Fatigue Life Prediction of Machined Specimens with the Consideration of Surface Roughness

The fatigue strength and fatigue life of high-strength steels are greatly affected by their surface roughness. This study investigates the underlying mechanisms responsible for fatigue failure of the high-strength steel 42CrMo. Bending fatigue tests of stepped shafts with different levels of surface roughness were conducted to observe the fatigue live reduction affected by surface topography. Besides, the mechanical properties of 42CrMo and its strain–life relationship were established. Moreover, the analytical formulas to describe the stress concentration factor (SCF) and fatigue notch factor (FNF) induced by surface topography were introduced. To estimate the fatigue life of machined specimens with the consideration of surface roughness, the elastic portion of the total strain–life curve of the material was revised with the proposed analytical FNF imposed by surface topography. Comparisons between the estimated fatigue lives and experimentally obtained fatigue lives show that the effect of surface roughness on fatigue lives could be estimated effectively and conveniently by the proposed procedure.

Fatigue Life Prediction of AA6063-T6 under Erosion Condition

The study of fatigue behavior of aluminum alloy 6063 exposed to periodic fatigue stresses was studied in laboratory conditions under the two conditions of the presence of the first indentation in the first test, then the presence of the phenomenon of erosion in the second test resulting from the projection of pure water Jet on samples of the same metal used in The first test. The purpose of these tests was to estimate the practical life of these samples and the resulting accumulation by using upward and downward variable stresses. A mathematical model was built to calculate the life of the samples in the above conditions, and the results of the estimated life of the samples calculated by the model showed a large convergence with the results of the estimated life of the samples practically calculated. So this mathematical model can be used to estimate the life of samples made of different minerals under these same conditions, after knowing the (S-N) curve for each metal and the amount of the value of the fatigue notch factor (Kf), which can be calculated from special tables without referring to performing practical tests for them.

Fatigue Life Prediction of Fan Blade Using Nominal Stress Method and Cumulative Fatigue Damage Theory

Fan blade is one of the key parts used in aircraft engine and its failure is mainly caused by fatigue fracture. This paper aims to predict fatigue life of fan blade during its service operation. First, the effective load and stress of fan blade are obtained by using finite element analysis and simulation. Second, the fatigue notch factor of fan blade is determined by using the nominal stress method. Then, the material properties of fan blade are used to correct and obtain the $S - N$ curve of fan blade. Finally, according to the actual load spectrum of three working loading cycles in 900h, the Miner’s damage accumulation rule is employed to predict the fatigue life of fan blade.

A fatigue life prediction method from magnetic pole material to simulation part

Due to the higher reliability needs of the large moving component motor-generator rotor, the assessment of the service life has drawn more and more attention. After finite element analysis of the rotor, the simulation part which can represent the magnetic pole with the most dangerous position of the rotor was designed to investigate the S–N curves. Compared with the conventional specimen, considering the main influencing factors of fatigue life for simulation part, the comprehensive factor was proposed to establish the fatigue life relationship between magnetic pole material and simulation part. It was found that the calculation method of fatigue notch factor based on the notch sensitivity factor is relatively simple and practical, and there is no significant effect of surface roughness on high and low cycle fatigues for low roughness ( R a is about 1 µm), and the dimension factor changes linearly with the scale factor. Based on those results, a fatigue life prediction method was proposed and validated, and the predicted results were in good agreement with the experimental data. This study will provide a reasonable reference to determine the fatigue life prediction of large moving components.

Strain-Based Fatigue Reliability Analysis of a Load-Carrying Fillet Welded Cruciform Joint

The objective of this work is to perform a fatigue reliability analysis of a load-carrying fillet welded cruciform joint based on the local strain approach. The effective notch stresses of the weld root and toe of the cruciform joint, where the fatigue cracks are usually initiated, are estimated by the finite element method and the fatigue notch factors of these two locations as a function of the weld leg and slit lengths are explicitly represented by response surface models. Within the context of the local strain approach, a critical fatigue notch factor that can exactly trigger fatigue failure is proposed. The statistical descriptors of the critical fatigue notch factor are determined by using the Monte Carlo simulation method, in which the nominal stress range, material properties and fatigue damage at failure are treated as random variables. The limit state functions of the weld root and toe are formulated based on the response surface models and critical fatigue notch factors. The first order reliability method is applied to evaluate the reliability against the fatigue damage. Finally, the cruciform joint system, composed by the two fatigue-prone locations, is evaluated as a series system of components.

Fatigue Reliability Assessment of Fillet Welded Cruciform Joints Based on the Fatigue Notch Factor and Local Strain Approach

The objective of this work is to analyze the fatigue reliability of fillet welded cruciform joints considering the uncertainty of the load and capacity. The weld shape is defined by multivariate normally distributed variables, which represent the position variations of the shape control points on the fillet welds. Finite element analyses are performed to calculate the fatigue notch factors of the weld root and toe, where the fatigue crack is usually initiated. Various weld shapes associated with various correlation conditions and weld quality levels are generated and the corresponding probability distributions of the fatigue notch factors are obtained by using the Monte Carlo simulation method. Sensitivity analyses are carried out to identify which location is more important for the fatigue notch factors. Within the context of the local strain approach, a critical fatigue notch factor that can exactly trigger fatigue failure is proposed. Its statistical descriptors are determined by using the Monte Carlo simulation method, in which the nominal stress range, material properties and fatigue damage at failure are treated as random variables. The limit state functions of the weld root and toe are formulated based on the actual and critical fatigue notch factors. The first order reliability method is applied to evaluate the fatigue reliability. The cruciform joint, composed by two fatigue-prone locations, is evaluated as a series system of components. Two different loading conditions, which make the cruciform joints load-carrying and non-load-carrying respectively, are considered.

Fatigue Behaviour of Friction Stir Welded AZ31B Magnesium Alloy Joints

Friction stir welding (FSW) is a relatively better joining technique particularly for magnesium and aluminum alloys that are difficult to weld by fusion welding techniques. Fusion welding of these alloys is not preferable due to hot cracking, formation of porosity, etc. However solid state welding techniques, such as, friction sitr welding are found to offer solution to the above problems. Many research papers available in open literature focusing tensile properties, microstructural characteristics, and corrosion behaviour of friction stir welded AZ31B magnesium alloys but fatigue behaviour of these welds are not yet investigated. Hence, in this investigation, an attempt has been made to evaluate fatigue behaviour of friction stir welded rolled plates of AZ31B magnesium alloys.Fatigue experiment was conducted using servo hydraulic controlled fatigue testing machine. Fatigue strength, fatigue notch factor and notch sensitivity factor were evaluated. It is found that the fatigue strength of AZ31B welded joints is 46 MPa at 2x106cycles which is approximately 34 % lower than that of the base metal fatigue strength.

Applicability of Empirical Formulae for the Fatigue Notch Factor to Estimate Riveted Lap Joint Fatigue Strength

A semi-empirical fatigue life prediction model under development by the present authors for riveted lap joints used in aircraft structures is outlined. In contrast to existing models, it will account for the influence of the rivet squeeze force on the fatigue life of riveted joints. To determine the effect of rivet-hole interference on the fatigue behaviour of a riveted joint, a series of fatigue tests on filled hole coupons with different amounts of interference will be carried out under loading conditions representing the bypass load, transfer load and secondary bending. These experiments will allow evaluating of the dependency of the fatigue notch factors on rivet hole expansion. Preliminary results obtained so far are presented in this paper.