X-ray-diffraction approach for studies on fatigue and creep

It is well known that X-ray diffraction is one of the most powerful means for investigating the microscopic structure of crystalline materials. X-ray diffraction is advantageous when it is applied to metallic materials; it responds very sensitively to changes in the metal's crystalline structure. Another characteristic advantage of the X-ray-diffraction approach is its nondestructive nature in the measurement of crystalline-material parameters, enabling us to observe the process of mechanical phenomena of metals, such as fatigue and creep. The X-ray-diffraction patterns obtained on a deformed material include a great deal of information covering the microscopic and macroscopic characters consistent with the nature of the existing material. Residual stress measured by means of X ray is called the macroscopic-material parameter. It is evaluated by measuring the shift of the peak of a diffraction profile. The diffusiveness of the profile corresponds to the irregularity in microscopic structure of deformed crystalline material and it is noted as the submacroscopic material parameter. The X-ray-microbeam diffraction technique supplies information on the change in microscopic structure such as subgrain size, misorientation and microlattice strain. Profile analysis is another way to evaluate the microscopic-material parameters: particle size and microscopic strain. By appropriately combining these techniques in the study of mechanical behavior of materials, the parameters that control the phenomena may be extracted to facilitate discussion of their mechanism. In this lecture, X-ray-diffraction techniques to evaluate the macroscopic, submacroscopic and microscopic-material parameters are presented and the approach is demonstrated by exhibiting a case of studies on fatigue and creep of carbon steels at room and elevated temperature, where phenomena are discussed in terms of the change in the material parameters. Initiation and propagation of fatigue crack in steel at room temperature, the change in microstructure during isothermal and thermal fatigue, and also that in creep at elevated temperature under variational load are presented.     

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.     

The nonlinear mechanics of continual damage and its application to problems of creep and fatigue

The results of investigations of the mechanics of continual damage of materials and structural elements are reviewed. The focus is on nonlinea models of damage accumulation for problems of creep and high-cyclic fatigue. Nonlinearity criteria are formulated, and methods of construction of nonlinear models are analyzed. Approaches to the solution of problems of prediction of long-term strength and lifetime under creep and interaction of creep and fatigue are considered. S. P. Timoshenko Institute of Mechanics, national Academy of Sciences of Ukraine, Kiev. Translated from Prikladnaya Mekhanika, Vol. 36, No. 3, pp. 31–66, March, 2000.     

Radiation effects on creep rupture and fatigue strength of pure aluminum

Paper describes an experimental investigation on the effect of nuclear radiation on the creep rupture and fatigue strength of pure aluminum for the ratios of alternating stress to mean stress ranging from zero to infinity. It was found that not only was the strength of the material tested affected by radiation over a range of ratios of stresses, but also the brittle-ductile transition was influenced.Based upon the experimental results obtained, an analytical model was developed to correlate creep rupture, fatigue strength and radiation effect for various ratios of stresses. This information may prove to be valuable in dealing with the stress analysis of nuclear reactors.Paper was presented at 1966 SESA Annual Meeting held in Pittsburgh, Pa., on November 6–9.     

钢管混凝土圆弧桁架拱平面内徐变稳定分析

核心混凝土的徐变会增加钢管混凝土拱肋的屈曲前变形,降低结构的稳定承载力,因此只有计入屈曲前变形的影响,才能准确得到钢管混凝土拱的徐变稳定承载力。基于圆弧形浅拱的非线性屈曲理论,采用虚功原理,建立了考虑徐变和剪切变形双重效应的管混凝土圆弧桁架拱的平面内非线性平衡方程,求得两铰和无铰桁架拱发生反对称分岔屈曲和对称跳跃屈曲的徐变稳定临界荷载。探讨了钢管混凝土桁架拱核心混凝土徐变随修正长细比、圆心角和加载龄期对该类结构弹性稳定承载力的影响,为钢管混凝土桁架拱长期设计提供理论依据。     

可靠性敏度分析方法及其在非线性蠕变疲劳失效模型中的应用

针对非线性极限状态方程,发展了两种基本随机变量为非正态情况下的可靠性敏度分析方法:基于改进一次二阶矩的近似解析法和基于Monte-Carlo的数字模拟法.近似解析法中非正态变量首先被等价变换为正态变量,然后用正态变量的敏度分析法和隐函数求导法则来得到失效概率对非正态变量分布参数的灵敏度,求解敏度的数字模拟法是从计算失效概率的所有样本点中选取合适的抽样点,利用回归分析和隐函数求导法则来求取可靠性灵敏度的.所提方法被用于非线性蠕变疲劳失效模式的可靠性灵敏度分析,近似解析法和数字模拟法结果的一致说明了所提方法的合理可行.蠕变疲劳失效的可靠性灵敏度随参数的变化趋势分析为工程设计提供了有益指导.     

Approximate creep rupture lifetimes for butt welded ferritic steel pressurised pipes

The creep rupture of butt welded ferritic steel pipes composed of a range of weld and heat affected zone materials has been previously analysed in detail (Leckie and Hayhurst, 1994). These analyses required substantial computational resources which cannot be justified during the preliminary phases of the design process. To reduce cost and improve speed an approximate method for the analysis of kinematically determinate structures, known as the modal method (Leckie and Hayhurst, 1974), has been developed to compute creep rupture lifetimes. This paper reports an extension of the method for the analysis of multi-material structures, such as weldments, and its implementation as a post processor to a stationary state finite element creep analysis. Histories of stress and damage have been determined using the modal method for weldments with a range of heat affected zone and weld material combinations. The lifetimes determined in this way are compared with those determined using complete continuum damage mechanics analyses, and are shown to be conservative; in addition the regions of intense damage have been shown to be accurately predicted. The modal method is also shown to be superior to more approximate reference stress methods.     

Model based online diagnosis of unbalance and transverse fatigue crack in rotor systems

A model based technique for online identification of malfunctions in rotor systems is discussed. Presence of fault changes the dynamic behavior of the system. This change is taken into account by equivalent loads acting on the undamaged system model. Equivalent loads are fictitious forces and moments acting on the undamaged system model, which generate a dynamic behavior identical to that of the real damaged system. The mathematical representation of equivalent loads is referred to as Fault Model. The work focuses on developing a fault model for a transverse fatigue crack in shaft and testing it through simulated studies. The basic principle of the technique is validated for unbalance identification, through numerical simulations as well as by experiments on a real rotor system.     

A new model for fatigue crack propagation in 4340 steel

An investigation on fatigue crack propagation under mode I loading has been performed. Fatigue crack growth data in the case of plane strain mode I have been obtained by performing experiments on compact tension specimens of 4340 steel for increasing ΔK_I, decreasing ΔK_I, and constant ΔK_I loading conditions. Fatigue crack extension predictions have been obtained for Khan's proposed equation [1989], together with the widely used and [1963] and [1967] equations. Khan's equation overcomes the drawbacks associated with the Paris and Forman equations, such as the inclusion of all the three stages of fatigue crack propagation, the nondimensionality of the constant, “C”, and also the accuracy in life prediction. Within the context of Linear Elastic Fracture Mechanics and the types of loading performed at the crack tip, it has been shown that Khan's model can accurately predict the crack growth data from near threshold value to the unstable fracture; these predicted values are much more accurate than those from the Paris and Forman equations.     

Transient stress of steel phase transformation and transformation-induced fatigue

Expansion changes the size of any machine element and can thus generate severe stress or distortion failures during its operation if this phenomenon is not taken into account during the design process. On the other hand, most steels contract during heating within a short temperature range as a result of microstructural phase transformations. This transient contraction may generate further short- and long-term dysfunction of a metallic structure. The current paper focuses on this issue; investigating the effects of the microstructural transformation on the transient stress response of a metallic structure. Analysis is carried out by means of finite element simulation employing material data from typical constructional steel. The paper emphasizes the necessity of considering material phase changes when dealing with the transient stress or displacement response of a metallic structure and demonstrates the differences obtained when the transformation effects are ignored or accounted for in the design process. The mechanism of transformation-induced fatigue is thus discussed and proposed to the engineering community for establishment.     

既有铁路钢桥剩余疲劳寿命的概率性评估

中国铁路线上大量既有钢桥承受着日益繁重的交通荷载,其疲劳剩余寿命与使用安全已受到桥梁管理部门的高度重视,为避免不必要的维护与更换,应建立合理的既有钢桥使用安全评估方法。在建立能反映铆接钢桥疲劳破坏机理的脆断和韧断概率失效模型基础上,给出了疲劳可靠性分析的极限状态方程,并合理确定了随机变量的参数取值。建立了铆接钢桥构件双角钢概率疲劳破坏模型,基于Monte Carlo算法实现了铆接钢桥构件双角钢疲劳断裂失效概率的计算,编制了相应的概率断裂分析程序DAPFF。进而将概率断裂力学评估方法应用于京九线赣江桥的时变疲劳可靠度分析,并给出了赣江桥概率疲劳剩余寿命评估结果与维护对策。     

An experimental study of uniaxial creep,cyclic creep and relaxation of aisi type 304 stainless steel at room temperature

Following previous work (Krempl, 1979), a servocontrolled testing machine and strain measurement at the gage length were used to study the uniaxial rate(time)-dependent behavior of AISI Type 304 stainless steel at room temperature. The test results show that the creep strain accumulated in a given period of time depends strongly on the stress-rate preceding the creep test. In constant stress-rate zero-to-tension loading the creep strain accumulated in a fixed time-period at a given stress level is always higher during loading than during unloading. Continued cycling causes an exhaustion of creep ratchetting which depends on the stress-rate. Periods of creep and relaxation introduced during completely reversed plastic cycling show that the curved portions of the hysteresis loop exhibit most of the inelasticity. In the straight portions, creep and relaxation are small and there exists a region commencing after unloading where the behavior is similar to that at the origin for virgin materials. This region does not extend to zero stress.The results are at variance with creep theory and with viscoplasticity theories which assume that the yield surface expands with the stress. They support the theory of viscoplasticity based on total strain and overstress.     

Very high cycle fatigue for GCr15 steel with smooth and hole-defect specimens

Very high cycle fatigue (VHCF) properties of a low temperature tempering bearing steel GCr15 with smooth and hole-defect specimens are studied by employing a rotary bending test machine with frequency of 52.5 Hz. Both smooth and hole-defect specimens break in VHCF regime with some difference in fatigue crack initiation. For smooth specimens, a fine granular area (FGA) is observed near the grain boundary in the fracture surface of the specimens broken after 10⁷ cycles. But no FGA is observed in the hole-defect specimens broken in VHCF regime, and the VHCF crack does not initiate from the small hole at the surface as it does at low or high cycle fatigue regime. Internal stress is employed to explain the VHCF behavior of these two types of specimens. At last, an advanced dislocation model based on Tanaka and Mura model is proposed to illustrate the internal stress process and to predict fatigue crack initiation life with FGA observed in the fracture region.     

Fatigue and fretting fatigue of ion-nitrided 34CrNiMo6 steel

This work is concerned with the surface treatment (ion nitriding) of fretting fatigue and fatigue resistance of 34CrNiMo6. Tests are made on a servo-hydraulic machine under tension for both treated and non-treated specimens. The test parameters involve the applied displacements δ±80–±170 μm; fretting pressure σ_n=1000–1400 MPa; fatigue stress amplitude σ_a=380–680 MPa and stress ratio R=−1. The ion nitriding process improves both fatigue and fretting fatigue lives. Subsurface crack initiation from internal discontinuities was found for ion-nitrided specimens.