良好的市场开发前景。     

钢筋混凝土梁长期变形的计算

基于受弯等效原则对混凝土徐变和收缩引起的截面曲率增大系数进行了分析,进而得到了钢筋混凝土梁的附加变形增大系数,计算公式形式简单、概念清楚、适用于以受弯荷载为主的钢筋混凝土梁的长期变形计算。与该文中3根试验梁和文献中31根试验梁的试验结果对比分析表明,建议方法计算结果与试验结果吻合较好,且优于文献其它方法。     

PREDICTION OF FATIGUE LIFE BASED ON LEVEL CROSSINGS AND A STATE VARIABLE

Abstract The differences between the cycle count approach and the level crossing approach are discussed from the general fatigue life estimation assumptions as formulated by Holm and de Maré. It is concluded that the differences are related to the interpretation of the assumption of order independence, i.e. the neglecting of sequential effects. A revised level crossing model is proposed where damage accumulation depends on the level crossing and the stress history condensed in a state variable. In order to formulate a mean fatigue life the stationarity and ergodicity conditions on the involved processes are outlined. In this revised model sequential effects can be taken into account and in an example the state variable is chosen as the opening stress of a fatigue loaded crack. The dynamics of the opening stress is described by a simple two parameter auto-regressive model. The entering parameters are estimated from published experimental results and the model is applied to different variable amplitude results from the literature. Calculated results for fatigue life are promising compared to experimental results.     

多级载荷耦合下的镍钛合金血管支架疲劳强度和寿命预测

为探究镍钛合金血管支架植入下肢动脉后产生断裂失效的原因,对Absolute Pro下肢动脉支架在多级载荷耦合作用下的疲劳性能进行研究。利用有限元方法对镍钛合金支架在一级(生理脉动、轴向拉伸、压缩、弯曲、扭转)、二级(拉-弯、拉-扭、压-弯、压-扭、弯-扭)和三级(拉-弯-扭、压-弯-扭)载荷下分别进行数值模拟,基于应变法评价支架的疲劳强度,采用名义应力法和断裂力学进行疲劳寿命预测。经疲劳性能分析发现,一级载荷和部分多级载荷下的支架疲劳强度均满足10年疲劳寿命的要求,其中二级载荷的拉-弯和三级载荷的拉-弯-扭下最大交变应变大于疲劳极限,易产生应力集中导致疲劳失效;寿命云图和安全系数显示,在一级载荷下,压缩载荷对支架寿命的影响最大,脉动载荷最小,在二级载荷下,拉-弯载荷影响最大,弯-扭载荷影响最小,在三级载荷下拉-弯-扭对支架寿命的影响大于压-弯-扭;基于断裂力学寿命预测发现,初始裂纹的大小对支架的寿命有显著的影响。该研究结果揭示了多级载荷对支架疲劳强度和寿命的影响,为支架的临床断裂失效机理提供理论参考。     

FATIGUE LIFE PREDICTION OF NOTCHED COMPOSITE COMPONENTS

Abstract— The local stress/strain approach has been used to predict the fatigue lives of notched composite components. The method was based on a microstress analysis and the application of a multiaxial fatigue parameter incorporating the alternating strain components on the critical plane. This parameter was able to correlate the fatigue lives obtained under a variety of multiaxial loading and geometrical configurations, enabling a generalized fatigue life curve to be determined on the basis of limited experimental data.
The ability of the multiaxial fatigue parameter to relate the fatigue behaviour of composites was illustrated by predicting the locations of crack initiation sites in a unidirectional silicon carbide fibre reinforced titanium plate containing a circular hole tested under constant amplitude cyclic loading. The same approach was also successfully employed to predict the fatigue lives of graphite reinforced epoxy composite tubes with circular holes tested under several combinations of cyclic tension and torsion.     

FRACTURE AND LIFE PREDICTION UNDER THERMAL-MECHANICAL STRAIN CYCLING

Abstract— Thermal-mechanical cyclic strain tests were carried out under in-phase and out-of-phase conditions on a ferritic 1 CrMoV steel in the temperature range 315 to 565°C and two different batches of a 316 stainless steel in the temperature range 400 to 625°C. The results were compared with isothermal data.
In general, lives for in-phase thermal-mechanical tests were lower than for equivalent isothermal tests. This was substantiated by metallographic examination which indicated greater intergranular damage for the in-phase specimens. The addition of a tensile dwell to the in-phase condition caused even greater differences.
Three life prediction methods were used: the ductility exhaustion and a modified strain range partitioning approach gave reasonable estimates of life, whilst the linear life fraction method produced poor prediction.     

属性综合评价系统在岩爆发生和烈度分级中的应用

应用属性数学理论,建立了岩爆发生预测和烈度分级的属性识别模型。选取影响岩爆的主要因素:如最大切向应力σθ、单轴抗压强度σc、单轴抗拉强度σt以及弹性能量指数Wet,以σθ/σc、σc/σt以及Wet作为岩爆评价指标和烈度分级标准;构造了属性测度函数;定义了单指标属性测度和样本综合属性测度的计算方法。利用国内外一些重大岩石地下工程实例对模型进行了验证,由于评判结果与实际情况基本一致,并且与模糊综合评判法、物元可拓评判法等的评判结果有较好的一致性,从而验证了属性识别模型应用于岩爆预测预报的可行性和有效性。此外,利用该模型对某水电站工程和秦岭隧道工程的岩爆情况进行了评判,结果与人工神经网络法、距离判别法等的评判结果一致。     

BIMODAL CONCEPT OF HIGH-CYCLE FATIGUE LIFE PREDICTION FOR CONSTANT AND VARIABLE AMPLITUDE FATIGUE

A bimodal concept for the prediction of the high-cycle fatigue life of structural details subjected to constant- or variable-amplitude loading is considered in this paper. The total fatigue life was separated into two phases: crack initiation and crack propagation. The portion of life spent in crack initiation was estimated by using S–N data obtained on smooth specimens. A fracture mechanics concept was used to calculate the portion of life spent in crack propagation, and the S–N curve, including the fatigue limit of a structural detail, was determined by using material properties and the geometry of the detail. The bimodal concept was applied to a welded stiffener and the results are compared with experimental data reported in the literature.     

复合材料层合板低速冲击后压-压疲劳试验研究及寿命预报

进行了复合材料层合板低速冲击和冲击后压-压疲劳试验。在疲劳试验过程中详细测量了损伤扩展情况,获得了损伤扩展规律。将冲击损伤等效为一圆形开孔,应用含椭圆形夹杂的杂交应力单元分析含圆孔有限大板的应力分布,采用特征曲线和点应力判据相结合的方式并通过引入损伤扩展规律建立了含低速冲击损伤复合材料层板压-压疲劳寿命预测模型。通过与试验数据的对比,证明了该模型的有效性。同时,该模型还可预报在疲劳载荷下含冲击损伤层板的剩余压缩强度。     

PREDICTION OF FATIGUE LIFE BY X-RAY DIFFRACTION METHODS

Abstract— X-ray double crystal diffractometry and reflection topography were employed to examine the defect structure induced by fatigue of A1 2024 specimens. Analysis after various amounts of tension compression cycling revealed that the excess dislocation density in the surface layer increased rapidly early in the fatigue life, and maintained virtually a plateau value from 20 to 90% of the life. Beyond 90%, the excess dislocation density increased rapidly again to a critical value at failure. Investigation of the defect distribution in depth showed that the excess dislocation density in the bulk material, by contrast to the surface layer, increased more gradually during the life. Using deeply penetrating molybdenum radiation, the grains in the bulk could be analyzed, and thus the fatigue life and onset of failure could be predicted nondestructively. It was also shown that, after removal of the surface layer, the defect structure induced in the bulk by prior cycling was unstable. The resultant extension of fatigue life by surface layer removal was explained on the basis of this structural instability of the bulk material.     

线图法预测疲劳寿命的改进

线图法预测工程的疲劳寿命简单，方便。在已有预测公式的基础上，提出了预测低应力高周疲劳寿命的新公式，并进行了严密的公式推导，对一实例进行了计算并进行了验证，给出了新旧公式的预测结果，并进行了比较。     

FATIGUE LIFE PREDICTION OF WELDED JOINTS—A RE-ASSESSMENT

Abstract— Fatigue life prediction of welded joints needs an accurate and exhaustive theoretical Fracture Mechanics characterization of weld toe crack propagation. The method proposed by Albrecht et al. leads rapidly to accurate solutions of the LEFM δK-parameter. However, non-LEFM short crack behavior within the notch (weld toe) plastic zone must be taken into account. Available information on notch fatigue is surveyed, and practical cases where short crack growth is likely to occur are identified. Based on an elastoplastic finite element analysis, the LEFM validity limits and errors resulting from the misuse of LEFM in fatigue life prediction are quantified.     

考虑棘轮效应的疲劳寿命预测方法

多轴棘轮加载时轴向加载的恒定应力、剪切应变幅对轴向棘轮应变和疲劳寿命有很大的影响.考虑棘轮效应影响的Coffin模型将棘轮效应与循环部分相结合来计算疲劳寿命,预测结果较好,绝大部分预测结果分布在2倍分散带内.     

PREDICTION OF THE FATIGUE LIFE OF LASER WELDED STAINLESS STEEL JOINTS

Abstract— A fatigue scatter band has been computed for laser welded austenitic stainless steel joints. These laser welded steels have a very small heat affected zone. The unified scatter band provided by standards for welded structural steels does not adequately describe the trend of the experimental data of laser welded steels and this makes their design parameters scarcely realistic. The scatter band proposed in this paper has been computed by re-sorting experimental data relative to joints with high stress concentration factors and has subsequently been assessed with data relevant to butt welded joints.     

A SHEAR STRESS BASED CRITICAL-PLANE CRITERION OF MULTIAXIAL FATIGUE FAILURE FOR DESIGN AND LIFE PREDICTION

Abstract— The use of a previously presented general criterion of failure for high cycle multiaxial fatigue, τ_a/ t _A,B+σ_n.max/2σ_T= 1, is extended to cases where the shear and normal stress on the critical plane are non-proportional and also to give life predictions in the range of 10⁴ to 10⁶ cycles. The criterion takes account of whether case A cracks, growing along the surface, or case B cracks, growing in from the surface, occur.     

一个新的接触疲劳寿命预测模型的研究

本文给出了一个新的针对应力控制接触疲劳问题的寿命预测模型。在该模型中，选用Ｍｉｓｅｓ等效应力作为接触疲劳应力控制参量；引进了非零的接触疲劳应力强度值，根据损伤力学理论，分析了接触疲劳应力强度值与基体材料性质，第二相夹杂之间的关系，同时考虑了表面起裂和亚表面起裂两种失效形式，并将对这两种失效形式的寿命预测统一到一个公式中，引进一个与油膜系数Λ有关的系数ｆ（Λ），来表征润滑程度对接触疲劳寿命的影响。应用该模型来预测一些实验的接触疲劳寿命，计算值与实测值吻合很好。     

航天介质材料寿命的电性能评估技术

由于轨道空间环境与地球表面环境存在巨大差异,应用在航天环境的介质材料老化机理与一般地表环境介质材料的老化机理完全不同.如何在地面对其进行寿命评估,进而采取材料改性技术,延长航天介质材料可靠性与寿命,是摆在相关科研人员面前的重要课题.在分析航天器介质老化机理的前提下,提出采用介电性能、特别是材料的表面带电与深层带电与放电性能,结合机械性能的退变速率对航天介质材料进行评估的方法.