基于飞行数据的高压涡轮叶片蠕变寿命评估

针对民用航空发动机的特点,提出一种基于机载飞行数据的民用航空发动机高压涡轮叶片蠕变寿命评估方法。从机载飞行数据中提取发动机的实际使用载荷谱,通过有限元仿真得到高压涡轮叶片的应力和温度载荷谱,借助寿命评估模型计算一个飞行循环对高压涡轮叶片造成的累计损伤量,进而得到该发动机高压涡轮叶片的蠕变寿命。该方法对民航发动机的寿命预测、避免因高压涡轮叶片提前到寿而影响飞行安全和维修方案的制定具有重要意义。     

An approximate description of the creep rupture of structures

The economic design of high temperature structures requires an accurate estimation of the creep rupture life. Earlier work by the authors has shown that it is possible to bound the life of a structure subject to fixed loading. The bounds, however, are well separated and it is important to establish where the actual lives lie relative to the two bounds. In this paper, detailed finite element calculations and experimental work on plate structures are reported. These results, together with the concept of local kinematic determinacy, lead to a correction factor on the upper bound life prediction. This enables a good measure of the load-carrying capacity of any ductile structure to be obtained from knowledge of the limit load, the elastic stress concentration factor and material stress rupture data.     

The application of an approximate analysis of the diffusion process for a description of creep and creep rupture

An analysis of the environmental effect on creep and creep rupture of metals is presented. It is pointed that the investigation of the diffusion processes leads to significant computational difficulties. These difficulties arise on representing the solution of the diffusion equation with variable boundaries in the form convenient for analysis.The process of damage accumulation is modelled for a material, which is subjected to the combined action of mechanical loads and the aggressive environment. The Rabotnov kinetic theory is applied in which two parameters are taken into account: the damage of the material and the concentration of chemical elements reducing the resistance of the material to mechanical loads. An approximate solution of the diffusion equation is suggested for rod or shell subjected to axial tension. This solution is based on dividing the cross-section of rod or shell into the disturbed and undisturbed parts and determining the motion of the boundary between these parts. The system of constitutive equations which describe the interaction between the diffusion and rupture fronts during the creep process until failure is obtained. A good quantitative agreement between experimental and theoretical results has been received.     

Dynamic creep rupture of a rotating disk of variable thickness

An analysis is provided of the finite strains of rotating disks followed by damage and creep rupture front motion as described by Kachanov's hypothesis. The creep is represented by the generalized Norton-Odqvist law for the true stresses and logarithmic strains. An iterative method of solution is proposed. It allows for the inclusion of inertia forces in the formulation of equilibrium. Two numerical examples illustrate the deformation process, ductile and ductile-brittle creep rupture.     

Improvement of long-term creep life extrapolation using a new master curve for Grade 91 steel

This study aims to improve the long-term creep life extrapolation of Grade 91 (Gr. 91) steel using a new master curve of a hyperbolic sine (“sinh”) form. In the master curve for extrapolating creep rupture life, a controversial point of a polynomial form, which has usually been used in time–temperature parametric methods, was preliminarily indicated. The validity of the new master curve was verified. A large body of creep rupture data for Gr. 91 steel was prepared from different sources of available worldwide data at 500 °C to 700 °C for Gr. 91 steel. The material constants of the Larson–Miller, Orr–Sherby–Dorn, and Manson–Harferd parameters were obtained using the polynomial and sinh forms on the basis of the rupture data. Long-term creep rupture life was extrapolated for up to 60 years using the two forms that defined the master curves. Results of long-term creep life extrapolation showed that the master curve of the polynomial form exhibited overestimation due to the divergent curves in the low stress ranges, whereas the master curve of the sinh form revealed goodness in the low stress ranges beyond the experimental data. The proposed sinh form was superior to the polynomial form. The sinh form could be applied to extrapolate the long-term creep rupture life of other heat-resistant steels. The creep rupture stresses predicted by the sinh function were comparable to those of RCC-MRx code.     

The sensitivity of the creep rupture of a pressurized cylinder to variations in the multiaxial rupture surface

The sensitivity of creep rupture life to variations in the multiaxial creep rupture surface is investigated for a long, thick-walled, internally pressurized circular cylinder. The cylinder is initially undamaged and accumulates damage at a rate dependent on both maximum principal stress and equivalent stress. It is shown that failure can initiate at the inner or outer radius, or at an intermediate radius, depending on the parameters in the creep and damage laws, and that the ratio of rupture time to initial failure time depends strongly on these parameters. The results are compared with estimates based on the mean diameter formula and in almost all cases a longer rupture life is predicted.     

The influence of stress concentration on creep rupture at non-stationary loading

A method is given for the calculation of creep rupture strength of parts containing stress concentrations. The creep theory takes account of the damage and anisotropy of materials. which arise during deformation. For the example of a plane specimen with a notch, results of calculations are compared with experimental data. The results of an investigation into the creep strength of the fir-tree root of a turbine blade at non-stationary loading is given.     

The determination of ductile creep rupture tim for orthotropic sheets and cylindrical shells

Equations describing the deformation of orthotropic materials for which a logarithmic creep rate in the form of a power function of the stresses are obtained. These equations are then used for the determination of the ductile creep rupture time of orthotropic sheets and cylindrical shells in biaxial tension. They can also be used in superplastic orthotropic metal forming calculations.     

Estimates of the creep rupture lives of structures subjected to cyclic loading

Constitutive relationships are discussed for materials which undergo creep rupture due to cyclic loading. Relationships are proposed which describe the strain rates in terms of the current stress and a single state variable. An approximate method is derived which enables a lower bound on the rupture life to be obtained for kinematically determinate structures subjected to cyclic load and isothermal conditions. The bound on the rupture life is expressed in terms of the energy dissipation rates within the structure corresponding to stationary-state creep. The effect of multi-axial stress creep-rupture upon the structural performance is examined and bounds are derived for materials which obey maximum tension and octahedral shear stress criteria. For both multi-axial stress rupture laws and structures subjected to cyclic and reverse load conditions formulae are derived which express the lower bound rupture life in terms of the behaviour of a steady-load uni-axial creep rupture test. Results of experiments which have been carried out on copper and aluminium beams are presented for cyclic and reverse load conditions. For both rupture laws the experimental rupture times are closely predicted by the representative rupture stresses and uni-axial data.     

A lower bound to the creep rupture time of pressurised thick cylinders

A lower bound to the creep rupture time of internally pressurised thick cylinders is derived. Material behaviour is described by a phenomenological creep rupture theory that accounts for all phases of creep, and for the full coupling between the deformation and damage processes. To obtain the desirable lower bound, the effective stress and the equivalent rupture stress, which represent the effects of multiaxial stress states on the creep strain and damage rates, respectively, were approximated by the maximum shear stress in the constitutive equations. By comparing the lower bound estimations for a wide range of cylinder dimensions and different engineering materials with the rupture times determined from accurate finite element calculations, it is shown that the lower bound estimations provide quite conservative lifetime predictions.     

Calculating process control parameters from steady state operating data

The tutorial paper will show that chemical process control parameters can be obtained from steady state operating data in combination with simple algebraic equations. The parameters include steady state process gain, feedforward control factor and approximate process time constant. Here, we treat ‘self-regulating’ processes first. Integrating processes and unstable processes are treated separately, unlike the standard method of setting up linear differential equations followed by Laplace transformations. With the standard method, a steady state portion of dynamic response, if there is any, appears as a part of equations of a complex variable. Many control engineers in the chemical industries who might have studied the method, do not use it because of mathematical difficulties. Instead, they may resort to plant tests that are often time consuming and costly. The parameters obtained by this paper's method will aid improvement of control systems. We will address the relation of this method with that of commercial software, which implement advanced process controls based on plant tests.     

Multi-axial creep rupture of a model structure using a two parameter material model

A simplified, two parameter creep curve model is developed, which represents primary-secondary-tertiary creep behaviour. The two parameters are related via the secondary strain respectively to: the sum of secondary and primary strains; and, the sum of secondary and tertiary strains. Techniques are described for fitting the model to laboratory data; and, for the determination of the parameters which characterize primary-secondary and secondary-tertiary creep. The single state variable theory used to describe tertiary creep is compared with mechanisms based models and shown to closely predict the effect of stress-state on rupture strain. A two bar plane strain model component subjected to steady load is studied and used to determine the effect on the component lifetime of primary creep; and, of the multi-axial creep rupture criterion. The representative rupture stress is found to be weakly dependent on primary creep and strongly dependent on the multi-axial rupture criterion of the material.     

Upper bounds on the creep rupture life of structures subjected to variable load and temperature

Upper bounds on the rupture life and time to initial rupture of a body obeying the Katchanov creep rupture equations are derived for cyclic load and temperature. The bounds are derived from the creep rupture equation without reference to the deformation properties. The relationship to previously derived bounds of this type are discussed, and the circumstances when the bounds can be both very good and very poor are demonstrated through simple examples.     

The prediction of creep rupture of pressurized tubes using a model for void growth and coalescence

A phenomenological formulation based on a theory of plasticity for voided solids, and power-law creep, to estimate the rupture times and strains for pressurized tubes is presented. Realistic values of the damage parameters involved are selected by satisfying the creep rupture uniaxial data for the same materials. Void growth and coalescence and hence the loss of load-carrying capacity of the tube is taken as a failure criterion. Computed values of rupture times for thin- and thick-walled tubes are shown to be mostly in close agreement with the experimental data found in the literature. Only fair agreement among predicted and experimental fracture strains is observed.     

Optimal design of a thin-walled cross-section subject to bending with torsion against creep rupture

Optimal structural design of a thin-walled closed bar cross-section under bending and torsion is considered. The material is assumed to be governed by the Norton-Odqvist non-linear steady creep law. Minimal cross-sectional area is the design objective, middle line of the profile and wall-thickness are design variables and the constraint refers to brittle creep rupture as described by the Kachanov-Sdobyrev hypothesis.     

陶瓷材料超声波辅助蠕动磨削工艺参数优选试验研究

为探索利用小直径磨轮对陶瓷叶片型面进行数控展成型面超声磨削,通过对Al2O3陶瓷进行蠕动磨削和超声波辅助蠕动磨削对比试验研究,分析各加工参数对磨削表面质量的影响规律,优选工艺参数.结果表明:蠕动磨削和超声波辅助蠕动磨削所获得较低表面粗糙度值的优选工艺磨削参数均相同,但工艺参数对加工表面粗糙度的影响顺序不同;工艺参数对表面粗糙度的影响规律均相同,且存在着最优工艺参数,但在相同的工艺参数下,蠕动磨削所获得的表面粗糙度值一般均低于超声波辅助蠕动磨削.     

Identification of modal parameters from nonstationary ambient vibration data using the channel-expansion technique

The identification of modal parameters from the response data only is studied for structural systems under nonstationary ambient vibration. In a previous paper by the authors, the modal parameters of a system were identified using the correlation method in conjunction with the curve-fitting technique. This was done by working within the assumption that the ambient excitation is a nonstationary white noise in the form of a product model. In the present paper, the Ibrahim time-domain method (ITD) is extended for modal-parameter identification from the nonstationary ambient response data without any additional treatment of converting the original data into the form of free vibration. The ambient responses corresponding to various nonstationary inputs can be approximately expressed as a sum of exponential functions. In effect, the ITD method can be used in conjunction with the channel-expansion technique to identify the major modes of a structural system. To distinguish the structural modes from the non-structural modes, the concept of mode -shape coherence and confidence factor is employed. Numerical simulations, including one example of using the practical excitation data, confirm the validity and robustness of the proposed method for identification of modal parameters from the nonstationary ambient response.     

Tension and creep rupture behaviors of Alloy 617 thermally aged for a year at 900 °C

Tension and creep rupture behaviors of Alloy 617 after a year of thermal ageing at 900 °C were investigated in comparison with those of unaged (virgin     

利用点云数据的法矢及曲率估算

测量数据的微分几何属性估算是逆向工程中区域分割以及曲面拟合的基础,对逆向工程研究具有重要意义。简单介绍了基于坐标转换法的法矢和曲率估算方法;以迭代的方式对估算值进行优化,提高了点云法矢和曲率估算的精度;试验结果表明,迭代优化方法简单可行,能够计算出精确的结果。     

利用IGES的人工骨三维结构生成过程中的数据转换

为了解决人工骨三维结构生成过程中编程生成数据与CAD/CAM系统之间的数据转换问题,设计了基于初始化图形交换规范(IGES)的文件格式数据转换模块,实现了将程序所生成人工骨三维结构数据转换为标准数据格式的IGES文件,该文件可以被CAD/CAM系统正确读取。经过测试,几何数据转换完整,结果可靠。