Fatigue analysis of airfoil under different working conditions

The drive shaft of the conveyor system of a car assembly shop is broken due to fatigue damage. The finite element theory and nCode fatigue life analysis software are used to analyze the fatigue life and fatigue damage of the drive shaft. Firstly, it uses Catia to build 3D models, imports the established drive axis model into ANSYS/Workbench, performs finite element static analysis, and then imports the analysis results into the fatigue analysis software of nCode. Fatigue life analysis is carried out, and the fatigue damage diagram and life diagram of the drive shaft of the conveyor system are obtained by the S-N analysis method of nCode Design-life. The most vulnerable position and life of the drive shaft of the conveyor system are found, and the method of improving service life of the conveyor system’s drive shaft is proposed.

The solar power tower (SPT) receiver design should be able to stand with fatigue damage caused by the passage of clouds, start-up and shut-down. In this paper, to investigate the impacts of incident heat flux distribution and SPT site weather data on the thermal stress, fatigue life and allowable flux density (AFD) of the molten salt receiver, the relationship between the fatigue life and AFD of the tube wall is developed based on the coupled thermal–structural analysis and Miner linear damage theory. The results show that the cosine effect of the circumferential heat flux distribution considered has a significant influence on the location and magnitude of the maximum thermal stress of the tube wall, which lead to the difference in the tube wall fatigue damage. The AFD are, respectively, 829 kW/m2 and 1037 kW/m2 under uniform and cosine circumferential heat flux distributions for the site of Barstow, USA, when the design lifetime of the tube is 30 years. Compared with the SPT site of Barstow, USA, the fatigue damage of the tube wall in Sevilla, Spain, and Delingha, China, are lower under the same conditions due to lower insolation hours of direct normal irradiation in the range of 750–1100 W/m2. The AFD are, respectively, 829 kW/m2, 973 kW/m2 and 997 kW/m2 for the site of Barstow, USA, Sevilla, Spain, and Delingha, China, with 30 years design life. These findings give guidelines for the operation reliability of the SPT molten salt receiver tube.

Jack-up platforms are designed to work in three conditions: Transit, Preloading and Operating. The fatigue life of the jack-up platforms in operating condition will be determined to be similar to that of offshore fixed steel structures. In preloading conditions, fatigue damage is usually ignored. Up to now, the fatigue damage of the jack-up leg structure in transit conditions has been calculated at approximately 20% of the total fatigue damage of the jack-up leg structure in two conditions (transit and operating). The approximate calculation method is usually accepted by consultants and register agencies. If the approximation is used, the fatigue life of the jack-up leg structure will be calculated only as for the jacket structure of the fixed offshore structure, with 80% of the allowable fatigue life based on standards. The approximation proved to have many disadvantages: the different travel times of each jack-up rig cannot be mentioned; hot spots that need to be maintained during the transit condition have not been pointed out; it is difficult to guarantee the safety of the jack-up leg structures in the transit condition. In order to overcome the limitations of the approximation method, this paper will propose a method to predict the overall fatigue life of the jack-up leg structure in three main problems. Firstly, we use the analysis method of fatigue of fixed steel offshore structures for jack-up leg structures in operating conditions. Secondly, we suggest a method to analyze the fatigue of the structures in transit conditions. Herein, motion analysis and determination of inertia forces on the leg structure are performed by the Boundary Element Model (BEM) in SACS software. Then the inertia forces are assigned to a Finite Element Model (FEM) in SACS to decide the internal forces of the structures. Hotspot stresses are determined by combining nominal stress from FE analysis results with a concentrated stress factor from the analysis of joint local models in the ANSYS program. Then, fatigue damage and fatigue life of hotspots of the structure are determined in the transit condition. Finally, a formula is suggested to determine total fatigue damage in operating conditions and transit conditions with different cases in relation to different fraction factors. These results are used to predict fatigue life corresponding to the most dangerous cases of structural joints. These new suggestions are applied to fatigue analysis for jack-up Tam Dao 05. Currently, the Tam Dao 05 platform has been operating in the Vietnam East Sea. Doi: 10.28991/CEJ-2022-08-03-06 Full Text: PDF

Based on the typical aerospace frame structure, the fatigue life law of the structure under multi-axis correlation time-domain random excitation is studied. By constructing multi-axis time-domain random excitation with different correlation coefficients, multi-axial fatigue theory and structural vibration fatigue life time-domain method are used to calculate the fatigue life of the structure under different excitations; by constructing the structural finite element model in MSC. Pastran and nCode-DesignLife, and Co-simulation calculates the structural fatigue life. The results show that there is a negative correlation between the correlation coefficient of the applied multi-axis time-domain random excitation and the structural fatigue life, that is, the larger the correlation coefficient, the shorter the fatigue life.

\nThe purpose of this study is to lower deviation of workpiece by meeting high stability and rigidity to prevent the resonance in producing procedure of the CNC universal cylindrical grinding machine. Using finite element analysis software ABAQUS in grinder machine tools for numerical simulation of several analyses for the following: structural rigidity analysis, optimized design, vibration frequency analysis and fatigue damage analysis. This work aims on state of the transmission of outer diameter spindle to proceed in stress and fatigue life analysis by FE-SAFE Subroutine. The max values of equivalent stress and average amount of displacement in structural rigidity analysis are 0.67(Mpa) and 0.92(µm). Optimization design effectively reducing extreme value of stress, the largest decline of about 5.43%. Modal analysis compared with the experimental, the average error percentage was less than 10% of parts. The whole structure error does not exceed 3%. The fatigue life of approximately 1,193,988 times, estimates into real life time can use more than sixty years, from the viewpoint of structural strength, spindle has a good high breaking strength is designed to be safe.\n

SummaryAn efficient approach for predicting wind‐induced fatigue in large horizontal axis wind turbine coupled tower–blade structures subject to aeroelastic and yaw effects is presented. First, aerodynamic loads under yaw conditions are simulated based on the harmonic superposition method and modified blade element momentum theory, in which wind shear, tower shadow, tower–blade interactions, aeroelastic, and rotational effects are taken into account. Then, a nonlinear time‐history of wind‐induced responses under simulated aerodynamic loads is obtained. Finally, based on these results, wind‐induced fatigue damage and lifespan are predicted according to linear cumulative damage theory. For completeness, the influences of mean wind speed, aeroelasticity, and yaw angle on horizontal axis wind turbine fatigue life are discussed. The results indicate that the aerodynamic loads and residual fatigue life can be estimated accurately by the proposed model, which can be used to simulate the 3D wind fields of wind turbines under given wind conditions. The wind energy of the wind turbine blade is mainly concentrated at its edge and is weaker at the hub. Estimation of wind turbine fatigue life is therefore suggested to be based on the component with the shortest life, being the blade root. Furthermore, yaw conditions significantly shorten fatigue life and should not be ignored. Fatigue life is also rather sensitive to mean wind speed.

Evaluation of ice-induced fatigue life for a vertical offshore structure in the Bohai Sea

Multiaxial variable amplitude fatigue life analysis including notch effects

Structural integrity of aircraft, nuclear power plants, space rockets, ships, automotive structures, biomedical devices, and many other applications, is a major design purpose and concerns various components subjected to cyclic loads for which fatigue is generally the dominant factor leading to its failure. The fatigue damage cumulation phenomenon is a process which may lead to cracks initiation and possibly to the structure failure under the action of variables stresses cycles. The mechanical components are generally subjected to multiaxial stress states. Taking into account this stress states triaxiality generally deals with multiaxial fatigue criteria. They are suitable tools for assessing the material fatigue resistance against periodical stress states, especially when they are multiaxial. The study carried out in this paper aims to analyze the fatigue behaviour of pre-stressed bolts involved in a so-called tee-stub bolted assembly that is subjected to variable loads. A multiaxial fatigue post-processing tool using two multiaxial fatigue approaches (integral approach and critical plan approach) is developed and implemented on Matlab software in order to assess the bolt fatigue damage and then its fatigue life through an iterative process. The tool is validated by fatigue test results on bolted assemblies found in the literature, which are additionally compared with those obtained by standards (Eurocode, VDI). A parametric study on the tee-stub is then performed.

Background: Fatigue damage is one of the main failure modes in ship structures. This type of damage usually starts from weak point of the structure as welded joints, sites of stress concentrations and cracks, whose propagation can lead to the failure of the ship structures. Cyclic loadings that ships encounter during their service life are one of the main causes that can produce fatigue damage, especially loads due to wave, mainly analyzed in this work. Once the crack is started, also lower stress cycles, which would have negligible effects on intact components, can propagate the crack. Objective: This paper wants to resume the most used fatigue strength assessment approaches, in order to analyze drawbacks and advantages and to provide the necessary background knowledge for the development of a future and reliable theoretical/numerical models for predicting the fatigue life of ship structures subjected to collision events and different sea states. Methods: This scientific work will thus consider the main theoretical approaches in time and frequency domain by using energy spectral methods. Collision rules are also assessed to check the hull girder ultimate bending capacity in the damaged state. Results: Cyclic stresses determination in specific structural details of the hull girder and welded joints is discussed, in order to evaluate the relevant maximum stress range useful for the subsequent fatigue studies, performed by finite element analysis. In a collision scenario, also in minor damages, fatigue cracks may appear and propagate, reducing structural strength and structures fatigue life. Conclusion: Structural loads determination can be categorized in different methods, but the frequency-domain method is the most extensively used methods in fatigue analysis procedure, because it requires significantly less computational efforts than the time-domain method. Once determined the stress distribution, in most cases fatigue damage is calculated using the Palmgren–Miner cumulative damage rule in combination with the S–N curves of materials, structural details and welded joints. In a scenario where a damaged ship encounters large wave amplitudes, the damage accumulation could lead to low-cycle fatigue.

Welding defects are known to cause crack propagation and reduce structural fatigue performance. Based on the Paris theory of fracture mechanics, research is conducted on evaluation methods for analyzing fatigue crack propagation by adopting random loads with long-term distribution that follows the Weibull distribution for the stress ranges of fatigue loads. This approach is combined with the corrective stress intensity factor (SIF) equation and the method for calculating the reference stress and failure criterion. A large container ship is selected for a simulation, and fatigue crack propagation analysis is conducted on typical critical locations. A detailed comparison of the forecasted fatigue life is carried out between fracture mechanics theory and the S-N curve. The results indicate that the fatigue life values obtained using the two methods are of the same magnitude. In general, for the welded structure, the fatigue life value obtained via the fracture mechanics method is shorter than that obtained via the S-N curve method, while, for the free edge of the structure and the unwelded structure, the predicted fatigue life value is closer than that predicted via the S-N curve method. Moreover, the influence of initial crack defects on the fatigue life is investigated, and the results show that the depth of the initial crack will greatly affect the fatigue life of the target ship in typical locations, but the influence of the shape ratio on the fatigue life is limited. Therefore, in the actual ship construction process, controlling the initial crack depth of components is effective for limiting crack propagation and improving fatigue life. The above conclusions and suggestions can serve as a reference for the structural design and fatigue life evaluation of large container ships.

Bonded patches are widely used in several industry sectors for repairing damaged plates, cracks in metallic structures, and reinforcement of damaged structures. Composite patches have optimal properties such as high strength-to-weight ratio, easiness in being applied, and high flexibility. Due to recent rapid growth in the aerospace industry, analyses of adhesively bonded patches applicable to repairing cracked structures have become of great significance. In the present study, the fatigue behavior of the aluminum alloy, repaired by a double-sided glass/epoxy composite patch, is studied numerically. More specifically, the effect of applying a double-sided composite patch on the fatigue life improvement of a damaged aluminum 6061-T6 is analyzed. 3D finite element numerical modeling is performed to analyze the fatigue performance of both repaired and unrepaired aluminum plates using the Abaqus package. To determine the fatigue life of the aluminum 6061-T6 plate, first, the hysteresis loop is determined, and afterward, the plastic strain amplitude is calculated. Finally, by using the Coffin-Manson equation, fatigue life is predicted and validated against the available experimental data from the literature. Results reveal that composite patches increase the fatigue life of cracked structures significantly, ranging from 55% to 100% for different applied stresses.

Multiaxial fatigue life assessment of sintered porous iron under proportional and non-proportional loadings

Multiscale-based multiaxial fatigue model of short fiber reinforced polymer composites under high-cycle proportional loading

The established linear fatigue life prediction model based on the Miner rule has been widely applied to fatigue life prediction under constant amplitude uniaxial and multiaxial loading. Considering the physical significance of crack formation and propagation, a multiaxial equivalent linear fatigue life prediction model is put forward based on Miner rule and critical plane method under constant amplitude loading. The essence of this approach is that the equivalent strain, which consists of the shear strain and normal strain on the critical plane, replaces the relevant parameter of uniaxial nonlinear fatigue damage model. The principal axes of stress/strain rotate under non-proportional loading. Meanwhile, the microstructure of material and slip systems change, which lead to additional hardening effect. The ratio of cyclic yield stress to static yield stress is used to represent the cyclic hardening capacity of material, and the influence of phase difference and loading condition on the non-proportional hardening effect is considered. The multiaxial fatigue life is predicted using equivalent stain approach, maximum shear stain amplitude model, CXH model, and equivalent multiaxial liner model under proportional and/or non-proportional loading. The smooth and notched fatigue specimens of four kinds of materials (Q235B steel, titanium alloy TC4, Haynes 188, and Mod.9Cr-1Mo steel) are used in the multiaxial fatigue experiments to verify the proposed model. The predicted results of these materials are compared with the test results, and the results show that these four models can achieve good effect under proportional loading, but the proposed model performs better than the other three models under non-proportional loading. Meanwhile, it also verifies that the proposed enhancement factor can reflect the influence of phase difference and material properties on additional hardening.