Probabilistic fatigue life prediction methodology for notched components based on simple smooth fatigue tests

The methodology of probabilistic fatigue life prediction for notched components based on smooth specimens is presented. Weakestlink theory incorporating Walker strain model has been utilized in this approach. The effects of stress ratio and stress gradient have been considered. Weibull distribution and median rank estimator are used to describe fatigue statistics. Fatigue tests under different stress ratios were conducted on smooth and notched specimens of titanium alloy TC11. The proposed procedures were checked against the test data of TC11 notched specimens. Prediction results of 50 % survival rate are all within a factor of two scatter band of the test results.     

Prediction of multiaxial fatigue life for notched specimens of titanium alloy TC4

Both the proportional and nonproportional multiaxial fatigue tests were conducted on two kinds of notched specimens of titanium alloy TC4. The multiaxial fatigue critical area of notched specimen is considered as the location experiencing the maximum damage. It is unsatisfactory to predict the multiaxial fatigue life with the local stress and strain in the fatigue critical area. The critical distance concepts are employed in the multiaxial life prediction method for notched specimens. The proposed method was checked by the test data of TC4 notched specimens. The prediction results are almost within a factor of three scatter band of the test results.     

含缺口结构压疲劳失效研究

结构的疲劳失效与内部的应力水平有着紧密的联系。这里阐述了压应力作用下结构内部裂纹的萌生和扩展机理，并介绍了循环压应力下的裂纹扩展计算方程，结合现代疲劳研究方法的新成果新方法对压应力条件下疲劳破坏研究的未来提出展望。     

Prediction of lubrication regimes of concentrated contacts

Friction experiments are performed on lubricated concentrated contacts (LCCs) to obtain the transitions EHL-ML and ML-BL as a function of the operational conditions under which these contacts operate. A transition diagram is developed to determine the lubrication mode of an LCC as a function of the operational conditions. In this investigation the LCCs operated macroscopically in the lubricants' liquid state regime.     

Prediction of lubrication regimes of concentrated contacts

Friction experiments are performed on lubricated concentrated contacts (LCCs) to obtain the transitions EHL-ML and ML-BL as a function of the operational conditions under which these contacts operate. A transition diagram is developed to determine the lubrication mode of an LCC as a function of the operational conditions. In this investigation the LCCs operated macroscopically in the lubricants' liquid state regime.     

Fretting fatigue case studies of engineering components

Fretting fatigue has been studied for nearly a century and a great deal of progress has been made in formulating design practices based on the understanding that has emerged during this period. Nonetheless, many fretting induced fatigue failures still occur and often could have been prevented if knowledge that exists were applied. This paper provides seven brief case studies in various fields of interest. Each case is presented briefly and then the manner used to deal with the issue to prevent its continued occurrence is given. The paper demonstrates that if fretting fatigue failure mode is anticipated and predicted based on knowledge of the factors that lead to fretting fatigue then adequate prevention and control practice can be used to avoid it in the design and use of components. One of the great challenges ahead is to assure all engineering students around the world get educated as to the factors and parameters that cause fretting fatigue to occur and thus design teams could take positive action to prevent it in almost all cases.     

多轴加载下缺口件弹塑性有限元分析及疲劳寿命预测

利用有限元分析软件ANSYS,得到了承受拉扭循环载荷的缺口试件缺口的处精确应变数值,给出了引起疲劳破坏的危险点。在临界平面法的基础上,考虑到法向正应变对疲劳寿命的影响,建立了一个新的多轴疲劳寿命预测模型,该模型可以较好地预测缺口试件的疲劳寿命。     

螺栓疲劳寿命预测

通过材料疲劳寿命方程回归、有限元计算,应用局部应力应变法,预测了螺栓试样疲劳寿命,螺栓试样疲劳试验验证了该方法的适用性及螺栓接触有限元计算结果的精确性.     

多轴低周疲劳寿命预测软件

通过Visual Basic6.0各种控件和算法的运用，编制的“多轴低周疲劳寿命预测软件（MLCFP1.0）”，涉及多种疲劳寿命预测方法，特别适用于结构中出现非比较、多轴应力状态的情况，为多轴疲劳下的寿命预测提供了解决的有力工具。本软件还留有新模型的嵌入接口，数据处理过程实现多文件操作。后处理绘图软件LCF FIGURE1.0，能给出标准的疲劳寿命预测图形，其保存文件形式多样。     

Prediction of the fatigue life of pneumatic tires

A method of the prediction of the fatigue life for parts of pneumatic tires based on the computation of the stress-strain state of the stress concentration areas in a rubber matrix and the self-heating temperature by means of the finite-element method and experimental fatigue curves for rubbercord composites has been developed. Multiple tensile tests of single-layer rubber-cord samples with a squared shape have been carried out, and fatigue curves under various test conditions and types of stress-strain states have been constructed. The fatigue life of the area of the edges of a breaker strip of a 9.00R20HC12 tire under a maximum working load and other parts of pneumatic tires is determined.     

Prediction of fretting fatigue behavior under elastic-plastic conditions

Fretting fatigue generally leads to the degradation of the fatigue strength of a material due to cyclic micro-slip between two contacting materials. Fretting fatigue is regarded as an important issue in designing aerospace structures. While many studies have evaluated fretting fatigue behavior under elastic deformation conditions, few have focused on fretting fatigue behavior under elastic-plastic deformation conditions, especially the crack orientation and fatigue life prediction for Ti-6Al-4V. The primary goal of this study was to characterize the fretting fatigue crack initiation behavior in the presence of plasticity. Experimental tests were performed using pad configurations involving elastic-plastic deformations. To calculate stress distributions under elastic-plastic fretting fatigue conditions, FEA was also performed. Several parametric approaches were used to predict fretting fatigue life along with stress distribution resulting from FEA. However, those parameters using surface stresses were unable to establish an equivalence between elastic fretting fatigue data and elastic-plastic fretting fatigue data. Based on this observation, the critical distance methods, which are commonly used in notch analysis, were applied to the fretting fatigue problem. In conclusion, the effective strain range method when used in conjunction with the SMSSR parameter showed a good correlation of data points between the pad configurations involving elastic and elastic plastic deformations.     

汽车关键零部件的寿命定量设计

汽车关键件疲劳寿命定量设计是一种现代设计方法，它把定量疲劳寿命理论和试验相结合，使疲劳寿命由定性设计提高到定量设计，提高了设计水平，对于汽车制造商来说极具经济价值。     

Prediction of fatigue life distribution of marine propeller materials

Engineering materials have been studied and developed for a remarkably long time, but there are few reports about marine propeller materials. Recently, some researchers have studied the material strength of marine propellers. However, studies on parametric sensitivity and probabilistic distribution of fatigue life of propeller materials have not yet been carried out. In this study, we have evaluated strength characteristics of AlBC3 and HBsC1, both of which have been used for marine propellers using air jet chisel. Then a method to predict the probabilistic distributions of fatigue life of propeller materials is presented and the influence of several parameters on the life distribution is discussed. This paper was recommended for publication in revised form by Associate Editor Jooho Choi Han-Yong Yoon received his B.S. degree in Mechanical Engineering from DanKook University in Seoul, Korea, in 1981. He then received his M.S. and Ph.D. degrees from The University of Tokyo in Tokyo, Japan in 1985 and 1988, respectively. Dr. Yoon is currently a Professor at the Department of Mechanical Engineering at Mokpo National University in Jeonnam, Korea. And, he serves concurrently as the director of Library at his University. His research interests include reliability, fatigue, and fracture mechanics. Jianwei Zhang received his B.E. degree from the School of Electro-Mechanical Automobile Engineering from Yantai University, China, in 2005. He then received his M.E. from the Department of Mechanical Engineering Graduate School of Mokpo National University, Korea, in 2008. Mr. Zhang is currently studying for his doctorate at the Department of Mechanical Engineering, Graduate School of Mokpo National University, Jeonnam, Korea. His research interests include metal fatigue, weld residual stress, and composite materials fatigue.     

Prediction of subsurface stress in elastic perfectly plastic rough components

Understanding and anticipating the effects of surface roughness on subsurface stress in the design phase can help ensure that performance and life requirements are satisfied. One approach used to address this problem is to simulate contact between digitized real, machined surfaces, and then analyze the predicted subsurface stress field. Often, elastic-perfectly plastic contact models are used in these simulations because of their relative computational efficiency. Reported here is an analysis of the magnitude and location of maximum stress predicted using an elastic-perfectly plastic model. Trends are identified which then enable estimation of the upper bound of the simulation results based on surface discretization, operating conditions, and material properties. These estimations can be used as an effective and efficient tool for rapid prediction of maximum subsurface stress in real surface contact.     

Fatigue life evaluation of mechanical components using vibration fatigue analysis technique

Unit brackets attached on a cross member and subjected to random loads often fail due to self-vibration. To prevent such failures, it is necessary to understand the fatigue failure mode and to evaluate the fatigue life using test or analysis techniques. The objective of this study is to develop test specifications for components, which are applicable to predict fatigue life at the stage of initial product design, for the unit brackets by using a vibration fatigue technique. For this objective, the necessity of a fatigue analysis considering resonant effect was reviewed. Also, a series of vibration fatigue analyses were carried out by changing the acceleration’s direction and magnitude. Then, a methodology was proposed to determine the optimum vibration fatigue test specification of the component, which gives an equivalent failure mode with the vehicle test condition.     

Creep stress redistribution in notched bars

The finite element method has been used to determine stress and strain histories in circumferentially notched bass which undergo creep due to steady loading. A semi-circular notched bar and a British Standard V-notched bar are studied. Stationary-state stress distributions for the circular notch are relatively smooth; those for the B.S. notch are non-uniform and peaks develop adjacent to the notch tip where plane strain conditions prevail. High values of the first stress invariant can be obtained from both types of notch. Strain gradients are shown to occur for both notches. Times required to achieve a stationary-state of stress have been determined for both notches; in general the values are much higher for the B.S. notch.     

On size and technological effects in fatigue analysis and prediction of engineering materials and components

Analytical approaches concerning size, stress gradient and technological effects such as surface roughness and residual stresses induced during manufacturing processes are presented and discussed in this paper. Their implementation into the Short-Crack-Model for fatigue-life (lifetime to initiation of cracks of a size of 0.5–1 mm) prediction of engineering components subjected to cyclic loading is explained in detail. The procedures to consider the aforementioned effects are demonstrated by using an example of a forged and tempered steering shaft made of low-alloyed steel subjected to variable amplitude bend loading. The corresponding experimental results are used to check the accuracy of the analytical fatigue-life prediction. The comparison between analytically calculated and experimentally determined fatigue-life values emphasises the significance of technological effects (surface roughness, residual stresses) on fatigue-life estimation and the usefulness of the Short-Crack-Model for fatigue-resistant design of engineering components.     

Improving machine reliability and efficiency by increasing the fatigue life of the components

Analytical determination of the γ life of machine parts in terms of parameters of a set of finite size is considered. The γ strength values of various steels obtained by the proposed method and by the Dubey method are compared.     

Determination of fragments of multiaxial service loading strongly influencing the fatigue of machine components

The paper presents a method for determination of multiaxial load segments from original service histories, where the loaded machine part is meaningfully subjected to fatigue damage. These load segments are directly separated from a service-loading history, which can be of a random character. Typical procedures used for fatigue life assessment under multiaxial random loading are implemented to perform this task. While reduction from the multiaxial stress state to the equivalent uniaxial one, application of the linear multiaxial fatigue failure criteria was proposed. The equivalent stress history is subjected to the rainflow cycle-counting method, which allows to determine the amplitudes and mean values of counted cycles, their occurrence moment and time of duration. Influence of the stress mean value was taken into consideration with the Morrow's model. On the assumption of the linear Palmgren–Miner hypothesis of damage accumulation and the stress–life fatigue characteristics of the material, the damage-time function was determined. The load segments, where the influence on the material fatigue was significant, were determined on the basis of the fixed damage-intensity level and the proposed damage-intensity function. The presented method was studied on a hook loaded with two independent forces. FEM program which has the possibility to perform fatigue analysis was used during the computation for determining the expected place of crack initiation. The service-loading course was compressed to shorter one, so that only a small decrease of the fatigue damage in comparison with full length of the original service loading was observed. The proposed method seems to be right for preparing multiaxial loading histories in order to cut down the fatigue tests, especially in the case when the correlation between particular loading channels is very important.     

Fully plastic analyses of unequally notched specimens in bending

This paper proposes slip line fields for bending of unequally notched specimens in plane strain, that have a sharp crack in one side and a sharp V-notch in the other side. Depending on the back angle, two slip line fields are proposed, from which the limit moment and crack tip stress fields are obtained as a function of the back angle. Excellent agreement between slip line field solutions with those from detailed finite element limit analysis based on non-hardening plasticity provides confidence in the proposed slip line fields. One interesting point is that, for the unequally notched specimen, the difference between the crack tip triaxial stress for tension and that for bending increases significantly with the increase in the back angle. This suggests that such a specimen could be potentially useful to investigate the crack tip constraint effect on fracture toughness of materials. In this respect, the possibility of designing a new toughness testing specimen with varying crack tip constraint is discussed.