Showing papers on "Vibration fatigue published in 1998"

Application of Measured Loads to Wind Turbine Fatigue and Reliability Analysis

Abstract: Cyclic loadings produce progressive damage that can ultimately result in wind turbine structural failure. There are many issues that must be dealt with in turning load measurements into estimates of component fatigue life. This paper deals with how the measured loads can be analyzed and processed to meet the needs of both fatigue life calculations and reliability estimates. It is recommended that moments of the distribution of rainflow-range load amplitudes be calculated and used to characterize the fatigue loading. These moments reflect successively more detailed physical characteristics of the loading (mean, spread, tail behavior). Moments can be calculated from data samples and functional forms can be fitted to wind conditions, such as wind speed and turbulence intensity, with standard regression techniques. Distributions of load amplitudes that accurately reflect the damaging potential of the loadings can be estimated from the moments at any wind condition of interest. Fatigue life can then be calculated from the estimated load distributions, and the overall, long-term, or design spectrum can be generated for any particular wind-speed distribution. Characterizing the uncertainty in the distribution of cyclic loads is facilitated by using a small set of descriptive statistics for which uncertainties can be estimated. The effects of loading parameter uncertainty can then be transferred to the fatigue life estimate and compared with other uncertainties, such as material durability.

Fatigue reliability analysis of prestressed concrete bridges

Abstract: A model for the fatigue analysis of prestressed concrete bridges is developed. The uncertainties in the geometrical and mechanical variables involved in the problem are included. Traffic actions, thermal effects, and differential settlement are considered. The model also includes the nonlinear behavior under the action of external loads causing cracking. The \IS-N\N curves and the Palgrem-Miner hypothesis were chosen for the definition of the fatigue strength of the studied materials (reinforcing and prestressing steels). The fretting fatigue effect is considered in the definition of the resistance of posttensioning steel strands. This effect causes a very significant drop in the fatigue resistance of these elements predicted after tests in air. Weibull distributions are adopted for describing the uncertainty in the definition of the fatigue strength of the steel elements. The limit state functions for the fatigue assessment of reinforcing and prestressing steel are derived. Finally, the statistical definition of the variables affecting the fatigue resistance of posttensioning steel strands under fretting conditions is given. The variability of the random variable damage of Miner at failure is derived from the variability in the \IS-N\N curves and a model for the calculation of the parameters of the cumulative distribution function is also proposed. The paper shows that the calculation of the reliability index of posttensioned concrete bridges under fatigue is a straightforward process.

Fatigue Life Prediction of Nonlinear Plates Under Random Excitations

Abstract: An efficient method is presented for estimating the high cycle fatigue life of nonlinear structures under random excitations. The procedure is based on an application of the method of equivalent linearization for constructing the response of the stress of the structure in time domain. Fatigue estimates are obtained by processing the time domain signal using the Rain-Flow cycle counting scheme in conjunction with the linear accumulative damage theory. The estimated average fatigue life of a nonlinear plate under random excitations by the present method is compared with the result obtained by direct Monte Carlo simulations of the original nonlinear modal equations. The agreement is excellent for a wide range of levels of nonlinearity. The present method has the advantage of being much more computationally efficient than direct numerical simulations of nonlinear systems. The computational effort required of the present method for a nonlinear system is nearly the same as that for a linear system and is not affected much by the type and level of nonlinearity in the structure. The present method offers a practical means for predicting high cycle fatigue lives of complex nonlinear structures.

A unified approach to random-fatigue reliability quantification under random loading

Abstract: The random fatigue damage accumulation under stationary random loading, narrow-band or wide-band, has been quantified by numerous authors. It is shown in this paper that their results can be applied to random fatigue crack growth under random loading. The random fatigue life distribution is obtained, which is found to be a Birnbaum-Saunder's distribution. Then, the fatigue life statistics and the associated reliability are quantified.

Sensitivity study on material properties for the fatigue life prediction of solder joints under cyclic thermal loading

Abstract: A computational study is presented in this paper to investigate the effect of variation in material properties on the fatigue life prediction of solder joints subjected to cyclic thermal loading. The package under investigation was a plastic quad flat pack (PQFP) with gull‐wing leads. A commercial finite element code, ABAQUS, was employed to perform a two‐dimensional plane stress analysis. While all other constituents of the PQFP assembly were assumed to be linear elastic, the solder joint was considered to be elastic‐viscoplastic. The creep model was adopted from Norton’s equation and was implemented in the finite element analysis via a user‐defined subroutine. The maximum creep strain was evaluated and incorporated into the modified Coffin‐Manson equation to estimate the life cycles under thermal fatigue. It was found that the variation in material properties could have a significant influence on the fatigue life prediction of solder joints.

Expected Fatigue Damage of Road Vehicles due to Road Excitation

Abstract: A fundamental theoretical study is performed for the computation of the expected fatigue damage of the main structures of a road vehicle running on a randomly profiled road. The expected fatigue damage can be estimated according to Miner's rule and Rain-Flow counting employing Rychlik method. Referring to a 2 d.o.f. mechanical system, the standard deviations of the random forces on acting on the body and on the wheel are computed symbolically by means of the Power Spectral Density Method. The influence on damage of vehicle parameters is discussed.

Shape Design Optimization of Thermoelastic Structures for Durability

Abstract: Shape design sensitivity analysis (DSA) and optimization methods for the fatigue life of thermoelastic structural components are presented in this paper. A multiaxial fatigue life prediction method is used for crack initiation. The crack initiation life prediction is modeled using constant amplitude strain-life data and cyclic stressstrain curves. A hybrid DSA method is used for the fatigue life. The design sensitivities of the dynamic stress and the temperature field are obtained using analytical ap­ proaches. The design sensitivity is used to predict the dynamic stress and temperature of the perturbed design. Using predicted stress and temperature, the fatigue life of the perturbed structural design component is predicted. The predicted fatigue life is then used to obtain the design sensitivity of the fatigue life by utilizing the finite difference method. The proposed DSA method is applied to design optimization of an automotive exhaust manifold of an automotive vehicle, considering crack initiation lives as design constraints.

Fatigue Life Prediction for an S-Train Bogie

Abstract: A method of fatigue life prediction for railway vehicles based on computer simulation is presented Vehicle interaction forces are calculated from measured track irregularities with a multibody-system program Knowing the forces, stresses in certain bogie cross sections are evaluated and, using a cumulative damage theory, the fatigue life is predicted The procedures when using spectral analysis or covariance analysis are described Some results are compared with measured data, and a fatigue life prediction for two critical cross sections of an S-train bogie is shown

A Probabilistic Analysis on Variability of Fatigue Crack Growth Using the Markov Chain

Abstract: Understanding the stochastic properties of variability in fatigue crack growth is important to maintaining the reliability and safety of structures. In this study, a stochastic model is proposed to describe crack growth behavior considering the variability of fatigue crack growth rates due to the heterogeneity of material. Fatigue life distribution is then predicted based on this model To construct this model, fatigue tests are conducted on a high strength aluminum alloy 7075 T6 under constant stress intensity factor range control. The variability of fatigue crack growth rates is expressed by random variablesZ and Γ based on the variability of material constantsC andm of the Paris-Erdogan equation. The distribution of fatigue life under constant stress intensity factor ranges is evaluated by the stochastic Markov chain model based on the Paris-Erdogan equation. The merit of the proposed model is that only a small number of tests are required to determine this function, and fatigue life required to reach certain crack length at a given stress intensity factor range can be easily predicted.

Vibration reliability test of a PBGA assembly

Abstract: In order to understand vibration fatigue failures in plastic ball grid array (PBGA) solder joints, a series of constant-amplitude vibration tests of a PBGA assembly were conducted. The test vehicle comprises four 256 I/Os PBGA modules mounted on a printed circuit board (PCB). In the test, the assembly was clamped at two opposite sides on a fixture which was bolted to a vibration shaker, to simulate a boundary condition frequently encountered in practice. The shaker provided a constant-amplitude foundation vibration source for the PBGA assembly. During the test, the PBGA modules were continuously monitored so that the fatigue failure could be detected and the cycles-to-failure of the PBGA modules measured. In this paper, the test set-up and procedures are described, the results regarding the cycles-to-failure of the PBGA solder joints are presented, and the typical vibration fatigue failures of the PBGA solder joints are identified. Some problems regarding vibration testing of PCB assemblies are also discussed. Test results show that the PBGA solder joints are vulnerable to vibration.

Fatigue Life Prediction by Statistical Approach Under Constant Amplitude Loading

Abstract: In general, the experimental data of fatigue crack growth rates scatter very much even under identical experimental condition such as a constant amplitude loading condition It is, thus, essential to take into account the data scatter of crack growth rates by using statistical approach for a reliable fatigue crack, propagation analysis In this study, fatigue crack propagation tests were conducted on a 102 mm-thick 2024-T3 aluminum alloy under a constant amplitude loading condition The distribution of the fatigue crack propagation life is estimated by using the stochastic Markov chain model based on a modified Paris-Erdogan equation to consider the variability of the fatigue crack growth The fatigue lives estimated by using the Markov chain model are found to be agreed well with the experimental results

Fatigue properties of a glass-fibre-reinforced polyester material used in wind turbine blades

Abstract: Glass-fibre-reinforced polyester (GFRP) is a composite commonly used in the manufacture of wind turbine blades. In the present work, one such material has been subject to static and dynamic tests in order to obtain data that can be applied to the design of wind turbine blades and other machine elements. The results of the static tests established a basis for the determination of a set of tension-tension (constant amplitude and sinusoidal load) dynamic tests with the aim of establishing a mathematical model in order to predict life as a function of the load state and calculate the fatigue limit. The multiplicative model (y = axb) for y = log of life and x = transformed stress (a and b are characteristic parameters of the material obtained from data) matches the data quite well. The conclusion is that the GFRP studied has no fatigue limit. The possible decrease of fatigue strength of the material with solar radiation and moisture absorption was also investigated, with a negative result.

The Application of S-N Curves Considering Mismatch of Stress Concentration Between Test Specimen and Structure

Abstract: This paper presents a procedure for the adjustment of S-N diagrams for noncontinuous welded joints in the fatigue analysis of structural details. The adjustment takes into account the effects of differences in geometry between the test specimen underlying the S-N curve and the hull structural detail it is applied to, considering the inelastic response of material at the anticipated crack origin, multiple modes of loading, and their statistical correlation. The basis of the approach is a comparison of relative fatigue lives for the structural detail of interest and the corresponding baseline specimen, using the local strain approach to fatigue, with extensions to the high-cycle regime. An S-N diagram so corrected is used in the conventional manner to predict fatigue damage or to establish related allowable stresses. The procedure developed is thus entirely compatible with present fatigue life assessment methods. The developed procedure is illustrated through numerical examples.

High pressure fuel injection pipe material and manufacture thereof

Abstract: PROBLEM TO BE SOLVED: To provide a pipe material excellent in a durability for a repeated high pressure fatigue and the vibration of a car body by making an outer surface structure to a rolling structure and increasing a vibration fatigue strength by process-hardening the outer surface, in a fine diameter thick metal pipe consisting of a single pipe body dry drawn a final finish extending pipe by a surface reduction ratio specific value. SOLUTION: A fine diameter thick steel pipe consisting of a single pipe body is dry drawn at a surface reduction rate 1.2-6.4% in a final extending process after repeating an annealing and drawing process several times. Thereby, the outside of a pipe is made to a rolling structure by the effect of processing and process-hardening is carried out, but its inside is as being in the annealing state before final drawing process without the effect of processing and made to a pipe excellent in a vibration fatigue strength and inner pressure fatigue strength, even it is not annealed after drawing. When the surface reduction ratio is less that 1.2%, the hardness by the processing of the pipe outside is insufficient and when it is beyond 6.4%, the pipe outside is hardened sufficiently and the vibration fatigue strength is improved, by the pipe inside is hardened by the effect of processing and the hardness of the surface side in the pipe is increased and inner pressure fatigue strength is decreased.

Reliability Engineering. A Study of Method Utilizing Fatigue Life Data on Two Competing Fracture Modes.

Abstract: Many high strength materials, including high strength steels and high strength titanium alloys, show the complex characteristics of fatigue life distribution in the high cycle region. It is caused by the two competing fracture modes, fractures from surface and from interior of the specimen. A method to describe such a complex fatigue life data was proposed based on the competing-risk model. The characteristics of fatigue life distributions and S-N relations of each mode were successfully separated and extracted from whole data. These separated data was analyzed and the complex fatigue life distributions was well described by the competing-risk model. The method is helpful to understand the fatigue mechanisms because the fatigue life distribution of different two modes results from and reflects the fatigue mechanism of each mode.

Development of design margin decision system considering fatigue limit

Abstract: The experimental results of fatigue at low stress levels around the fatigue limit have large scatter. For the design of structures and machines, it is important to evaluate the scatter and to decide the design margin. In this paper, the statistical method to decide the fatigue limit is proposed. In the proposed method, the fatigue limit is decided with considering the distribution of fatigue strength. Because each fatigue data is transformed into normalized fatigue strength, this method has the advantage point that the distribution of fatigue strength and the fatigue limit can be found by relatively small samples. Using the proposed method, the computer system to decide the design margin for fatigue is developed. In this system, the S-N curve with fatigue limit and the distribution of fatigue strength are obtained at first. Then the design margin with a given probability of failure and a confidence level is calculated.The developed system is applied to some experimental results for fatigue test. As a result, the S-N curve with the design margin has obtained easily. It is recognized that the obtained S-N curve is decided after consideration of scatter.

A Method in Mixed Frequency time domain for Fatigue Life Estimation of Steel Girders of Cable stayed Bridges Due to Buffeting

Abstract: A method in the mixed frequency time domain for estimation the fatigue flife of steel girders of cable stayed bridges due to buffeting is developed in this paper. In this method, the joint probability density function of wind speed and wind direction at the deck level of the bridge is first established. The derived stress spectra are of wide band frequency properties when the background component of stress response is included. Thus, the time histories of the critical stress are simulated from its power spectra and the stress cycle distributions are estimated in terms of rainflow count method. The formulae derived based on the modified Miner′s law and the random vibration theory are used for estimating the fatigue life of the bridge girder due to buffeting considering the effects of wind direction. The Yangpu Bridge, a cable stayed bridge with a main span of 602m, is finally taken as an example, and the results show that the effects of wind direction on the fatigue life of the Yangpu Bridge is significant. The predicted fatigue life due to buffeting is much longer than the design life of the bridge.

Etudes Industrielles de vibrations sous écoulement dans des réseaux de tuyauteries

Abstract: To improve the allowance mode in the design stage for vibratory phenomena affecting main lines and nozzles under operating conditions have led Electricite de France to develop special engineering codes and study methodologies. The basic aim of the various actions undertaken has been to locate the causes of fractures of cracking of piping in auxiliary circuits, exomine the corrective actions taken, and define industrial vibration prevention practice. So far, vibration problems have been detected during start-up tests. As regards nozzles, the cracks sometimes oppear rapidly, and are in this case treated during the installation testing period, as called for by the RCC-M. Further more, the problem posed by vibration fatigue is hard to solve, because the operating configuration that gave rise to the problem must be known. Evolution of EDF's acceptance guide involves incorporation on the vibration acceptance criterion for piping systems. Knowing the operation of the circuit, we predict the dynamic Ioads generated by the flow, to predict the vibratory behavior of the piping, in particular that of nozzles, and so estimate the design margins with respect to the vibration risk either using the current acceptance criteria or a damage analysis. Two methodologies based on the modeling and calculation options of the CIRCUS code are proposed. One concerns studies based on operating design data and the other studies based on the operating diagnostic. The latter is based on an opproach combining numerical modeling and on-site tests.

Bimodal concept of high-cycle fatigue life prediction for constant and variable amplitude fatigue

Abstract: A bimodal concept for the prediction of the high-cycle fatigue life of structural details subjected to constant- or variable-amplitude loading is considered in this paper. The total fatigue life was separated into two phases: crack initiation and crack propagation. The portion of life spent in crack initiation was estimated by using S-N data obtained on smooth specimens. A fracture mechanics concept was used to calculate the portion of life spent in crack propagation, and the S-N curve, including the fatigue limit of a structural detail, was determined by using material properties and the geometry of the detail. The bimodal concept was applied to a welded stiffener and the results are compared with experimental data reported in the literature.

Fatigue life of randomly loaded notched components

Abstract: We generalize the results of the investigation of the fatigue life of structural components with notches under random loads. For this purpose, we analyze various blocks of random loads, both stationary and nonstationary, and their influence on the stress concentration near notches. An attempt is made to determine the critical sites of a structure according to the measured levels of strains on smooth surfaces. We propose a method for reducing stress concentration by applying localized plastic deformation of notches. This method is experimentally checked for specimens of low-carbon steel. In particular, it is shown that the realization of technological operations of this sort after long-term operation of a structure may significantly increase its safety.