Fatigue Damage Accumulation Rule Research Articles

Estimation of equivalent stresses and equivalents of the fatigue test programs of airframe composite elements

It is known that for metal components of aircraft structures, stress is one of the most important parameters used to assess the fatigue damage resistance of a structural element for a given program of its fatigue tests. The method and the procedure for estimating the values of this parameter for metal elements of aircraft structures are presented It is noted that for aircraft structural elements made of polymer composite materials, neither the concept of the stress parameter nor the methods for calculating estimates of the values of this parameter are currently defined in domestic and foreign studies of the fatigue strength of such elements. In order to achieve particular progress in the considered area, the definition of the stress parameter of fatigue test programs for composite aircraft structural elements is proposed. For the case of uniaxial tension-compression load of laminates from layered composites representing the upper and lower panels of composite wings of transport category aircraft, a method for calculating the estimation of this parameter is proposed. It is shown that using the stress parameter, the following main tasks can be solved: assessment of the damage resistance of a structural element with a given program of its fatigue tests; comparison of damages resistance of various programs, calculation of equivalents between the programs; calculation of fatigue life of layered composites samples and elements using the fatigue curve under regular load. It is noted that the fundamental meaning of the proposed method for calculating the equivalent stresses and equivalents of fatigue test programs is the use of a special hypothesis of a fatigue damage accumulation rule. An example of calculating the equivalent stresses and equivalents of various modifications of the quasi-random program for CFRP T300/5208 [45/0/-45/90] 2s samples of fiber-carbon composite with the open hole is presented. The fatigue life of these samples under load with the considered modifications was calculated using the obtained values of equivalent stresses and equivalents. Good agreement between the calculated results and experimental data is shown.

Effects of fatigue damage and wear on fretting fatigue under partial slip condition

The continuum damage mechanics model incorporating wear was developed in the previous paper by the authors [27] to predict the fretting fatigue crack initiation life for both cases of partial slip and gross sliding. In the present study, the effects of external parameters on the fretting fatigue crack initiation behaviour under partial slip condition are investigated systematically. Firstly, a different fatigue damage accumulation rule is used, which can predict a more reasonable damage process. Secondly, the effects of the relative slip amplitude and wear coefficient are evaluated based on the improved approach. It is significant to note that the impact of wear coefficient on the fretting fatigue life is non-monotonic, and the action mechanism of that is analysed in detail in this work. Finally, the combination and competition between fatigue damage and wear are presented. The results indicate that none of the fatigue damage and wear can be neglected in the fretting fatigue life prediction.

Probabilistic fatigue life analysis of welded steel plate railway bridge girders using S–N curve approach

In this article, a generalized procedure for estimating probabilistic fatigue life of steel plate railway bridge girders with welded connections, considering plate breathing and a loading spectrum, is presented. The procedure combines the probabilistic S–N curve with Palmgren-Miner’s fatigue damage accumulation rule. One of the features of the study is determination of effect of the modeling error associated with the S–N curve on estimation on the fatigue life of bridge girders. Expressions for probability density functions of number of cycles to failure and accumulated fatigue damage are obtained in the closed form and Monte Carlo simulation, for the cases of without and with modeling error considered in fatigue life estimation, respectively. The use of the proposed procedure is illustrated by considering two railway plate girder bridges designed according to Indian Railway Standards. From the results obtained, it is noted that plate breathing is an important mechanism to be considered while estimating the fatigue life of the railway bridges. The results of the present investigation clearly bring out the importance of carrying out fatigue reliability analysis of different bridge spans, while establishing railway line reliability (as reliability of the weakest span would govern the line reliability).

Reliability Calculation of Design Point Method for Orthotropic Plate

The design point method is a commonly-used method on fatigue reliability analysis. It is put forward, based on Palmgren-Miner linear fatigue damage accumulation rule, that the evaluation method of fatigue reliability for orthotropic plate in this article. It is analyzed the random variables of limit state equation of single component, established the three parameters of stress amplitude , fatigue life and constant , and derived the calculation method on reliability index of design point method, which would provide evidence for consequent design of Orthotropic Plate.

Fatigue Reliability Assessment for Long-Span Bridges under Combined Dynamic Loads from Winds and Vehicles

During a bridge’s life cycle, the stresses from multiple dynamic loads might be superposed and progressive fatigue damage might accumulate and induce serious fatigue damage issues. This paper presents a comprehensive framework for fatigue reliability analysis of long-span bridges under combined dynamic loads from vehicles and wind. After modeling the complicated structure details with equivalent orthotropic material, dynamic stress ranges of a long-span bridge are obtained via solving the equations of motion for the vehicle-bridge-wind dynamic system with multiple random variables considered, for instance, vehicle speed, road roughness conditions, and wind velocity and direction. After counting the number of stress cycles at different stress range levels using the rain flow counting method, fatigue damage increments are obtained using the fatigue damage accumulation rule. The probability of failure for the fatigue damage at the end of each block of stress cycles and the cumulative probability of failure can be obtained. As a result, the fatigue life and reliability for the given structure details can be obtained. Based on the results from a case study, the dynamic effects from vehicles are found to be relatively small for long-span bridges and the effects from vehicle speeds and road roughness conditions can be neglected. Nevertheless, even though the stresses from either the vehicle loads or wind loads may not be able to induce serious fatigue problems alone, the superposed dynamic stress ranges cannot be ignored for fatigue reliability assessment of long-span bridges.

Continuum damage mechanics application in low-cycle thermal fatigue

In this article, a fatigue model for low-cycle thermal fatigue formulated in a continuum damage mechanics framework is presented. The model is based on a unified damage law presented by Lemaitre for low-cycle fatigue, which has been extended to low-cycle thermal fatigue. The temperature dependencies of material parameters are considered in the damage evolution integration to take the non-isothermal condition of loading into account. This model considers the stress triaxiality and non-linearity of damage evolution, and it is developed to a fatigue damage accumulation rule in which the load sequence effect is also included. The stabilized structural response under thermomechanical loading motivates the use of uncoupled analysis approach making the model a fast tool suitable for design purposes in the costly and time-consuming field of thermomechanical fatigue life assessment. To demonstrate the capability and ease of use of this model for real industrial applications, the low-cycle thermal fatigue life of a stainless steel engine exhaust manifold which is in an early stage of design is assessed.

Theoretical Calculation of Ultimate Bearing Capacity and Fatigue of Safety Pin of Transmission Tower

In this paper, the hazard of normal operation power lines caused by transmission lines icing is analyzed. On the basis, several technology means of preventing the hazard of transmission lines icing in power grid in China are summarized. A new kind of overload protection fitting which is safety pin is designed in this paper, which can make transmission line dissociate from tower when ice thickness exceed the numerical value that ultimate load related. And then collapse of power transmission tower and disconnection of transmission are prevented and the goal of avoiding and decreasing the ice disasters is achieved. Ultimate bearing capacity of safety pin is researched based on elastic-plastic theory and elastic limit load and plastic limit load of safety pin are obtained. Finally, applying Palmgren Miner linear fatigue damage accumulation rule, fatigue life of safety pin is calculated.

Study on Fatigue Characteristic of Double Block Ballastless Track under Coupling Loads

Regarding double block ballastless track as research subject, three mechanical models were established, which include steel bar and concrete interaction mechanical model, spatial finite element static model of double block ballastless track, train-double block ballastless track vertical coupling dynamics model. According to concrete and steel bar stress time-dependant curves under axial even temperature and shrinkage load, temperature grads load and train load, fatigue load spectrum under coupling loads was obtained by using rain-flow counting method. Based on steel bar and concrete S-N curve, and Palmgren-Miner linear fatigue damage accumulation rule, track concrete layer fatigue life predicting model of double block ballastless track was established. Taking Changsha climatic condition for an example, the fatigue life of double block ballastless track is predicted, and the effects of design parameters to fatigue life are analyzed.

Stochastic fatigue damage modeling under variable amplitude loading

A general methodology for stochastic fatigue life prediction under variable amplitude loading is proposed in this paper. The methodology combines a nonlinear fatigue damage accumulation rule and a stochastic S–N curve representation technique to achieve this objective. The nonlinear damage accumulation rule proposed in this paper improves the deficiencies inherent in the linear damage accumulation rule and still maintains its simplicity in the application. The covariance structure of the stochastic damage accumulation process under variable amplitude loading is included into the methodology by a new stochastic S–N curve approach. A wide range of fatigue data available in the literature is used to validate the proposed methodology, which covers different type of metallic materials under constant and variable amplitude loadings. The prediction results are also compared with existing fatigue models.

A phenomenological fatigue damage accumulation rule based on hardness increasing, for the 2024-T42 aluminum

In this paper the problem of cummulative fatigue damage is considered. A non-linear fatigue damage accumulation rule, suitable for stepwise constant amplitude fatigue loading, is presented for the aerospace material Al-2024-T42. The proposed rule takes into account the previous damage history as well as the loading order effect. This damage function is specified with respect to damage parameters derived by the hardness increase during fatigue of the material Al-2024-T42. Theoretical results are correlated with experimental data for specimens subjected to two stage fatigue loading.

Equivalent life distribution and fatigue failure probability prediction

Equivalent fatigue life distributions under two-level cyclic stress are analyzed experimentally, and a model of describing equivalent life distribution is presented. Based on the equivalent life distribution model, an “equivalent damage – equivalent life distribution” method is developed to predict fatigue failure probability under variable amplitude loading. The underlying principles are fatigue damage accumulation rule and “load cycles – fatigue life” interference theorem. In the equivalent damage – equivalent life distribution method, two parameters, i.e. mean and standard deviation of the (equivalent) life distribution are used to describe cumulative fatigue damage effect. A basic feature of the equivalent damage – equivalent life distribution method is its capability of reflecting the loading history dependent change of the equivalent life distribution. Tests under multi-level stress show a good agreement between predicted fatigue failure probability and the test results.

Effect of external loading on wear maps of aluminium alloys

The continuum damage mechanics model incorporating wear was developed in the previous paper by the authors [27] to predict the fretting fatigue crack initiation life for both cases of partial slip and gross sliding. In the present study, the effects of external parameters on the fretting fatigue crack initiation behaviour under partial slip condition are investigated systematically. Firstly, a different fatigue damage accumulation rule is used, which can predict a more reasonable damage process. Secondly, the effects of the relative slip amplitude and wear coefficient are evaluated based on the improved approach. It is significant to note that the impact of wear coefficient on the fretting fatigue life is non-monotonic, and the action mechanism of that is analysed in detail in this work. Finally, the combination and competition between fatigue damage and wear are presented. The results indicate that none of the fatigue damage and wear can be neglected in the fretting fatigue life prediction.