航空发动机涡轮盘裂纹萌生寿命的估算方法

针对航空发动机涡轮盘裂纹萌生寿命的估算问题，给出一个全新的损伤参量。该损伤参量结合了分析多轴疲劳的临界面方法，“应力功”的概念和疲劳破坏的微观机制。依据该损伤参量，利用有限元程序和自编程序计算得出损伤参量和疲劳循环次数的关系，建立以这一损伤参量为基础的疲劳损伤累积模型，进而估算涡轮盘裂纹萌生寿命。     

基于加权应变因子的叶片振动疲劳寿命预测

通过循环应力-应变方程和应变-寿命方程推导了可预测材料高、低周疲劳寿命的非线性疲劳损伤累积模型。模型中引入弹性、塑性加权应变因子对疲劳延性指数和疲劳强度指数进行修正,以考虑疲劳过程中弹塑性应变共同作用对疲劳损伤累积速率的影响。以某型航空发动机压气机叶片为研究对象,利用所建立的模型预测其疲劳寿命,并与试验结果、Basquin方程、改进的Basquin方程的预测结果进行对比分析。结果发现:在低应力水平(≤700MPa)下,三种预测模型给出的高周疲劳寿命误差小于30%;但是当应力水平达到770MPa时,所建立模型的预测精度比Basquin方程和改进的Basquin方程分别提高89.14%和22.73%。     

涡轮盘榫槽疲劳/蠕变交互作用的寿命研究

本文对涡轮盘榫槽的疲劳/蠕变行为,采用非线性有限元进行了分析研究,根据发动机的工作特点,在计算应力-应变时,分别对疲劳和蠕变进行处理,在作寿命判断时,则考虑二者的交互作用,结果表明:无论用应变范围区分法(SRP),或应变能区分法(SEP),两种方法的计算寿命值与发动机的实际寿命值是很吻合的,它给研究发动机重要零件的寿命,提供了一种有价值的方法。     

缺口件多轴疲劳寿命预估新方法

局部应力应变法(LSSM)以危险点处的局部应力应变历程进行寿命评价,忽略了非比例附加强化及缺口附近应力梯度对疲劳损伤的影响.论文通过分析裂纹萌生面(临界面)上剪应力和正应力分布状态,提出一种计算缺口件多轴疲劳损伤影响区的数学计算方法,建立考虑法向正/剪应力梯度与非比例附加强化效应的寿命预估模型.首先,基于能量法和临界面理论,借助坐标变换原理将应变能密度最大的材料平面定义为临界面,建立确定裂纹萌生方向的计算方法.其次,以临界面上特定路径为积分方向,将缺口件三维问题转化为线性问题,简化计算过程.综合考虑峰值应力和等效应力场强对多轴疲劳寿命的贡献度,建立表征缺口件多轴疲劳损伤演化过程的损伤参量.最后,通过TC4合金和Q345钢多轴疲劳试验验证论文模型的可行性和精确度,并与LSSM法、FS模型、SWT模型进行对比分析.     

一种考虑非比例附加损伤的多轴低周疲劳模型

在实际工作环境中,机械结构往往承受着多轴非比例循环载荷.相比多轴比例循环加载,多轴非比例循环加载由于产生了附加强化现象,造成机械结构疲劳寿命下降.通过分析薄壁圆筒管件在非比例加载工况下应力应变变化规律和发生破坏位置,本文基于临界面法提出一种考虑多轴非比例附加损伤的疲劳模型.该模型将最大剪切应变幅平面作为临界面,提出一个新的附加强化因子,结合临界面上切应变幅和正应变幅组成新的多轴疲劳损伤参量.此参量不仅考虑了非比例加载下临界面上正应变幅和切应变幅对材料造成的疲劳损伤,还考虑到应变路径的变化和材料非比例加载敏感特性对材料疲劳寿命的影响.考虑到实际情况下模型所需材料附加强化系数有时难以获得的情况,给出了材料附加强化系数的有关近似计算公式.只需要材料基本力学参数便可得到材料附加强化系数,方便工程实际应用.采用8种材料的多轴疲劳寿命数据对提出的新模型进行检验,结果表明所提出的新模型与传统多轴疲劳模型相比预测寿命精度更高.     

转动惯量法在多轴疲劳本构关系预测中的应用

一些金属材料在承担多轴非比例加载过程时,会产生额外非比例附加强化或软化现象,这一现象往往会导致在评估疲劳寿命时因为材料本构关系的不确定而引起预测结果出现较大误差．因此基于单轴疲劳理论得出的寿命预测模型并不能准确地预测多轴非比例疲劳加载下的材料寿命．针对此问题,本文阐述了非比例附加强化效应产生的原因及结果,结合转动惯量法的理论和塑性增量法,建立了预测多轴低周疲劳加载下循环应力-应变曲线的数值计算模型．利用316L 不锈钢试样在5 种加载路径下的实验数据对预测结果进行了验证,结果表明该模型具有良好的预测有效性及精度．     

一种考虑拉伸保持效应的多轴疲劳寿命模型

通过定义考虑拉伸保载效应的CFI因子(creep-fatigue interaction factor),将拉伸蠕变损伤和疲劳损伤进行非线性耦合. 根据断裂实验的观察,针对拉伸主 导的裂纹萌生、扩展及破坏的多轴疲劳问题,给出了一个基于临界面方法的能量型高温多轴 疲劳寿命预测模型. 所给出的模型可对不同温度、不同载荷特点、不同保载时间的多轴疲劳 寿命进行预测,模型的材料参数不依赖于温度和载荷. 并且此方法可以很方便地推广到其它 因素主导破坏的高温多轴疲劳寿命预测. 通过拟合高温合金Udimet720Li单轴带保持时间的 低循环疲劳(low cycle fatigue, LCF)寿命试验数据,得到了材料常数. 结合黏 塑性有限元分析方法,对高温双轴带保载循环载荷下Cruciform试件的寿命进行了 预测,预测结果基本落在2倍分散带内,达到工程的要求,证明了该模型的有效性.     

一种考虑非比例附加损伤的多轴低周疲劳模型

在实际工作环境中,机械结构往往承受着多轴非比例循环载荷.相比多轴比例循环加载,多轴非比例循环加载由于产生了附加强化现象,造成机械结构疲劳寿命下降.通过分析薄壁圆筒管件在非比例加载工况下应力应变变化规律和发生破坏位置,本文基于临界面法提出一种考虑多轴非比例附加损伤的疲劳模型.该模型将最大剪切应变幅平面作为临界面,提出一个新的附加强化因子,结合临界面上切应变幅和正应变幅组成新的多轴疲劳损伤参量.此参量不仅考虑了非比例加载下临界面上正应变幅和切应变幅对材料造成的疲劳损伤,还考虑到应变路径的变化和材料非比例加载敏感特性对材料疲劳寿命的影响.考虑到实际情况下模型所需材料附加强化系数有时难以获得的情况,给出了材料附加强化系数的有关近似计算公式.只需要材料基本力学参数便可得到材料附加强化系数,方便工程实际应用.采用8种材料的多轴疲劳寿命数据对提出的新模型进行检验,结果表明所提出的新模型与传统多轴疲劳模型相比预测寿命精度更高.      

基于裂纹扩展的疲劳-蠕变寿命预测

为了提高疲劳-蠕变寿命的预测精度,首次从裂纹扩展角度出发引入有效应变能密度增量,利用应力-应变迟滞回线所围图形的正值面积对其进行了定义和计算。计算表明,有效应变能密度增量与加载的应力、速率、保载时间、材料性质及疲劳-蠕变的速率相关,且随压应力增加而增大。通过将有效应变能密度增量与裂纹的长度之积定义为裂纹扩展的控制参量,建立了疲劳-蠕变下的裂纹扩展速率方程,并由此导出了疲劳-蠕变寿命与有效应变能密度增量之间的关系式。该式中的疲劳与蠕变有效应变能密度增量交叉项恰好反映了疲劳与蠕变的交互作用。最后,采用该式对1.25Cr0.5Mo钢在540℃时不同应力控制下的疲劳-蠕变寿命进行了预测,发现83.3%的预测值在实验值的1.3倍分散带以内,预测结果良好。     

基于应变能的复杂应力场低周疲劳分析

塑性应变能使材料微观组织结构发生不可逆变化,从而引起等效宏观应力,该应力随循环加载而增大．假定材料疲劳源处破坏是由最大拉应力引起的,最大等效宏观应力与外加应力叠加达到材料本征断裂应力时形成微裂纹．微裂纹引起上述两部分应力变化,继续加载直至宏观裂纹出现,从而得到材料的疲劳寿命．本文所建立的多轴疲劳寿命公式包含材料参数、拉应力以及塑性应变能等,以上数据可通过单轴疲劳数据和有限元方法获得．通过对SM45C材料的计算验证,表明该模型对多轴随机应变加载低周疲劳寿命,具有良好的预测结果．     

Deformation and fracture behaviour under combined creep and fatigue

An examination of deformation and fracture has been made for the nickel base superalloy IN597 at 850°C for relatively high stress short life cyclic torsional loading programmes. The cycles combine low cycle fatigue with creep and relaxation dwell from the repeated application of a closed hysteresis loop under strain control. From a consideration of the changing shape of the hysteresis loop throughout cyclic life the effects of dwell stress, inelastic strain range and cycle number on creep deformation are examined together with the variation of peak stress softening with cycle number and relaxation time. Their effects on fracture in a high-temperature oxidising environment are appraised in relation to various predictive techniques; the Coffin-Manson equation, the universal slopes method, the linear damage rule, strain range partitioning and various incremental life prediction laws.Observations made on transgranular and intergranular crack paths are shown to provide the necessary metallographic evidence for the relative contributions to fracture from the repeated action of creep, fatigue and relaxation processes.     

Constitutive relations describing creep deformation for multi-axial time-dependent stress states

A theory of primary and secondary creep deformation in metals is presented, which is based upon the concept of tensor internal state variables and the principles of continuum mechanics and thermodynamics. The theory is able to account for both multi-axial and time-dependent stress and strain states. The wellknown concepts of elastic, anelastic and plastic strains follow naturally from the theory. Homogeneous stress states are considered in detail and a simplified theory is derived by linearizing with respect to the internal state variables. It is demonstrated that the model can be developed in such a way that multi-axial constant-stress creep data can be presented as a single relationship between an equivalent stress and an equivalent strain. It is shown how the theory may be used to describe the multi-axial deformation of metals which are subjected to constant stress states. The multi-axial strain response to a general cyclic stress state is calculated. For uni-axial stress states, square-wave loading and a thermal fatigue stress cycle are analysed.     

A study of the generation and creep relaxation of triaxial residual stresses in stainless steel

This paper presents results from a numerical and experimental research programme motivated by the need to predict creep damage generated by multi-axial states of stress in austenitic stainless steels. It has been hypothesized that highly triaxial residual stress fields may be sufficient to promote creep damage in thermally aged components, even in the absence of in-service loads. Two prerequisites to test this hypothesis are the provision of test specimens containing a highly triaxial residual stress field and an accurate knowledge of how this residual stress field relaxes due to creep. Creep damage predictions may then be made for these specimens and compared to damage observed in experiments. This paper provides solutions to both of these prerequisites. Cylindrical and spherical test specimens made from type 316H stainless steel are heated to 850 °C and then quenched in water. Finite element predictions of the residual stress state, validated by extensive neutron diffraction measurements, are presented which confirm the high level of triaxiality present in the specimens. The specimens are then thermally aged at 550 °C and numerical predictions of the residual stress relaxation are given, again validated by extensive neutron diffraction measurements. The results confirm the validity of the creep relaxation models employed. In addition, the results show the influence of specimen size and permit comparisons to be made between three different types of neutron diffractometers.     

Creep,plasticity, and fatigue of single crystal superalloy

Single crystal components in gas turbine engines are subject to such extreme temperatures and stresses that life prediction becomes highly inaccurate resulting in components that can only be shown to meet their requirements through experience. Reliable life prediction methodologies are required both for design and life management. In order to address this issue we have developed a thermo-viscoplastic constitutive model for single crystal materials. Our incremental large strain formulation additively decomposes the inelastic strain rate into components along the octahedral and cubic slip planes. We have developed a crystallographic-based creep constitutive model able to predict sigmoidal creep behavior of Ni base superalloys. Inelastic shear rate along each slip system is expressed as a sum of a time dependent creep component and a rate independent plastic component. We develop a new robust, computationally efficient rate-independent crystal plasticity approach and combined it with creep flow rule calibrated for Ni-based superalloys. The transient variation of each of the inelastic components includes a back stress for kinematic hardening and latent hardening parameters to account for the stress evolution with inelastic strain as well as the evolution for dislocation densities. The complete formulation accurately predicts both monotonic and cyclic tests at different crystallographic orientations for constant and variable temperature conditions (low cycle fatigue (LCF) and thermo-mechanical fatigue (TMF) tests). Based on the test and modeling results we formulate a new life prediction criterion suitable for both LCF and TMF conditions.     

纤维增强树脂基复合材料多轴疲劳研究进展

纤维增强树脂基复合材料结构在工程应用中常常承受复杂载荷的作用, 从而使材料处于多轴循环应力条件下. 为了更加合理地进行复合材料结构设计, 必须对复合材料多轴疲劳行为进行深入的研究, 建立比较理想的复合材料疲劳寿命预测模型. 首先简单回顾了复合材料单轴疲劳损伤判据及其寿命预测模型, 然后结合作者的研究结果, 总结了近年来国内外纤维增强树脂基复合材料多轴疲劳理论的研究成果, 对各种失效准则、疲劳寿命预测模型进行了较为深入的分析, 指出了它们各自的特点及其存在的问题, 并对复合材料多轴疲劳理论的研究趋势作出了展望. 最后, 对目前常用的复合材料多轴疲劳实验方法进行了总结, 并指出了各种方法的优缺点及其实验中存在的困难.      

边坡预应力锚杆蠕变的数值分析

运用非线性蠕变分析理论模型,采用MSC.MARC软件重点研究了锚杆的应力指数、蠕变系数和几何参数的变化对于锚杆应力松弛的影响.通过对预应力锚杆支护的边坡进行数值分析,探讨了锚杆的蠕变规律以及锚杆预应力松弛现象.锚杆的参数取不同数值时,比较数值模拟结果发现:锚杆的轴力和剪力主要分布在锚固段的前端;随着锚杆应力指数及蠕变系数的增加,蠕变应变增大,应力松弛加快.     

透平转子的可靠性设计

介绍透平转子的可靠性设计方法。该方法以传统的转子强度和振动的设计方法为基础,把透平转子的静应力、材料静强度、应力幅、尺寸系数、表面加工系数、应力集中系数、材料疲劳极限、热疲劳累积损伤程度、材料低周疲劳程度、蠕变寿命、固有频率、激振力频率、不平衡响应等处理为随机变量,使用机械概率设计法确定转子设计的可靠度。文中给出了透平转子静强度、疲劳强度、热疲劳、蠕变寿命、避开弯曲共振、避开扭转共振和限制不平稳响     

A thermodynamic approach to nonlinear ultrasonics for material state awareness and prognosis

We develop a thermodynamic framework for modeling nonlinear ultrasonic damage sensing and prognosis in materials undergoing progressive damage. The framework is based on the internal variable approach and relies on the construction of a pseudo-elastic strain energy function that captures the energetics associated with the damage progression. The pseudo-elastic strain energy function is composed of two energy functions—one that describes how a material stores energy in an elastic fashion and the other describes how material dissipates energy or stores it in an inelastic fashion. Experimental motivation for the choice of the above two functionals is discussed and some specific choices pertaining to damage progression during fatigue and creep are presented. The thermodynamic framework is employed to model the nonlinear response of material undergoing stress relaxation and creep-like degradation. For each of the above cases, evolution of the nonlinearity parameter with damage as well as with macroscopic measurables like accumulated plastic strain is obtained.     

Sequential creep-fatigue interaction in austenitic 316 L-SPH stainless steel

Stainless steel type 316 L-SPH are predamaged by fatigue from 20 to 70% of fatigue life or creep load from the primary to tertiary range. Their subsequent fatigue and creep properties are investigated at 600°C for different fatigue saturation and creep stresses. Results were evaluated by the time and cycle fraction rule.In this way, information can be gained on the effect of very long intermittent hold times for low cycle fatigue and on transitory loads under normal service at elevated temperatures. In the cases of creep-fatigue and fatigue-creep sequence of preloading, the subsequent resistance of the specimens remained unchanged, if not improved. Thin-foil specimens were also made to reveal that the dislocation substructure developed during preloading was quickly erased and replaced by that corresponding to the subsequent load type. More specifically, grain boundary cavitation did not occur until creep loadapplied well beyond the end of the secondary stage. Hence, preloading by creep up to approximately 80% of rupture life does not affect the subsequent fatigue properties. On the other hand, significant surface cracks were found in the prefatigued specimens after more than approximately 50% of life. The subsequent creep damage was found to be localized at the tip of the crack and the remaining creep life was fatigue damage with long intermittent hold times is less severe than continuously repeated loading.