Showing papers on "Vibration fatigue published in 2007"

A fatigue approach to wind turbine control

Abstract: Conventional design of wind turbine controllers is focused on speed and produced electric power. As fatigue loads is an important design consideration, the resulting design is evaluated also with respect to the fatigue loads inflicted on the turbine structure. This is normally done by performing simulations using tools like FLEX, HAWC or FAST, followed by rainflow counting in the resulting time series. This procedure constitutes an iterative design procedure involving realisations of the stress processes in order to obtain the time series needed for fatigue estimates. The focus of this paper is the elimination of the need for process realisation. To this end, known techniques for approximative fatigue load assesment based on the spectral moments of the inflicted stress histories are applied. Assuming a linearised system model, we present a novel scheme for efficient computation of these spectral moments. The scheme is applied to obtain rapid evaluation of cost functions including fatigue loads, hereby allowing efficient numerical optimisation of the controller. Three different controller design examples are given, all defined directly in terms of component life times.

Non‐local stress approach for fatigue assessment based on weakest‐link theory and statistics of extremes

Abstract: In the present paper a non-local stress approach for fatigue assessment based on weakest-link theory and statistics of extremes is presented. It is a non-local stress approach in the sense that it takes the complete stress field into account rather than just the highest local stress. The statistical distribution of fatigue strength data from smooth standard specimens serves as a starting point for the computation of the probability of fatigue failure of a mechanical component under cyclic loading. The probability of fatigue failure can be obtained by post-processing results from a standard finite element stress analysis. It is shown that the non-local stress approach can be linked to the probability of finding the fatigue critical defect in the most highly stressed volume of the component. A numerical procedure is presented that is fully compatible with the results from a standard finite element stress analysis. It is further shown how the fatigue strength distribution can be transformed into a fatigue life distribution by using Basquin's equation. Finally, the non-local stress approach is used for predicting the fatigue limit of several specimens and predictions are compared with test results.

Two scale damage model and related numerical issues for thermo-mechanical High Cycle Fatigue

Abstract: On the idea that fatigue damage is localized at the microscopic scale, a scale smaller than the mesoscopic one of the Representative Volume Element (RVE), a three-dimensional two scale damage model has been proposed for High Cycle Fatigue applications. It is extended here to anisothermal cases and then to thermo-mechanical fatigue. The modeling consists in the micromechanics analysis of a weak micro-inclusion subjected to plasticity and damage embedded in an elastic meso-element (the RVE of continuum mechanics). The consideration of plasticity coupled with damage equations at microscale, altogether with Eshelby–Kroner localization law, allows to compute the value of microscopic damage up to failure for any kind of loading, 1D or 3D, cyclic or random, isothermal or anisothermal, mechanical, thermal or thermo-mechanical. A robust numerical scheme is proposed in order to make the computations fast. A post-processor for damage and fatigue (DAMAGE_2005) has been developed. It applies to complex thermo-mechanical loadings. Examples of the representation by the two scale damage model of physical phenomena related to High Cycle Fatigue are given such as the mean stress effect, the non-linear accumulation of damage. Examples of thermal and thermo-mechanical fatigue as well as complex applications on real size testing structure subjected to thermo-mechanical fatigue are detailed.

Vibration Testing and Analysis of Ball Grid Array Package Solder Joints

Abstract: A methodology to characterize and predict fatigue failure of BGA package solder joints under vibration loading is presented. The results show that the board strain versus number-of-cycles-to-failure (or E-N) curve has a linear trend with little scatter in data points, similar to that of a classical fatigue theory using cyclic stress versus number-of-cycles-to-failure (or S-N) curves. Using finite element analysis (FEA), the solder joint stress was shown to be linearly correlated to the board strain. Therefore, board strain can indeed be used as an optimum engineering metric to study the fatigue of ball grid array (BGA) solder joints. In addition, the E-N curve approach was shown to be applicable to cyclic bend and cyclic shock loading conditions as well. The E-N curves of lead-free and leaded solder systems also have been generated and compared to demonstrate that the lead-free system has a better high-cycle fatigue performance. In addition, a fatigue-life prediction methodology based on the Miner's cumulative damage theory is proposed. The effectiveness of this methodology was demonstrated with promising results through random vibration testing of actual motherboards. Finally, a novel approach to study solder joint reliability (SJR) under vibration loading at the system level, using a fatigue curve generated at the component level is presented.

Development of an Improved High Cycle Fatigue Criterion

Abstract: An integrated computational-experimental approach for prediction of total fatigue life applied to a uniaxial stress state is developed. The approach consists of the following elements: (1) development of a vibration based fatigue testing procedure to achieve low cost bending fatigue experiments and (2) development of a life prediction and estimation implementation scheme for calculating effective fatigue cycles. A series of fully reversed bending fatigue tests were carried out using a vibration-based testing procedure to investigate the effects of bending stress on fatigue limit. The results indicate that the fatigue limit for 6061-T6 aluminum is approximately 20% higher than the respective limit in fully reversed tension-compression (axial). To validate the experimental observations and further evaluate the possibility of prediction of fatigue life, an improved high cycle fatigue criterion has been developed, which allows one to systematically determine the fatigue life based on the amount of energy loss per fatigue cycle. A comparison between the prediction and the experimental results was conducted and shows that the criterion is capable of providing accurate fatigue life prediction.

Lifetime simulation of thermo-mechanically loaded components

Abstract: The estimation of the lifetime of thermo-mechanically loaded components by testing is very costly and time-consuming, since the high temperature cycle time in practical application dominates the test duration. Common frequencies for TMF (thermo-mechanical fatigue) tests are at about 0.01 Hz compared to 10–100 Hz at HCF (high cycle fatigue) and about 0.1–1 Hz at isothermal LCF (low cycle fatigue) tests. Therefore, the simulation of fatigue life is an important design step in the fast moving and competitive automotive industry, where the steady rise of engine power and the demand for lightweight construction concurrent with enhanced reliability require an optimised dimensioning process. Methods and models are usually derived from results made on tests with specimens, since it is possible to systematically and exactly define loading parameters and measurement categories. After an extensive test programme (tensile tests, creep tests, low cycle fatigue tests and thermo-mechanical fatigue tests with different influences on specimens) it was possible to develop material models for the simulation of the time- and temperature dependent stress–strain hystereses and damage models for the simulation of the TMF lifetime. Based on this knowledge the whole simulation chain to determine the TMF life of a component is introduced: thermal calculation, mechanical calculation and lifetime calculation. Furthermore the transferability of specimen based simulation models to real components (an alternative test piece and a cylinder head) is investigated.

Prediction of the behavior of small fatigue cracks

Abstract: A modified linear-elastic fracture mechanics approach has been proposed and developed by McEvily and co-workers, with which the fatigue crack growth rates of both small cracks and large cracks can be quantitatively evaluated. This paper reviews the recent results of applications of this approach to the following fatigue problems involving the prediction of the behavior of small fatigue cracks: (1) in an examination of the use of Miner's rule in fatigue life estimation under variable amplitude loading, (2) in the verification of the a r e a parameter model, and (3) in the prediction of the fatigue life and strength of defect-containing components subjected to multiaxial stress.

A through process model of the impact of in-service loading, residual stress, and microstructure on the final fatigue life of an A356 automotive wheel

Abstract: Fatigue life is a key consideration in the design of cast aluminium alloy automotive wheels. In this investigation, a through process modelling methodology was used to predict the fatigue life of an A356 automotive wheel subject to bending fatigue. This methodology considers the evolution of microstructural features and stress state through the manufacturing process (including casting, heat treatment, and machining) and during service loading. This paper focuses on validating the in-service model and quantifying the interaction between the key factors influencing fatigue behaviour. The cyclic elastic strains measured on the wheel surface for a series of different bending loads were found to agree well with predictions. The predicted crack initiation sites and the number of cycles to cause failure in the wheel were in agreement with full scale fatigue tests on wheels. The fatigue life and associated scatter are shown to be a function of microstructure, residual stress and in-service loading. Both the pore size and loading level have a significant impact on fatigue behaviour, while residual stresses showed a moderate influence on fatigue life for the wheel.

An overview on fatigue analysis of aeronautical structural details: Open hole, single rivet lap-joint, and lap-joint panel

Abstract: Over the last years, many efforts have been dedicated to the understanding of fatigue damage of aircraft structures. In order to achieve this goal, attention must be focused on the fatigue phenomena of simpler structural details, conducting research at different scales in terms of specimen size and structural complexity. This paper discusses the fatigue behaviour of different types of specimens with increasing level of geometrical and mechanical complexity. Problems such as the effects of residual stresses due to the cold-working process of rivet holes, load transfer and stress intensity factor calibration of riveted lap-joints, and finally the problem of multiple site damage, are addressed. The residual stress field created by cold expansion was experimentally assessed by using the X-ray technique. Finite element analysis of the different structural details was performed in order to model the residual stresses, stress intensity factor, load transfer and finally fatigue crack propagation. Fatigue tests were supplemented by post-mortem analyses of fracture surfaces in order to measure the location and extent of fatigue damage and the spacing between fatigue striations. The work presented is part of the contribution of IDMEC, Porto to two European Union research projects on the fatigue behaviour of aeronautical structures: ‘SMAAC’ and ‘ADMIRE’.

Vibration Durability Assessment of Sn3.0Ag0.5Cu and Sn37Pb Solders Under Harmonic Excitation

Abstract: In this paper, the vibration durability of both SAC305 & eutectic SnPb interconnects is investigated with narrow-band harmonic vibration tests and finite element simulation. The tests are conducted at the first natural frequency of the test board using constant amplitude excitation. Compared to broad-band vibration durability test, the advantage of harmonic test is less complexity, hence, less uncertainty in the desired fatigue constant. A time-domain approach, reported in the literature 0, was adapted in this study for the fatigue analysis. The test board consists of daisy-chained components, to facilitate real-time failure monitoring. The tests are conducted at different loading amplitude to obtain the durability data points located at both low cycle fatigue and high cycle fatigue region. The SAC305 solder was found to have lower fatigue durability than the SnPb solder, under narrow-band harmonic excitation, which is consistent with results from broad-band vibration test 0, as well as with results of mechanical cycling studies and repetitive mechanical shock studies conducted earlier 0. The test board was first characterized before the vibration durability test. A modal test was conducted to determine the mode shape and natural frequency, which were used to decide the excitation frequency of durability test. Flexure strain amplitudes at critical locations on the PWB were first characterized with strain gages at each level used in the test excitation. Detailed local finite element analysis was then conducted to estimate the strain transfer function between the PWB flexure strain and the strain in the critical solder joints for selected components (BGA256, LCR1210 and LCR2512). Then the strain history in the critical solder joint was estimated from the measured PWB strain, using the strain transfer function. Finally the solder strain and the measured time-to-failure data were used to estimate durability model constants for a generalized strain-life fatigue model 0 for both SAC305 and eutectic SnPb solder. Destructive failure analysis (cross-sectioning, polishing and microscopy) was used to confirm that the failure was by solder fatigue.© 2007 ASME

Fatigue damage assessment of a car body-in-white using a frequency-domain approach

Abstract: This work presents an application of a frequency-domain methodology developed for the fatigue damage and service life assessment of mechanical components under multiaxial random loadings. The road-induced random loadings in a virtual laboratory bench test (four post test rig) are determined using an integrated Multi-Body/Finite Element (MB/FE) analysis. A method (i.e. the variance method) based on the statistics of the observed multiaxial loadings is used to determine the critical direction. The shear stress resolved in the critical direction is then assumed as the reference loading for the subsequent fatigue analysis. A frequency-domain approach recently proposed in the literature (i.e. the non-Gaussian TB method), capable to include the load non-normality into the fatigue assessment procedure, is used to estimate the loading spectrum. A comparison between the observed and the estimated loading spectrum, extrapolated from shorter to longer time (e.g. the entire vehicle service life), is shown. The presented results show how the proposed methodology could be a very useful tool for the reliable and quick analysis of components under multiaxial random loadings.

3D fatigue crack propagation analysis of a helicopter component

Abstract: In this paper, the analysis of fatigue crack propagation on the folding beams mounted on the rear fuselage of a helicopter is performed under complex flight and folding load spectra; a preliminary cumulative damage fatigue analysis is used to select the zone of the structure where cracking is most likely to occur. A damage tolerance analysis is then carried out in the selected zone using an FEM-based approach in conjunction with a Three-Dimensional (3D) automatic crack propagation algorithm developed by the authors. The submodelling technique is used to obtain accurate results in the assessment area with a limited increase in the amount of time required for calculations. The numerical approach is validated by the experimental results of full-scale fatigue tests on the rear fuselage of the helicopter.

Fatigue Damage Behavior of a Structural Component Made of P355NL1 Steel Under Block Loading

Abstract: The common design practice of pressure vessels subjected to variable amplitude loading is based on the application of a linear damage summation rule, also known as the Palmgren-Miner’s rule. Even though damage induced by small stress cycles, below the fatigue limit, are often taken into account in design codes of practice by two-slope S-N curves, the sequential effects of the load history have been neglected. Several studies have shown that linear damage summation rules can predict conservative as well as non-conservative lives depending on the loading sequence. This paper presents experimental results about the fatigue damage accumulation behavior of a structural component made of P355NL1 steel, which is a material usually applied for pressure vessel purposes. The structural component is a rectangular double notched plate, which was subjected to block loading. Each block is characterized by constant remote stress amplitude. Two block sequences were applied for various combinations of remote stress ranges. Three stress ratios were considered, namely R = 0, R = 0.15 and R = 0.3. Also constant amplitude fatigue data is generated for the investigated structural detail which is applied for indirect damage calculations. In general, the block loading illustrates that the fatigue damage evolves nonlinearly with the number of load cycles and is a function of the load sequence, stress levels and stress ratios. In particular, a clear load sequence effect is verified for the two block loading, with null stress ratio. For the other (higher) stress ratios, the load sequence effect is almost negligible; however the damage evolution still is non-linear. This suggests an important effect of the stress ratio on fatigue damage accumulation.Copyright © 2007 by ASME