Prediction and evaluation of fatigue life for mechanical components considering anelasticity‐based load spectrum

To accurately predict the degraded fatigue life of components under random loads, anelasticity effect of metal on life estimation needs to be explored. Weighting coefficient and dynamic elastic modulus are introduced to modify the calculation process of the model. By establishing an equal‐amplitude fatigue median surface and applying Miner linear fatigue cumulative damage theory, a fatigue life prediction model of the component under random loads is proposed. The model can be used to calculate the degraded fatigue life of components, which provides a theoretical basis for life estimation under random load spectrum. The two‐dimensional load spectrum is compared with the unprocessed two‐dimensional load spectrum and the one‐dimensional program spectrum in life prediction results. With the comparison of the target life and simulation life and the data calculated by the model of the automobile front stabilizer bar, it is concluded that the data obtained by the modified model are more reasonable and accurate.     

An improved damage model using the construction technology of virtual load spectrum and its statistical analysis

In this work, an improved damage model based on construction technology of virtual load spectrum is developed. A predetermined reliability index is used as the termination criterion to calculate the fatigue life, and the results of statistical analysis have shown that the accuracy of fatigue life prediction result has been greatly improved. The model is targeting cyclic loading applications. The main advantage of the model is the use of construction technology of virtual load spectrum for expanding load spectrum and the Monte Carlo method for correcting fatigue damage. Another advantage concerns the link between predetermined reliability criterion and the fatigue life, which puts forward a new method of calculating fatigue life. The convergence proof of the model is presented, and physical experiment data are shown for validation. Several illustrative examples are presented to confirm the accuracy and efficiency of the proposed damage model.     

Changes in mechanical properties of vehicle components after strengthening under low-amplitude loads below the fatigue limit

Changes in the mechanical properties of vehicle components were investigated after strengthening under low-amplitude loads below the fatigue limit (SLAL). The strengths of vehicle components including fatigue strength, static strength and yield strength were significantly increased by SLAL. The surface hardness of the front axle, composed of a kind of low strength steel without heat treatment, was enhanced with increased strengthening and damaging. The change of surface hardness was reversed completely for the transmission gear, composed of a kind of high strength steel with surface heat treatment. The natural frequency of the transmission gear was appreciably increased by SLAL.     

随机载荷作用下疲劳寿命分布预测模型

为了研究构件在随机载荷下的疲劳寿命分布,建立疲劳寿命分布预测模型.应用雨流计数法,将随机载荷-时间历程处理成以载荷幅值和均值为随机变量的二维联合概率密度函数,得到构件的二维疲劳载荷谱.从Miner累积损伤的定义出发,分析了累积损伤分散性的来源.以此为基础,通过建立等幅疲劳中值Sa-Sm-N曲面,提出了基于二维载荷谱的疲劳寿命分布预测模型,当累积损伤的概率分布已知时,可以估算出构件的疲劳寿命分布,进而得到零件在任一时刻的可靠度.最后,给出了一个具体应用实例.结果表明所提出的疲劳寿命分布预测方法具有较好的工程实用价值.     

The effects of load sequencing on the fatigue life of 2024-T3 aluminum alloy

Current fatigue life analysis of metallic rotorcraft dynamic components are based on a linear cumulative damage concept known as Miner's rule. This type of analysis assumes that there is no load sequence effect that occurs during the fatigue loading history. Past studies have shown that linear cumulative damage analysis of fatigue tests has produced life predictions that have been conservative as well as unconservative. Some of this uncertainty has been attributed to the fact that load sequence effects do exist in most fatigue load spectra. As a first phase the study reported herein was done to evaluate the load sequencing effects that could exist in commercial fixed-wing fatigue load spectra. To evaluate these effects a typical commercial wing spectra was reordered using a scheme that had previously been shown in fatigue block loading to produce the shortest fatigue lives. This scheme starts with the smallest load range in a load sequence and proceeds in ascending order until the largest load range is reached. Tests on open hole test specimens made of 2024-T3 aluminum alloy were conducted on the normal sequence of loads as well as the reordered scheme called lo–hi. Test results showed no significant differences between the fatigue lives of the normal load sequence and the reordered load sequence. A computer program called FASTRAN which calculates total fatigue life using only crack growth data was shown to predict the fatigue life of the spectrum tests with acceptable accuracy.     

Fatigue residual life estimation of jib structure based on improved v‐SVR algorithm obtaining equivalent load spectrum

Fatigue tests of truck crane jib structure have great difficulty, long testing cycle, and high cost. Therefore, with the portfolio strategy for “collection, prediction, measurement and simulation” being introduced, an improved support vector regression (v‐SVR) algorithm is proposed to predict the equivalent load spectrum based on the small measured load spectrum and the fatigue residual life evaluation model is built. First, the v‐SVR algorithm corrected in kernel function, decision function, and parameter optimization is utilized to acquire the equivalent load spectrum. Second, the distribution of critical points is determined by the case‐based reasoning technique. The simulation model of first main stress‐time histories for critical points is established, and the two‐dimensional stress spectrum is obtained by the rain‐flow counting method. Finally, the fatigue residual life of jib structure is estimated by the Forman formula. Taking ZLJ5551JQZ110V truck crane jib structure as an example, the effectiveness of the above method is validated.     

On the prediction of the fatigue life of thin-walled structures based on a statistical model

We present experimental results of fatigue tests of AMg6 aluminum alloy model specimens-plates with a single notch, subjected to fully reversed and asymmetric cyclic axial loading. The experimentally determined lives of model specimens are used as the results of the base experiment for predicting the fatigue strength characteristics of large-scale cylindrical shells with circumferential ring-shaped stress raisers loaded by axial forces of a cyclic nature. A numerical study has been performed of the statistical model adopted for predicting the fatigue life of such shells. __________ Translated from Problemy Prochnosti, No. 3, pp. 112–122, May–June, 2006.     

Methodology for fatigue life assessment of the structural integrity of fighter aircraft

This work describes the present methodology used on Portuguese Air Force (PoAF) aircrafts for fatigue life assessment to maintain its structural integrity. This methodology uses experimental flight data collected by airborne instrumentation systems. The recorded data are analysed for the prediction of the crack growth of defects at critical components of the aircraft and this information is then used for planning non‐destructive inspections, maintenance, logistical support and operational usage of the various fleets. This methodology is exemplified by one application on DASSAULT/DORNIER ALPHA‐JET aircraft.     

A reliability-based fatigue design for mechanical components under material variability

The mechanical components subjected cyclic load unusually fail due to fatigue. The traditional deterministic design method still has the risk of failure while the safety factor method sometimes is overconservative and uneconomic. In this study, reliability-based design optimization is applied in structural design of components under low cycle fatigue. A constitutive model (Jiang and Sehitoglu model) was written into user-defined material subroutine of finite element software to make simulation more accurate. In addition, an adaptive least squares support vector machines (LS-SVM)-based response surface method is employed to improve the efficiency of design process. After constructing the implicit life model, a hybrid directional step method is employed to implement the performance measure approach. Finally, a simple case (thickness optimization for cantilever tube) is used to demonstrate the whole procedure of proposed design procedure.     

Multiaxial fatigue life calculation model for components in rolling‐sliding line contact with application to gears

Important components such as gears, rollers, or bearings operate in rolling‐sliding contact loading conditions. Determination of their fatigue lives remains a challenging task due to complex states of stress and strain in the contact region, as well as complex contact conditions such as variable loading amplitude and complex geometry of contact. A mathematical model of rolling‐sliding line contact combined with a multiaxial fatigue life calculation model based on the Fatemi‐Socie critical plane crack initiation criterion is proposed. The developed model was applied to gears' teeth in mesh and compared with fatigue lives of gears reported in the literature. Good agreement was determined confirming the validity of the proposed model. A further advantage is obtaining locations of initiated cracks and the orientation of critical plane(s), which can subsequently be used for the estimation of crack shapes in initial phases of their growth and the damage type that they can be expected to develop into.     

An anisotropic damage model based on microstructure of boom–panel for the fatigue life prediction of structural components

Because of their simplicity, many isotropic damage models have been used to approximately predict the fatigue life of metallic engineering components. However, experimental observations confirm that the anisotropic damage evolves at probable failure sites even for isotropic materials. In this study, a model of microstructure of boom–panel is constructed to simulate a representative volume element (RVE), and the anisotropic damage of the RVE is described by the independent isotropic damage of boom and panel. Firstly, the constitutive equation of the RVE in terms of stiffness of boom–panel is deduced by the principle of deformation and static consistency. Then the expressions of damage‐driving force for boom and panel based on the principle of thermodynamics are introduced, and the damage evolution equations are constructed. The parameters of boom and panel are identified from fatigue test data of uniaxial tension and pure torsion, respectively. Finally, the aforementioned method is applied to predict the fatigue life of two structures: one is Pitch‐Change‐Link, which is a kind of structure in helicopter, and the other is a specimen under tension–torsion. The prediction results all fit well with the experimental data.     

Multiscale fatigue life prediction model for heterogeneous materials

A multiscale fatigue life prediction model is developed for heterogeneous materials. The proposed model combines a two‐scale asymptotic homogenization approach in time with a ‘block cycle jump’ technique into a unified temporal multiscale framework that can be effectively utilized for arbitrary material architectures and constitutive equations of microphases. The unified temporal multiscale approach in combination with a spatial multiscale approach based on the reduced order homogenization is characterized for high temperature ceramic matrix composites. Copyright © 2012 John Wiley & Sons, Ltd.     

考虑载荷作用次数的机械零部件可靠性灵敏度分析方法

 为了研究载荷多次作用时,机械零部件的可靠度及可靠性灵敏度变化规律,从灵敏度角度修改零件的设计参数,降低制造成本,建立一种可靠性模型结合了随机摄动法、Edgeworth级数技术,并考虑了载荷的作用次数．摄动法和Edgeworth级数可以在基本随机参数的前4阶矩已知的情况下,研究具有任意分布参数的机械零件的可靠性灵敏度设计问题,顺序统计量理论考虑了载荷作用次数在可靠度和灵敏度计算中的影响．使用这种模型计算出的可靠度会随着载荷作用次数而变化,这与静态的可靠度计算方法存在差别．以某一型号的螺栓为算例,应用此模型计算了其可靠度、随机变量均值和方差的可靠性灵敏度．由提出的方法得到了可靠度和可靠性灵敏度值及其随载荷作用次数变化的曲线．可靠度及可靠性灵敏度随载荷作用次数变化的规律是：载荷作用次数增加,可靠度值降低,变化趋势单调;载荷作用次数达到最大时,可靠度达到最小;随机变量均值和方差的灵敏度随载荷作用次数变化出现不同的变化趋势,其中螺栓截面直径的均值和方差灵敏度随载荷作用次数的变化最大,随作用次数的增加,螺栓截面直径的参数将对螺栓的可靠性起主要的决定作用．     

Proposed life prediction model for laser-formed high-strength low-alloy curved components

Techniques employed for material processing using laser technology are progressing at a rapid pace. One such technique is that of forming sheet metal plates. This high‐intensity localized heating process allows for forming of metallic sheet materials without the need for expensive tools and dies or any mechanical assistance. The fundamental mechanisms related to this process are reasonably well understood and documented but there remain areas that require further research and development. One such area is the fatigue behaviour of sheet materials manufactured by this novel process. Hence, the proceeds of this paper deal with fatigue life prediction of sheet metal components laser‐formed to a radius with a curvature of approximately 120 mm. The approach to this proposed model considers the mean stress relationship as given by Gerber and a prediction model derived from combining the aspects of life prediction models according to Collins and Juvinall & Marshek.     

A multiaxial high-cycle fatigue life evaluation model for notched structural components

A model for multiaxial high-cycle fatigue life evaluation of notched structural components is proposed, which considers the impact of the stress field on fatigue life by utilizing the Theory of Critical Distances (TCD) and Finite Element Method (FEM). The maximum shear stress range plane is defined as the critical plane, and the damage parameters are the maximum effective shear stress amplitude and the maximum effective normal stress, which are obtained by averaging the stress in the hemisphere volume around the maximum stress point. To validate the accuracy of the model, multiaxial fatigue tests are carried out for both smooth and notched specimens of Aluminum–Silicon alloy. The results indicate that the evaluated life and experimental life have a good agreement.     

A multi‐axial low‐cycle fatigue life prediction model considering effects of additional hardening

It is generally accepted that the additional hardening of materials could largely shorten multi‐axis fatigue life of engineering components. To consider the effects of additional hardening under multi‐axial loading, this paper summarizes a new multi‐axial low‐cycle fatigue life prediction model based on the critical plane approach. In the new model, while critical plane is adopted to calculate principal equivalent strain, a new plane, subcritical plane, is also defined to calculate a correction parameter due to the effects of additional hardening. The proposed fatigue damage parameter of the new model combines the material properties and the angle of the loading orientation with respect to the principal axis and can be established with Coffin‐Manson equation directly. According to experimental verification and comparison with other traditional models, it is clear that the new model has satisfactory reliability and accuracy in multi‐axial fatigue life prediction.     

超深矿井提升机卷筒动态应力与疲劳寿命研究

超深矿井提升机卷筒在钢丝绳多层缠绕时所受钢丝绳的缠绕力错综复杂,在长期作业过程中卷筒结构可能会萌生裂纹进而发生疲劳破坏,所以有必要对卷筒结构在多层缠绕实际工作条件下的疲劳寿命展开研究。基于板壳理论,将超深矿井提升机卷筒视为均匀受压的旋转对称壳体,通过对卷筒结构负荷构成进行分析,建立了钢丝绳对卷筒作用力的数学模型,并根据提升系统动力学原理建立了钢丝绳满载上提和空载下放的动力学模型,确定了卷筒结构的动态载荷。通过对卷筒结构作业过程强度分析得出卷筒结构在整个工作循环中的应力-时间历程曲线。结合不同存活率的S-N曲线和累积损伤理论,分析了卷筒结构在不同负载下的疲劳寿命。研究结果表明：提升机满载上提和空载下放过程中卷筒结构的最大等效应力均出现在等速阶段,且其最大等效应力出现在卷筒内壁支轮与支环的中部;在给定的存活率下卷筒结构的疲劳寿命与工作负载密切相关。研究结果为多层缠绕复杂工况下卷筒的力学分析与科学设计提供了理论支撑。     

Energy density zone model and fatigue life prediction considering microscopic effects

An energy density zone (EDZ) model is developed for the prediction of fatigue life. The microscopic effects can be involved in the EDZ model. Three scale transitional functions in the model are utilized to describe the trans‐scale behaviours of fatigue failure from micro‐scale to macro‐scale. Fatigue failure behaviours of a low‐alloy and ultra‐high‐strength steel material (i.e. 40CrNi2Si2MoVA steel) is investigated. Two fatigue parameters in the model are determined from the experimental S–N curves for the smooth cylinder specimens (the stress concentration factor, SCF, K_t = 1). Then, fatigue lives of notched specimens with SCFs K_t = 2 and K_t = 3 are predicted respectively by the proposed model. The predicted S–N curves are satisfactory in comparison with the experimental results. Scatter of the fatigue test data can be depicted when the microscopic effects are considered. Influences of microscopic effects on the fatigue behaviours are explored by means of numerical simulations.     

316L不锈钢的高温疲劳蠕变行为和寿命预测

进行316L不锈钢在单级和两级载荷作用下的高温疲劳蠕变试验,研究了载荷历程效应对材料行为的影响.在已有统一的疲劳蠕变损伤演化模型基础上,得到了316L高温单级载荷作用下非线性损伤演化曲线.同时,建立了一种耦合载荷历程效应的多级疲劳蠕变载荷作用下的材料破坏准则.基于该破坏准则,结合材料的非线性损伤模型对316L不锈钢高温两级载荷作用下的疲劳蠕变寿命进行了预测,预测结果与试验数据符合得比较好.     

An equivalent uniaxial fatigue stress model for analyzing landing gear fuse pins

We study the biaxial stress state conditions in landing gear fuse pins in the fuse groove. This biaxial state comprises a combination of shear stresses which are usually the largest stresses in the fuse pin by design, and compressive stresses which keep the half-section of the fuse pin in equilibrium. Conventional fatigue analysis techniques use an equivalent uniaxial stress, based on the Mises stress of a pure-shear condition. The respective predicted fatigue damages are much higher than those obtained from fuse pin cyclic tests. A new equivalent uniaxial fatigue stress model is proposed that includes the additional compressive stress as a relief on the fatigue damage in the fuse groove, thereby explaining the observations from fuse pin tests. The model is used in conventional uniaxial strain-life fatigue software (Goodrich Aerospace’s Fatigue Life V2) to predict the fatigue damage on a landing gear fuse pin with a sample load spectrum. The results are then compared to the pure shear model, and to a biaxial finite element fatigue analysis. As compared to the equivalent Mises model, the proposed model provides less conservative estimation of the fuse pin fatigue life, the latter value being higher than that provided by the two-dimensional finite element calculation. __________ Translated from Problemy Prochnosti, No. 3, pp. 85–98, May–June, 2006.