Papers by Keyword: Random Loading

Abstract: A method of fatigue damage accumulation based upon application of energy parameters of the fatigue process is proposed in the paper. Using this model is simple, it has no parameter to be determined, it requires only the knowledge of the curve W–N (W: strain energy density N: number of cycles at failure) determined from the experimental Wöhler curve. To examine the performance of nonlinear models proposed in the estimation of fatigue damage and fatigue life of components under random loading, a batch of specimens made of 6082 T6 aluminium alloy has been studied and some of the results are reported in the present paper. The paper describes an algorithm and suggests a fatigue cumulative damage model, especially when random loading is considered. This work contains the results of uni-axial random load fatigue tests with different mean and amplitude values performed on 6082 T6 aluminium alloy specimens. The proposed model has been formulated to take into account the damage evolution at different load levels and it allows the effect of the loading sequence to be included by means of a recurrence formula derived for multilevel loading, considering complex load sequences. It is concluded that a ‘damaged stress interaction damage rule’ proposed here allows a better fatigue damage prediction than the widely used Palmgren–Miner rule, and a formula derived in random fatigue could be used to predict the fatigue damage and fatigue lifetime very easily. The results obtained by the model are compared with the experimental results and those calculated by the most fatigue damage model used in fatigue (Miner’s model). The comparison shows that the proposed model, presents a good estimation of the experimental results. Moreover, the error is minimized in comparison to the Miner’s model.

Abstract: This study discusses fatigue properties of low carbon steel under multiaxial non-proportional loading and an evaluation of failure life. Multiaxial fatigue tests under non-proportional loading with various stress amplitudes were carried out using a hollow cylinder specimen in low and high cycle regions at room temperature. In the test, three types of strain/stress path were employed. They are a push-pull, a reversed torsion and a combined push-pull and reversed torsion loadings in which stress amplitudes used were constant and random. This study evaluates an effect of non-proportional loading on fatigue life in the high cycle fatigue region to discuss the applicability of Δε_NP proposed by Itoh et al. on life evaluations in the high cycle region and under random loading.

Abstract: This study focuses on the method of predicting the fatigue life of materials subjected to random loading. Since random stress caused by random loading is rigorously expressed in the frequency domain as stress power spectral density (PSD), fatigue life should be predicted using stress PSD. We propose two adjustment methods of improving the accuracy of fatigue life prediction using stress PSD in the frequency domain. The method proposed by Dirlik is widely used for predicting the fatigue life in the frequency domain; however, it overestimates fatigue damage caused by large stress amplitude when the slope of the fatigue resistance curve is large. To prevent this overestimation, we applied our two adjustment methods to fatigue life prediction for typical random stresses observed on mechanical products. As a result, the adjustment methods worked well in improving prediction accuracy. Lightweight and reliable products can be therefore designed by applying the proposed methods to the evaluation of fatigue life under random loading.

Abstract: It is a common practice in the automotive industry to expose products to accelerated vibration tests, that simulate the load, predicted to occur during the products service time. To avoid long testing times, higher amplitudes are used. Usually such tests come late in the development process, and can result in unexpected costs. A common tool for predicting time-to-failure or expected fatigue-life of the product is the time-domain method, using the rainflow counting algorithm and the Palmgren-Miner summation method. However, if one chooses to apply this method inside a FEM environment on a large amount of nodes with different time histories dependent on the structure excitation, the time-domain method becomes computationally complex. This has led to more effective methods, that estimate the time-to-failure in frequency-domain but are less accurate, compared to the time-domain approach. In this research, a group of such methods is presented and compared using real signals, namely: Tovo-Benasciutti, Wirsching-Light, Petrucci-Zuccarello, empirical α_0.75, Dirlik and Gao-Moan method. Separately, only some of those methods were already compared side by side. Usually the comparison was made on simulated random signals, while this research compares them based on a real signal, collected by measuring different groups of spectra (e.g. typical vibration test profiles, different background noise levels, spectral width, number of modes etc.). In existing studies, Dirlik is usually identified as most accurate but in this research, conclusions show, that the Tovo-Benasciutti and Zhao-Baker methods can be more accurate than the Dirlik method and should therefore also be considered for vibration fatigue analysis.

Abstract: The wind turbine tower is to bear wind load and work load which is brought by the rotating blades during operation. Therefore, analysis of dynamic characteristics of wind turbine tower is a complicated task in the wind turbine reliability design. The dynamic characteristic of wind turbine tower plays an important role in its vibration control. This paper presents fluid-structure coupled dynamics equations accounting for the blade wheel load and wind load applying on the tower. By the model analysis, a feasible method for evaluating the dynamic characteristics of wind turbine tower under the wind turbine wind load and the rotating blade random load is obtained. The model is verified in the 1.5MW wind turbine.

Abstract: Precision of fatigue life calculations of structural elements in programmed loading conditions is connected with proper elaboration of loading spectrum and assumption of a proper fatigue characteristic. On the base of literature data and own research there has been elaborated an algorithm for fatigue life calculations in random loading conditions with wide spectrum. Calculations were performed with the usage of chosen mathematical models of two-parametric fatigue characteristics. Results calculated with accordance to the described procedure were validated with experimental test results of specimens made of 41Cr4 steel with a method of programmed fatigue life tests.

Abstract: A fatigue reliability method has been developed for strain-based fatigue under random loading. The adaptive kernel method is employed to establish the probabilistic random stress model. The probability-based cyclic stress-strain curve considering entire material constants as random variable is proposed. The probabilistic model of cyclic strain life relations is established by introducing a standard normal distribution. Based on these probabilistic models, the random characteristic value of fatigue life is obtained by Monte Carlo method. According to the mean value first order second moment reliability method, the probability of failure can be given out. The example of application is demonstrated.

Abstract: This paper presents the comparative study between the strain signal and its edited signal. In this study, a fatigue strain loading was measured on an automobile coil spring, and the purpose to analyse this component is because it been identified as one of the critical component in an automobile. The strain signal editing process was performed by removing low amplitude cycles contained in the original signal using the short-time Fourier transform (STFT) method. This low amplitude cycles were eliminated based on the cut-off level of the signal energy distribution in the time representation. The original and edited strain signals were then analysed for predicting the fatigue damage of the coil spring. A comparison study of the fatigue damage and the most damaged zone obtained from the strain signal and the edited strain signal was also carried out. It was found that, the prediction of the fatigue damage and the most damages zone for both signals was same. Hence, the shortened signal can be used in the laboratory fatigue testing for the purposes of accelerated fatigue testing.

Abstract: A durability related analysis has been performed using a new statistical-based method, called the Hybrid Integrated Kurtosis-based Algorithm for Z-notch filter (Hybrid I-kaz) Technique. This method provides a two dimensional graphical representation of the signal amplitude and the Hybrid I-kaz coefficient that were used to measure the degree of data scattering. For validation purposes, road tests have been conducted in order to measure the strain and vibration signals on a coil spring. Three road surfaces were used for the road test and data collection purposes. This study used test signals which excited based on three different road surfaces. The 5-mm strain gauge and an accelerometer were fixed on the outer surface of the coil spring for measuring the variable amplitude strain loadings together with the vibration response. The time domain strain and vibration signals were then analysed using global signal statistics and Hybrid I-kaz coefficients. Finally, it was found that the respective coefficients for each signals showed an increment with the total value of strain range and vibration amplitudes.

Abstract: The paper presents a definition of a new energy based parameter which allows to better describe fatigue proprieties of materials, especially cyclically unstable materials in comparison with the Lagoda-Macha parameter. The proposed parameter distinguishes positive and negative work of external force and depends on the sign of the stresses in paths of strain in materials. It is used in laboratory tests for control of a fatigue process on hydraulic stand. A close-loop control system has been equipped with computer program based on MATLAB/Simulink module.