Research on Fatigue Curves for Pre-corroded Aircraft Structures

To quantitatively evaluate the effects of corrosion during grounding onfatigue life of aircraft structures, a new power equation is proposed using two-variable linearregression method. That the slope is a constant and the logarithmic intercept is a linear functionof pre-corrosion time makes this equation advantageous: it has a simple form, its parameters haveunambiguous technical and geometrical meanings, and it facilitates engineering applications.Three-parameter equations after pre-corrosion are obtained from back-calculation of fatigue limits,which have been successfully used to predict safe life of aircraft structures in corrosiveenvironment.

Fatigue Life Prediction for SiC/Al Materials Based on Path Planning Algorithm Considering Residual Stress

To explore the influence of path deflection on crack propagation,a path planning algorithm is presented to calculate the crack growth length.The fatigue crack growth life of metal matrix composites(MMCs)is estimated based on an improved Paris formula.Considering the different expansion coefficient of different materials,the unequal shrinkage will lead to residual stress when the composite is molded and cooled.The crack growth model is improved by the modified stress ratio based on residual stress.The Dijkstra algorithm is introduced to avoid the cracks passing through the strengthening base and the characteristics of crack steps.This model can be extended to predict crack growth length for other similarly-structured composite materials.The shortest path of crack growth is simulated by using path planning algorithm,and the fatigue life of composites is calculated based on the shortest path and improved model.And the residual stress caused by temperature change is considered to improve the fatigue crack growth model in the material.The improved model can well predict the fatigue life curve of composites.By analyzing the fatigue life of composites,it is found that there is a certain regularity based on metal materials,and the new fatigue prediction model can also reflect this regularity.

Combine S-N curve and fracture mechanics for fatigue life analysis of welded structures

Based on the evolution of fatigue cracks in welded structures,the fatigue life of welded structures was defined as the sum of the crack initiation life Ni and the crack propagation life Np.Correspondingly,a fatigue-life analysis method combining S-N curves and fracture mechanics theory was proposed.The equivalent structural stress method and the lower 99%boundary of the master S-N curve were used to evaluate Ni,and cracks at the end of the initiation stage were considered as semi-elliptical surface cracks.Moreover,Paris equation and the stress intensity factor range of the cracks were used to evaluate Np.Furthermore,the fatigue test results obtained from the running girder of cranes were used as a reference for comparison and verification of the results.The results revealed that the equivalent structural stress is a good indicator for the crack initiation behavior of complex welded structures.In addition,the predicted fatigue life corresponded closely to the testing life.

GENERALIZED FATIGUE CONSTANT LIFE CURVE AND TWO-DIMENSIONAL PROBABILITY DISTRIBUTION OF FATIGUE LIMIT

According to the traditional fatigue constant life curve, the concept and the universal expression of the generalized fatigue constant life curve were proposed. Then, on the basis of the optimization method of the correlation coefficient, the parameter estimation formulas were induced and the generalized fatigue constant life curve with the reliability level p was given. From P-S-a-S-m curve, the two-dimensional probability distribution of the fatigue limit was derived. After then, three se, of tests of LY11 CZ corresponding to the different average stress were carried out in terms of the two-dimensional up-down method. Finally, the methods are used to analyze the test results, and it is found that the analyzed results with the high precision may be obtained.

Hardness-Based Non-destructive Method for Developing Location Specific S-N Curves for Fatigue Life Evaluation

This paper first describes the importance of using location specific S-N curves for fatigue damage assessment of existing steel structures. It discusses the existing concepts and methods for developing S-N curves using empirical formulae and monotonic strength parameters, such as the ultimate tensile strength and hardness. It also discusses relationships among these monotonic parameters. Then it presents formulae for developing hardness-based full range S-N curves for medium strength steels. The formulae are verified using experimental data obtained from both monotonic and cyclic testing. Finally, it describes the advantages of these hardness-based formulae for developing location specific S-N curves as hardness testing is a non-destructive test which can be carried out on specific locations in structures.

A Model of the Fatigue Life Distribution of Composite Laminates Based on Their Static Strength Distribution

The reasons of the static strength dispersion and the fatigue life dispersion of composite laminates are analyzed in this article. It is concluded that the inner original defects, which derived from the manufacturing process of composite laminates, are the common and major reason of causing the random distributions of the static strength and the fatigue life. And there is a correlative relation between the two distributions. With the study of statistical relationship between the fatigue loading and the fatigue life in the uniform confidence level and the same survival rate S-N curves of material, the relationship between the static strength distribution and the fatigue life distribution through a material S-N curve model has been obtained. And then the model which is used to describe the distributions of fatigue life of composites, based on their distributions of static strength, is set up. This model reasonably reflects the effects of the inner original defects on the static strength dispersion and on the fatigue life dispersion of composite laminates. The experimental data of three kinds of composite laminates are employed to verify this model, and the results show that this model can predict the random distributions of fatigue life for composites under any fatigue loads fairly well.

Vibration Fatigue Probabilistic Life Prediction Model and Method for Blade

Vibration fatigue is one of the main failure modes of blade.The vibration fatigue life of blade is scattered caused by manufacture error,material property dispersion and external excitation randomness.A new vibration fatigue probabilistic life prediction model(VFPLPM)and a prediction method are proposed in this paper.Firstly,as one-dimensional volumetric method(ODVM)only considers the principle calculation direction,a three-dimensional space vector volumetric method(TSVVM)is proposed to improve fatigue life prediction accuracy for actual threedimensional engineering structure.Secondly,based on the two volumetric methods(ODVM and TSVVM),the material C-P-S-N fatigue curve model(CFCM)and the maximum entropy quantile function model(MEQFM),VFPLPM is established to predict the vibration fatigue probabilistic life of blade.The VFPLPM is combined with maximum stress method(MSM),ODVM and TSVVM to estimate vibration fatigue probabilistic life of blade simulator by finite element simulation,and is verified by vibration fatigue test.The results show that all of the three methods can predict the vibration fatigue probabilistic life of blade simulator well.VFPLPM &TSVVM method has the highest computational accuracy for considering stress gradient effect not only in the principle calculation direction but also in other space vector directions.

The Influence of Some Key Parameters on Fatigue Life of Common Metallic Components

The effect of key parameters on fatigue life of common metallic components, e.g. steel, aluminium alloy, has been analysed quantitatively. The influential coverage and degree of these parameters have been investigated systematically, some phenomena which can′t be discovered with qualitative analysis method have been revealed, a series of valuable conclusions has been obtained, which would be very beneficial to fatigue resistant design and improvement of anti fatigue ability of metallic components.

IMPROVEMENT OF THE PLOT METHOD OF FATIGUE LIFE PREDICTION FORMULA

The fatigue life for components can be predicted by the plot method which is easy and convenient in engineering. Based on the usual fatigue life prediction formula, a new formula for estimating the low stress and high cycle fatigue life is proposed and deduced, which has adopted more accurate SN curve relationship——WeibullSN curve formula. The solution of the new formula is given, too. In addition, an example has been calculated and proved in practice. The results of the new formula and the old one are given and compared.

Study on Fatigue Lifetimes and Their Variation of Mg Alloy AZ61 at Various Stress Ratios

In this study, fatigue tests under different R ratios were conducted on the AZ61 Mg alloy to investigate its fatigue lifetimes and fatigue crack growth (FCG) behavior. The fracture surface of the failed specimens was investigated using a scanning electron microscope to study the size of the intermetallic compounds from which the pioneer fatigue crack initiated and led to the final failure of the specimen. To determine the maximum size of the intermetallic compounds existing within the cross section of the specimen at higher risk, Gumbel’s extreme-value statistics were utilized. In the present study, the intermetallic compounds contained within the specimen were assumed to be the initial cracks existing in the material before the fatigue tests. A modified linear elastic fracture-mechanics parameter, M, proposed by McEvily et al., was used to analyze the short FCG behavior under different stress ratios, R. The relation between the rate of FCG and M parameter was found to be useful and appropriate for predicting the fatigue lifetimes under different R ratios. Moreover, the probabilistic stress-fatigue life (P-S-N) curve of the material under different R ratios could be predicted with this method, which utilizes both the FCG law and a statistical distribution of sizes of the most dangerous intermetallic compounds. The evaluated results were in good agreement with the experimental ones. This correspondence indicates that the estimation method proposed in the present study is effective for evaluation of the probabilistic stress-fatigue life (P-S-N) curve of the material under different R ratios.

An improved fatigue life prediction model based on loading sequence

Purpose–In view of the difficulty in determining the key parameters d in the Corten-Dolan model,based on the introduction of small loads,damage degrees and stress states to the Corten-Dolan model and the existing improved model,the sequential effects of the adjacent two-stage load were further considered.Design/methodology/approach–Two improved Corten-Dolan models were established on the basis of modifying the parameter d by two different methods,namely,increasing stress ratio coefficient as well as considering the effects of loading sequence and damage degree as independent influencing factors respectively.According to the test data of the welded joints of common materials(standard 45 steel),alloy materials(standard 16Mn steel)and Q235B steel,the validity and feasibility of the above two improved models for fatigue life prediction were verified.Findings–Results show that,compared with the traditional Miner model and the existing Corten-Dolan improved model,the two improved models have higher prediction accuracy in the fatigue life prediction of welding materials whether under two-stage load or multi-stage load.Originality/value–Because the mathematical expressions of the models are relatively simple and need no multi-layer iterative calculation,it is convenient to predict the fatigue life of welded structure in practical engineering.

疲劳分析软件MSC Fatigue的工程应用

为实现交叉杆的疲劳寿命预测,应用实测载荷和材料数据,进行有限元的应力分析,采用名义应力法,基于MSC Fatigue进行交叉杆的疲劳寿命预测,与交叉杆的实际使用寿命较为吻合.表明基于虚拟疲劳设计软件的疲劳设计方法合理可行,为交叉杆的结构优化设计和疲劳寿命预测提供参考.

CONSTANT AMPLITUDE FATIGUE STRENGTH AND P-S_a-S_m-N CAMBER FAMILY

Functions of the P-Sa-Sm-N camber family in constant amplitude cyclic loading are established by combining the P-S-N curve family and constant life diagrams, which is feasible in engineering application. Meanwhile, the preconditions of mechanics and probability-statistics are clearly presented. According to Weibull's assumptions i.e., monotone decreasing and nonintersecting about individual S-N curves, the relation of isostructural measure preserving transformation (IMPT) between the probability spaces of constant amplitude fatigue life and constant amplitude fatigue strength is proved based on the measure theory. Therefore, an individual specimen possesses the same percentile value in different probability spaces of fatigue life and fatigue strength under constant amplitude loading. The P-Sa -Sm-N cambers of individual, population and sample are also distinguished.

Effect of Loading Frequency on the High Cycle Fatigue Strength of Flax Fiber Reinforced Polymer Matrix Composites

Among natural fibers,flax fiber reinforced polymer matrix composites show excellent dynamic/fatigue properties due to its excellent damping properties.Knowledge about fatigue limit and effect of loading frequency on fatigue limit is very crucial to know before being used a member as a structural component.Fatigue limit of fiber reinforced composite is measured through high cycle fatigue strength(HCFS).The effect of loading frequency on the HCFS of flax fiber reinforced polymer matrix composites was investigated using stabilized specimen surface temperature based thermographic and dissipated energy per cycle-based approaches.Specimens of unidirectional flax fiber reinforced thermoset composites were tested under cyclic loading at different percentages of applied stresses for the loading frequencies of 5,7,10,and 15 Hz in order to determine the stabilized surface temperature of the specimen and dissipated energy per fatigue cycle.Both approaches predicted similar fatigue limits(HCFS)which showed a good agreement with experimental results from Literature.HCFS of flax fiber reinforced composites decrease little with increasing loading frequency.Furthermore,effect of loading frequency on stabilized specimen temperature and dissipated energy per fatigue cycle was also investigated.Although specimen surface temperature increases with loading frequency,dissipated energy per-cycle does not change with loading frequency.Thermal degradation at higher loading frequencies may play a significant role in decreasing HCFS with increasing loading frequency.

Ultrasonic Fatigue Endurance Investigation on Plastic Material Nylon 6

Ultrasonic fatigue tests were carried out on the plastic material Nylon 6. Special attention was devoted to the temperature control in order to avoid physic-chemical transformation of this low melting point material. Under ultrasonic fatigue tests, important heat dissipation takes place at the narrow section of hourglass shape specimen leading to high temperature at this zone. The specimen was calculated to meet the resonance condition with the smallest dimensions at its narrow section, with aim to reduce the temperature gradient at this zone of this non heat conducting material. Temperature at narrow section was maintained lower than 45℃ using a cooling system with cooling air;under this condi- tion the ultrasonic fatigue tests were performed. Experimental tests were carried out at low loading range (9 - 12.5% of the elastic limit of material) in order to control the highest temperature and to avoid that specimen was out of resonance condition. Experimental results are analyzed together with the fracture surfaces and conclusions are presented concerning the ultrasonic fatigue endurance of this polymeric material.

EH36钢焊接接头焊趾处应力集中对高低周复合疲劳的影响

为研究船舶结构用钢EH36钢焊接接头焊趾处应力集中对高低周复合疲劳的影响,对不同焊缝形貌20 mm厚EH36钢双面全熔透对接接头进行高低周复合疲劳试验.研究发现,高低周复合疲劳对接头寿命的降低幅度随着高周应力比的增大而增大;结合数字图像相关法测量结果分析,发现焊趾处应力集中系数越大,接头的高低周复合疲劳寿命越低,其降低幅度随着高周应力比的增大而增大.基于试验数据对模型进行验证,结果表明,应力集中系数高的接头疲劳寿命低且寿命预测结果分散性增大;Palmgren-Miner模型和Zhu模型由于未充分考虑高低周耦合损伤作用,导致寿命预测结果相对危险.修正后Zhu模型在充分考虑低周疲劳对高低周复合疲劳寿命的影响后,预测寿命大于实际寿命的预测点较少,预测结果相对安全,且分散性较小,误差也较小.

考虑静强度的复合材料疲劳寿命预测及概率特性分析

为考虑初始静强度对疲劳寿命的影响,基于经典剩余强度理论,构建同时考虑初始静强度和应力的复合材料疲劳寿命预测模型。建立的寿命预测模型能够表征疲劳寿命与初始静强度和应力之间的内在关系,可退化为经典S-N曲线。疲劳寿命与初始静强度呈非线性关系,疲劳寿命与应力呈高度非线性关系,疲劳寿命的概率分布可根据初始静强度和应力的概率分布利用MCS获得。算例表明,初始静强度和应力的概率特性对剩余强度和疲劳寿命的概率特性均有影响,相比初始静强度,应力对疲劳寿命概率特性的影响更加显著。

基于静强度分布参数的复合材料剩余强度及寿命预测

为探讨剩余强度及疲劳寿命与初始静强度分布参数之间的关系,构建基于初始静强度分布参数的剩余强度和疲劳寿命计算模型。在模型构建过程中未涉及复合材料结构的层数、铺层厚度和铺层方向,适应性强。模型参数可通过静力试验和剩余强度试验获得,试验成本相对疲劳试验较低。剩余强度和疲劳寿命与初始静强度变异系数及其分布参数有关,当初始静强度服从威布尔分布时,疲劳寿命亦服从威布尔分布,可为开展疲劳可靠性提供借鉴和参考。算例表明,基于建立的模型,剩余强度计算结果最大误差为-1.58%,疲劳寿命与试验数据吻合较好。

基于多轴疲劳理论的冶金起重机主梁母材疲劳失效机理研究

冶金起重机金属结构件的主要失效形式是疲劳破坏,疲劳失效多发于疲劳性能薄弱、应力最大或应力集中处。以往的疲劳研究往往只考虑垂直和水平方向的载荷受力,理论设计疲劳强度余量较大。由于实际工况比较复杂,使用一定年限的冶金起重机母材会出现主梁开裂现象,高温环境下、垂直载荷、水平方向冲击载荷、大车轨道高差、钢丝绳吊载晃动等因素会导致拉伸-扭转多轴疲劳载荷。鉴于此,文中基于实际工况背景,以常用主梁箱体钢结构材料Q345B为研究对象,在实验室进行不同加载参量下的多轴高周疲劳试验,从而研究冶金起重机多轴高周疲劳失效的机理。