On the estimation of fatigue failure under fretting conditions using notch methodologies

ABSTRACT According to experimental evidence, the early stages of fatigue crack propagation under fretting conditions are strongly influenced by the stress gradient generated in the material near the contact zone. This suggests that the crack growth process can be analysed using methodologies similar to those employed to predict the fatigue behaviour of notched elements. This paper assesses the applicability of a number of models originally developed for notched components to fretting fatigue problems. The ability of such models to predict fatigue failure is discussed and compared with experimental results for Al 7075‐T6 specimens that were subjected to fretting fatigue under spherical contact.     

基于单轴缺口试样的一种低周疲劳测试方法与应用

对于漏斗式小曲率半径的缺口圆棒与缺口薄片试样提出了一种采用轴向测量应变的低周疲劳测试方法.通过有限元分析建立小曲率半径的漏斗式缺口圆棒或薄片试样的轴向测试应变到缺口根部Tresca等效应变的应变转换模型,并由疲劳损伤等效假设建立基于缺口试样疲劳实验数据的材料低周疲劳寿命估算模型;完成了大曲率半径漏斗光滑试样和小曲率半径缺口试样的Ti-4Al-2V合金低周疲劳试验,基于缺口试样的疲劳试验所建立的材料低周疲劳寿命估算模型与基于大曲率半径漏斗试样的低周疲劳寿命估算模型有良好的符合度.新方法也方便应用于N18锫合金缺口薄片试样的材料高温单轴低周疲劳行为试验研究,可推荐于材料低周疲劳测试规范修订采用.文中获得的Ti-4Al-2V合金和N18锆合金的不同温度下单轴低周疲劳行为试验数据与寿命估算模型有宜于工程应用.     

坦克扭力轴表面缺陷对疲劳寿命的影响

为探求坦克扭力轴过早产生疲劳断裂的原因,应用弹性有限元方法模拟了坦克扭力轴表面各种形状、尺寸的凹坑处产生的应力集中现象,结合坦克在某一训练科目下的载荷谱,使用疲劳分析软件计算了具有不同形状和尺寸缺陷的扭力轴的疲劳寿命(存活率为99%).结果表明:一旦扭力轴表面出现了凹坑,尤其是出现了狭长的裂纹状凹坑时,其疲劳寿命会急剧下降;当出现狭长的裂纹状凹坑时,将其扩展成等径凹坑(不增大凹坑深度),可使凹坑对扭力轴疲劳寿命的影响降至最低.     

The validation of some methods of notch fatigue analysis

This paper is concerned with the testing and validation of certain methods of notch analysis which the authors have developed theoretically in earlier publications. These methods were developed for use with finite element (FE) analysis in order to predict the fatigue limits of components containing stress concentrations. In the present work we tested and compared these methods using data from standard notches taken from the literature, covering a range of notch geometries, loading types, R -ratios and materials: a total of 47 different data sets were analysed. The greatest predictive success was achieved with critical-distance methods known as the point, line and area methods: 94% of these predictions fell within 20% of the experimental fatigue limits. This was a significant improvement on previous methods of this kind, e.g. that of Klesnil and Lucas [(1980) Fatigue of Metallic Materials , Elsevier Science]. Methods based on the Smith and Miller [(1978) Int. J. Mech. Sci. 20 , 201–206] concept of crack-like notches were successful in 42% of cases; they experienced difficulties dealing with very small notches, and could be improved by using an ElHaddad-type correction factor, giving 87% success. An approach known as 'crack modelling' allowed the Smith and Miller method to be used with non-standard stress concentrations, where notch geometry is ill defined; this modification, with the same short-crack correction, had 68% success. It was concluded that the critical-distance approach is more accurate and can be more easily used to analyse components of complex shape, however, the crack modelling approach is sometimes preferable because it can be used with less mesh refinement.     

Limits to the applicability of LEFM in notch fatigue

A comparison is made between elastic-plastic, and linear-elastic, fracture mechanics parameters, for notch fatigue problems. It is shown that in high-strength aluminium alloy 2024-T351, at normal working stress levels, the errors involved in using LEFM for any crack detectable during non-destructive testing, is likely to be small.The applicability of the method to structural steels is also discussed, and some problems identified.     

Evaluation of fatigue crack initiation life from a notch

Local strain at the notch-root and its effect on fatigue crack initiation was investigated in four metals by the real-time, fine-grid method. Special attention was focused on local notch-root strain behaviour until crack initiation. From the application of strain hysteresis at the notch root, the maximum strain under loading conditions during each cycle was investigated in detail. One of the main results was that the maximum strain value at the first cycle of the fatigue test coincided with that at crack initiation. Maximum strain defined from the cyclic strain changes at the notch root was proposed as one possible parameter for estimating fatigue crack initiation life. Based on the curvilinear relationship between maximum strain and number of cycles to crack initiation, a new life evaluation method for fatigue crack initiation is proposed. This approach differs fundamentally from the usual fracture mechanics method based on the stress intensity factor.     

The effect of notch geometry on critical distance high cycle fatigue predictions

A critical distance method for predicting the fatigue limit stresses of notched specimens was implemented for notched specimens with a wide range of notch dimensions. Circumferentially notched cylindrical specimens (k_t=1.97–4.07) taken from Ti–6Al–4V forged plate were cycled to failure (R=0.1 and 0.5) using a step loading method for estimating the 10⁶ cycle fatigue limit stresses. These experimental data were used in combination with finite element solutions for all specimen geometries to determine a ‘critical distance’, a quantity or parameter determined from the stress distribution surrounding the notch in combination with fatigue limit stress data from unnotched specimens. A unique parameter was not found for all of the specimen geometries. However, predictions for the fatigue limit stresses of the larger notch geometries may be made with some amount of accuracy using a single value of the critical distance parameter, while reasonable predictions for the specimens with the smallest notch dimensions may be made upon the recognition of an apparent size effect.     

Effect of notch severity on fatigue fracture in a rubber-modified glassy polymer

The research concerned the effect of notch severity on the fatigue behaviour of series of rubber-modified glassy polymers, consisting of a styrene-acrylonitrile copolymer with different amounts of an olefin rubber. Tests were conducted under displacement control and two different loading conditions. Both stages of fatigue lifetimes, that is, fracture initiation and crack propagation, were examined. It was observed that the initial notch severity determines the duration of the crack-initiation stage, while crack propagation does not depend on it. The crack velocity appears to be controlled by the maximum applied stress intensity factor, and the correlation does not depend on the rubber content. The results obtained have been interpreted by considering three different zones in the specimen during the fracture process: a far-field viscoelastic continuum, a process zone and a failure zone.     

Effect of notch root radius on the initiation and propagation of fatigue cracks

Under the conditions of constant nominal applied stress, increasing notch root radius causes an increase in the number of cycles to initiate a fatigue crack at a notch root. An explanation of the effect is given in terms of the effective stress concentration factor of the notch. Data are presented which indicate that variations in notch root radius may cause changes in the crack growth rate during the initial stages of propagation from a notch.     

Effect of small notch and absorbed hydrogen on the fatigue fracture in two-step stress test within fatigue limit diagram

It is usually regarded as a common understanding that fatigue failure would not occur if all stresses were kept within fatigue limit diagram. However, it was shown that fatigue failure occurred in some special cases of variable amplitude loading condition even when all stresses were kept within fatigue limit diagram in the case of small-notched specimen. The cause of such a phenomenon was examined using two-step stress pattern for low alloy steel SCM440H. In the case of constant stress amplitude loading, non-propagating crack was formed only at low mean stress and not formed at high mean stress. However, in the case of two-step stress pattern in which the first step stress was chosen as  R  =−1 and the second step stress was with high mean stress, a non-propagating crack was formed by the first step stress. This crack functioned as a pre-crack for the second step stress with high mean stress. Consequently, fatigue failure occurred by the stresses within fatigue limit diagram. In this study, the effect of notch size and shape were examined. The effect of absorbed hydrogen was also investigated. Absorption of 0.3 ppm hydrogen caused more reduction of fatigue limit.     

X20CrMoV12.1钢的组织结构和缺口疲劳强度

用扫描电镜和透射电镜观察X20CrMoV12.1耐热钢在545℃长期运行后的微观组织结构变化,研究了不同运行时间后材料的缺口疲劳强度和硬度.结果表明:经过高温长期运行后,X20CrMoV12.1钢的组织明显退化,其显微组织由原来的马氏体板条结构和弥散分布的细小碳化物逐步转变为亚晶粒和分布在晶界上的粗化球形碳化物组织;随着在高温下运行时间的增加,耐热钢的显微硬度和疲劳强度逐渐降低.其原因是基体中固溶体主要合金元素Cr,Mo,V发生贫化,固溶强化效果降低,沿晶界分布的粗化球形碳化物弱化了材料晶界强度,从而导致X20CrMoV12.1耐热钢高温服役寿命下降.     

The tensile fatigue behavior of CT-specimens with small notch root radius

Fatigue tests with sharply notched compact tension (CT) specimens of 7075-T6 aluminum were conducted for constant ratios of R=σ _min/σ _max and constant notch root radius, ρ. By monitoring the crack propagation throughout the tests, the number of cycles at which the characteristic state of stress near the notch root has no further influence on crack tip velocity could be determined. The results are discussed on the basis of Neuber's theory of macro-support effect. An analogy to linear elastic fracture mechanics is pointed out.     

Detection of a fatigue crack in a rotor system using full-spectrum based estimation

The force due to crack switching has multiple harmonic components of the spin speed. These components excite the rotor both in the same and reverse directions of the rotor spin. A full-spectrum method using complex Fast Fourier transform equations is developed to obtain force coefficients and displacement coefficients (forward and reverse). These coefficients are then used as an input to developed identification algorithms. Fault parameters identified are the additive stiffness due to crack, disc eccentricity due to unbalance, and system viscous damping. An extended algorithm estimates the crack forces. The algorithms are numerically tested.     

Microwave photonic notch filter based on a dual-Sagnac-loop structure

A new single-wavelength, coherence-free microwave photonic notch filter is presented. The concept is based on a dual-Sagnac-loop structure that functions with a new principle in which the two loops operate with different free spectral ranges, and which generate noncommensurate taps. It has the ability to generate a narrow notch response and can operate to high frequencies. Experimental results demonstrate a notch filter with a narrow notch width, a flat passband, and high stop-band attenuation of over 40dB.     

Effect of notch geometry on short fatigue crack growth in 8090 Al–Li alloy

The short-crack propagation behaviour of 8090 Al–Li alloy under different ageing conditions has been investigated. The effect of notch geometry on short fatigue crack growth was also studied. The results show that the geometrical configuration of the notch significantly affects the growth behaviour of the short crack, the growth rates of notched short cracks being much higher than those of long cracks at the same stress intensity factor range ΔK level. The orientations of the specimens had a stronger effect on the growth rate of long cracks than on that of short cracks. This revised version was published online in November 2006 with corrections to the Cover Date.     

Fracture and fatigue analysis of an agitator shaft with a circumferential notch

This paper is concerned with the fracture analysis of an agitator shaft of a large vessel and predicting its high cycle fatigue life. The agitator shaft has a circumferential notch around it and is subjected to remote bending and torque created by the mixing operation. The problem is comprised (i) the analyses of the bending force and torque acting on the agitator by using the analytical method, (ii) calculation of stress intensity factors under mode I and III loading conditions by using finite element method and, (iii) fatigue analysis of the agitator shaft failed in service.An agitator model is set up and data obtained from the agitator are processed to make more realistic approximations for bending forces, since they form a base for stress analysis, in which mode I stress intensity factors are evaluated. Mode I stress intensity factors obtained by finite element analysis are compared with the results provided by using the body force method.