Papers by Keyword: Vibration Fatigue

Abstract: Stiffened Thin Wall Panel Structure is a widely used structural configuration in the design of aircraft’s inlet duct and large open bay. Vibration fatigue failure of stiffened panel structure will quickly emerges under strong broadband random loading environment which caused by shock waves moving at the inlet lip and cavity vortex and shear layer oscillation during flight. At present, a main research focus on dynamic strength design of aircraft’s light-weight structure is to reduce the dynamic stress response level in broadband random vibration environment ,so as to improve the ability of thin wall panel’s vibration fatigue property with light structure weight. Taking a stiffened thin panel of inlet duct as a typical case under random vibration excitation environment, an Advanced dynamical topology optimization methods is established based on the parametric modeling technique . Within the main frequency domain of external dynamic load, the maximum root mean square of stress (MRMSS) for global element of stiffened panel structure is calculated and optimized under the weight constraint with Genetic Optimization Algorithm (GOA). The comparison of structural stress response before and after optimization design shows that the maximum element RMS dynamic stress is reduced by 38% with the weight increased by about 9.8% and the purpose of improving vibration fatigue property is reached.

Abstract: Traditional fatigue analysis approaches could not solve the structure fatigue failure caused by resonances. The time domain simulation approach which took the structural intrinsic property into account was presented to solve vibration fatigue problems. The vibration fatigue failure of the frame of vehicle equipment was studied. Broadband vibration loads were put into the finite element model of the frame as excitations, the stress power spectral density of a dangerous point was calculated. At last, the Dirlik method and the time domain simulation approach were used to calculate the fatigue life. The results show that the damage caused by the first-order vertical flexural vibration is the largest; the result of the time domain simulation approach has a good agreement with the experimental result when compared with the frequency domain method.

Abstract: The fatigue problem of structures under concurrent thermal and vibration loading has not been thoroughly studied even though it is common in applications of aero-engine combustor liners. Here we attempt to explore such a problem using a simplified combustor liner model that is implemented by the commercial finite element software ANSYS Workbench. The modal parameters at various temperatures are calculated and the fatigue behavior under stochastic base excitation and thermal environment are analyzed. The results show that thermal loading not only has an effect on dynamic characteristics but also reduces the vibration fatigue life of the structure.

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: Socket welds are widely used in nuclear power plants. According to industry experiences, vibration fatigue of socket welds is becoming one of the major ageing issues. It is very important for the operating engineers to monitor the vibration condition and detect the damage before leak occurs. This paper studies the damage identification technology applying wavelet transform method. Small bore pipes with socket welds are installed on the vibrating table and vibrate at the frequencies that near its first natural frequency. The acceleration response signal is acquired and decomposed by wavelet method. The energy of decomposition signal shows a suddenly variation when crack occurs. From this study, a parameter that is sensitive to vibration fatigue is defined and has the potential application value in the nuclear power plants to identify the vibration fatigue damage.

Abstract: In order to determine the fatigue limits of two kinds of titanium alloy joint pipes connected by welding and rolling, the fatigue tests were carried out by test standard which defines the test method and failure rule. One cantilever-beam specimen was fixed on the shaker by the special fixture. The laser-displacement-sensor was fixed at the free end of pipe and the strain-gauge was adhesive at the root of the pipe. The first frequency of specimen was measured by these sensors. The first resonance frequency fell rapidly with the increment of the excitation loads of the shaker. The failure rule (1% drop of resonance frequency) of the standard was not same with the fatigue limit test of these connected pipes at the test beginning. In order to define the lives of two kinds of pipes, the stair method of sweep test was used to eliminate the effect of connection gap. Then the vibration fatigue of every specimen was tested at the last resonance frequency. And all the fatigue tests were finished using the failure rule which defines 1% drop of the steady resonance frequency.

Abstract: In order to determine the fatigue limits of two kinds of titanium alloy pipes connected by welding and rolling, fatigue tests were carried out by the Aero-Criterion which gives vibration fatigue test method and failure criteria. A laser-displacement-sensor was used at the free end and a strain-gauge at the root of the pipe specimen. The test result shows that the fatigue limit of the welded pipe is higher than the rolled one. In the end some new findings are listed according to the test.

Abstract: A new analytical method for vibration fatigue life of cracked beams was proposed with the structural response changes as crack growth. In analysis, the damping loss factor was introduced by the complex elastic modulus, and the crack growth was simulated by employing a Paris equation, and the coupling effect of vibration and crack growth was considered through vibration analysis and estimation of fatigue crack propagation cycles by cycles. Results indicate that impacts of exciting frequency and damping on the crack growth are obvious. The resonance fatigue crack growth rate decreases rapidly when big damping is involved, and the first mode is more important to the crack growth than that of other mode.

Abstract: There are different modes of damage in any engineering structures, and most of them are cracks. In order to study the influence of coupling effect on the fatigue life, a calculation method of structure vibration fatigue life with crack propagation is proposed. In analysis, a series of finite element model with crack of different length is built to simulate the crack propagation, and Paris equation is employed to calculate the vibration fatigue life by stepwise method. The crack initiation life is got based on the change law of natural frequency from test results, and the total life is calculated in the end. Results indicate that the simulation results identical with the experimental results well.

Abstract: Based on the fatigue behaviors of welded structure and the distinction between static fatigue and vibration fatigue and sound fatigue, the general fatigue analysis methods of static fatigue and vibration fatigue are introduced in this paper, such as nominal stress method, structure stress method, hot stress method and power spectral density method. Especially application status and common software of finite element analysis are expounded and analyzed in fatigue analysis of the welded joint. In recent years, finite element technology is applied widely on analysis all kinds of welded joints, the main problems include two points, built suitable model and generated reasonable mesh. Nowadays, finite element software has many kinds of usage and different software has different functions. According to the stress analysis process of static fatigue and vibration fatigue, three large-scale general finite element software ANSYS, MSC.NASTRAN/PANTRAN and ABAQUS, some comparative analyses have been done in building model, partitioning mesh and applying method At last some important conclusions are given. Different finite element software has equivalent development background and reliable analysis performance. Different finite element software have different application platform. In order to get better analysis result, each other’s merits are drew from different software and matched with the application.