Showing papers on "Vibration fatigue published in 1995"

A high‐cycle fatigue criterion applied in biaxial and triaxial out‐of‐phase stress conditions

Abstract: A high-cycle fatigue criterion suitable for multiaxial non-proportional stress loading is proposed in this paper. The criterion is based on some microscopic considerations related to the crystalline structure of metals. The purpose of the present paper is mainly the application of this criterion in two loading cases : (a) biaxial loads involving two normal stresses or one normal and one shear stress, and (b) triaxial load with two normal stresses and one shear stress. Stress states of these kinds are very common in piping assemblies. Application of the proposed criterion in the case of triaxial loading, where the three stress components are of the same frequency, but out-of-phase, leads to a simple analytical formula. This formula is the equation of a bounding surface that delimits in the space of the above three stresses the safety domain against fatigue crack initiation. A remarkable theoretical result concerns the phase difference of the shear stress, which does not appear in the derived formula. Consequently, according to our proposal the safety domain (i.e. the limiting fatigue endurance) under combined out-of-phase biaxial normal stress loading and torsion is independent of the phase difference of the torsion. Obviously this result holds also for the simpler case of axial load and torsion. On the contrary the phase difference between the two normal stresses has a strong detrimental effect on the fatigue endurance of a metal. As is shown these theoretical conclusions are in good agreement with fatigue limit test data found in the scientific literature.

Fatigue damage calculation using the critical plane approach

Abstract: The critical plane approach is applied to model material fatigue behavior under any constant amplitude proportional loading. The most critical plane and the largest damage parameter are determined in closed form for six damage criteria that have been proposed in the literature. The correct procedures of utilizing these closed-form solutions to construct the damage parameter versus fatigue life curve are outlined. It is shown that the common practice of characterizing the material's fatigue behavior by plotting the damage parameter evaluated on the maximum shear plane against the observed fatigue the violates the principle of the critical plane approach, a problem which can arise during the calibration of any biaxial type of damage criterion. The study emphasizes that the critical plane approach should be consistently applied to both the initial calibration and the subsequent fatigue analysis

Vibration fatigue of surface mount technology (SMT) solder joints

Abstract: Recent trends in reliability analysis of electronics has involved developing structural integrity models for predicting the failure free operating lifetime under vibratory and thermal environmental exposure. This paper describes a test program which was performed to obtain structural fatigue data for SMT solder joints exposed to a random vibration environment. A total of eight printed circuit board specimens with nine surface mounted components were fabricated and tested. Vibration time to failure data for individual solder joints of the SMT components were recorded. These data became the basis for understanding the physics of "why and how" SMT solder joints fail under vibration loading. Using procedures similar to those developed for aerospace structures, a fatigue model was developed that is based on the physics of the problem.

A Strain-Based Fatigue Reliability Analysis Method

Abstract: A new fatigue reliability technique has been developed using a strain-based analysis. A probabilistic strain-life curve, where the variability in cycles to failure at constant strain range has been modeled with a three-parameter Weibull distribution, has been incorporated into the strain-based fatigue analysis. This formulation, which includes a notch strain analysis, rainflow cycle counting and damage accumulation according to Miner's rule, is used to estimate fatigue life to crack initiation for notched components using smooth specimen laboratory data. Unlike other probabilistic fatigue models, the technique developed here does not include a distribution model for stress peaks such as the commonly-used stationary narrow band Gaussian random process assumption but rather uses strain histories directly. Using this model, techniques have been developed to estimate the number of cycles to failure at a specified reliability and to predict the reliability and failure rate at a specified time in the analysis.

Numerical simulation of fatigue crack growth.

Abstract: This thesis describes the advances made by the author on the direct modelling of the fatigue growth of planar cracks A versatile step-by-step 3D finite element technique has been developed, which calculates the stress intensity factors at a set of points on the crack front according to linear elastic fracture mechanics principles and then applies a fatigue crack growth la\v to this set of points to obtain a new crack profile The software DUCK developed has a good capability of automatic remeshing so that the fatigue propagation of arbitrary shaped cracks can be conveniently followed The stress intensity factor calculation using the 3D finite element method has been improved and widely verified to be of good accuracy generally, which provides an important guarantee for the fatigue growth prediction. Several major problems associated with the stress intensity factor calculation, such as crack front definition, free surface layer and mesh abutting the crack front, as well as J-integral path independence, have been discussed A variety of cracked geometries of either theoretical or practical significance have been modelled by the numerical technique, including internal defects in infinite solids, surface cracks in finite thickness plates, round bars and pressure vessels, and initially multiple surface small cracks. Many results useful for the fatigue assessment of each kind of crack, such as stress intensity factor variations, fatigue shape changes and fatigue lives, have been obtained. By comparing with existing numerical methods and some limited experimental data found in the literature, it is shown that the numerical simulation technique is reliable and can predict the fatigue propagation of complex crack configurations, which is seldom possible for methods with assuming crack front configurations Meanwhile, due to the highly automated procedure or the technique, it has become straightforward to model the fatigue growth of practical cracked components, as demonstrated by the computations in this thesis. It is believed that such a numerical simulation technique has a great potential and will play an useful role in the area of fatigue study and assessment

Fatigue of fiberglass beam substructures

Abstract: Composite material beams representative of wind turbine blade substructure have been designed, fabricated, and tested under constant amplitude flexural fatigue loading. Beam stiffness, strength, and fatigue life are predicted based on detailed finite element analysis and the materials fatigue database developed using standard test coupons and special high frequency minicoupons.Beam results are in good agreement with predictions when premature adhesive and delamination failures are avoided in the load transfer areas. The results show that fiberglass substructures can be designed and fabricated to withstand maximum strain levels on the order of 8,000 microstrain for about 10{sup 6} cycles with proper structural detail design and the use of fatigue resistant laminate constructions. The study also demonstrates that the materials fatigue database and accurate analysis can be used to predict the fatigue life of composite substructures typical of blades.

Fatigue Equivalent Load Cycle Method A General Method to Compare the Fatigue Loading of Different Load Spectrums

Abstract: Fatigue is the main design driver for the calculation of the structural integrity of wind turbine components. In the JOULE II project `Load and Power Measurement Programme on Wind Turbines Operating in Complex Mountainous Regions` and in several other research projects there is a need to compare different fatigue load spectrums on a quantitative basis. A common way to compare two or more fatigue load spectrums is the use of an equivalent load range. The calculation of the equivalent load range is easy to perform. The fatigue behaviour of the material is formulated with a straight S-N curve on log-log scale. Different material behaviour may be characterised with different slopes of the S-N curve. A disadvantage of the above method is the neglecting of the mean level of a load cycle. In case of glass-polyester, glass-epoxy, cast steel, carbon epoxy, or wood laminates the mean level of the cycle effects the fatigue life. This could be avoided by calculating the fatigue stress reserve factor. This factor is defined as the factor by which the prevailing fatigue stress has to be multiplied in order that the calculated fatigue lifetime equals the design lifetime. The disadvantages of the fatigue stress reserve method are the need of detailed cross sectional data, the need of the specific fatigue formulae of the materials, the iterative calculation of the factor, and the fact that the results are not easy to generalize for other materials than considered. In this document an extension to the equivalent load range method is defined. With the extension the mean level of the load cycles is taken into account. The method is easy to apply and fully consistent with the equivalent load range method. In chapter 2 the formulae for calculating the equivalent load range are given. In chapter 3 the formulae for the equivalent load cycle method are given. An example is presented in chapter 4. Some conclusions are given in chapter 5. 2 figs., 3 tabs.

Life Prediction and Damage Acceleration Based on the Power Spectral Density of Random Vibration

Abstract: Fatigue life prediction and accelerated verification tests under a random vibration environment are important tasks for evaluating product reliability. This paper reviews the characteristics of random stress processes, discusses the methodology of life prediction and accelerated testing under various random loadings by using the stress power spectral density (PSD) function obtained from finite element analysis (FEA), and develops an engineering method to determine the acceleration level and test time in reliability verification tests. The discussions cover the narrow-band Gaussian processes, the wide-band Gaussian processes, and the nonGaussian processes. To illustrate the practical procedure of life prediction and accelerated testing based on the damage equivalent technique, the application example of an automotive component is presented.

Fatigue crack growth behavior of composites

Abstract: Fatigue crack growth data of discontinuously reinforced composites published in the literature has been re-evaluated using the two-parametric approach developed by the authors. The use of these two parameters involves †KandK maxas the driving forces, which are required simultaneously for fatigue crack growth to occur. These two parameters are intrinsic to fatigue deformation process. The first parameter is related to the degree of cyclic plasticity that results in fatigue damage near the crack tip, and the second (the peak stress) is required for initiating the local fracture processes in the fatigue-damaged region. Both driving forces are required simultaneously and have to exceed some critical minima for crack advancement. Thus, there are two fatigue thresholds instead of one as is normally assumed. However, in a given region, depending on the material and its crack-tip environment, one or the other parameter controls the growth behavior. Thus, normally for all materials (including composites) below a certain criticalR ratio, fatigue crack growth is Kmax controlled. Above the criticalR ratio, it is †K controlled. Although one parameter is the controlling factor in a given regime, both driving forces are needed to complete the fatigue description. This two-parametric approach is valid not only at the thresholds, but also at the higher crack growth rates. An understanding of fatigue process, then, requires a systematic evaluation of how these two driving forces vary with the reinforcement size, shape, volume fraction, and distribution, along with other material properties of the constituent phases, such as the interfaces. Finally, this article discusses some of the possible mechanisms and their effects on the two driving forces, using the limited available data in the literature.

Ultrasonic vibration welder

Abstract: A welder using an ultrasonic vibration includes a vibration direction converter for diversely converting the direction of vibration generated by a vibrator according to the properties of a welded material, so that welding efficiency is improved. The vibration direction converter is made to be disassembled or assembled, so that the replacement of the rod is easy when it experiences vibration fatigue. Efficiency is maximized because plural welding operations can be made simultaneously, since plural vibration-transmitting directions due to a symmetrical multiple structure are possible by means of the vibration-direction converter.

Discussion of the Fatigue Crack Model Used as Decision Basis for Probabilistic Inspection Planning

Abstract: Marine structures are often highly utilized structures and are operating in an environment dominated by dynamic loading. The fatigue limit state is of main concern and is governing the structural dimensions and the amount of resources utilized in inspection of several members and joints. There is a considerable uncertainty related to the parameters determining the fatigue life, and probabilistic analyses have therefore been used extensively in fatigue analysis. The models used both on describing the fatigue driving mechanisms and the deterioration mechanism, however, are always compromises between ability to describe nature and computational applicability. In a state-of-the-art probabilistic fatigue analysis, major assumptions are made when establishing the crack growth model, such as: (1) the relation between membrane and bending stresses is assumed to be constant for all stress ranges, and one common stress component for the total stress is used to calculate fatigue crack growth; (2) the crack growth is assumed to follow a pattern with constant relation between crack and depth and crack length. This paper investigates the foregoing fundamental assumptions for the fatigue driving mechanism and shows the effect on time to the first inspection based on state-of-the-art probabilistic crack growth analysis for the detail of interest, formore » a simple but realistic jacket structure.« less

Probabilistic material strength degradation model for Inconel 718 components subjected to high temperature, high-cycle and low-cycle mechanical fatigue, creep and thermal fatigue effects

Abstract: The development of methodology for a probabilistic material strength degradation is described. The probabilistic model, in the form of a postulated randomized multifactor equation, provides for quantification of uncertainty in the lifetime material strength of aerospace propulsion system components subjected to a number of diverse random effects. This model is embodied in the computer program entitled PROMISS, which can include up to eighteen different effects. Presently, the model includes five effects that typically reduce lifetime strength: high temperature, high-cycle mechanical fatigue, low-cycle mechanical fatigue, creep and thermal fatigue. Results, in the form of cumulative distribution functions, illustrated the sensitivity of lifetime strength to any current value of an effect. In addition, verification studies comparing predictions of high-cycle mechanical fatigue and high temperature effects with experiments are presented. Results from this limited verification study strongly supported that material degradation can be represented by randomized multifactor interaction models.

Fatigue damage considering whipping arising from slamming

Abstract: In floating vessels, there are many load processes that contribute to fatigue. Among these, present day fatigue design methods emphasize low-frequency wave-induced loads. The exact level of fatigue damage contributed by the load processes neglected, e.g. slamming-induced whipping, is at this time largely unknown. Full-scale measurements appear to indicate that the slamming-induced whipping stresses may be significant for certain vessel types, locations, and loading conditions. Further, the consideration of whipping is in principle important for design of the more slender, higher speed vessels, for systematic studies of slamming-induced fatigue damage, and for applications such as failure analysis. There thus exists a need for a fatigue damage calculation procedure addressing slamming. In this paper the authors outline and summarize the results of a study to develop a new integrated procedure for obtaining the statistics of the combined slamming and wave-induced stress response and resulting fatigue damage, accounting for phasing, hog-sag non-linearities, clustering, and hull flexibility. They also provide example calculations for a vessel.

Method and apparatus for predicting fatigue by introducing a continuous excitation to determine damping factor

Abstract: The fatigue integrity of metallic materials may be determined by either one of two methods. In a first method, an impulse of energy is introduced into the material, such as by striking the material (20), and the induced vibration is sensed and analyzed in order to compute the damping factor thereof, the damping factor being directly related to the fatigue thereof. With this method, a transducer (28) is coupled to the material and its output is amplified by an amplifier (30) before input to a computer (32) which determines the damping factor. In a second method, a continuous energy input is provided to the material, such as by utilizing a frequency generator coupled to a power amplifier whose output drives a transducer such as a speaker or the like for inducing a continuous vibration in the metallic material. This continuous vibration is measured with a transducer, an amplifier, and a damping factor calculated with a computer as in the first method.

Fatigue reliability under stochastic load

Abstract: In this paper a model developed by the authors for crack growth under stochastic loading incorporating sequence effects if inserted into fatigue reliability analysis. The goal is the estimation of the probability distribution of the number of cycles to failure. A probabilistic fracture mechanics crack growth model is used to quantify fatigue damage for a given number of cycles. The important sources of uncertainties include fatigue resistance, external loading and initial crack size. Suitable probabilistic models for the above variables are presented and the advanced mean value first order (AMVFO) methods is used to minimize the number of functions to be evaluated in the computation of the probability of failure. A numerical example showing the calculation of the probability distribution of the number of cycles to failure of a structural element is presented.

Fatigue life prediction of pressure vessels using an energy based approach

Abstract: Pressure vessels are subjected to pulsed loading, resulting in the occurrence of stress concentration zones, initiation and propagation of fatigue cracks, and subsequent failure Conventional approaches to fatigue life prediction are based on nominal stresses and strains Methods of prediction involving an energy criteria give the best results This paper presents the fatigue life prediction process of a pressure vessel designed by “Neftohim” — Bulgaria The energy dissipated over a cycle and equal to the hysteresis loop area was used as the fatigue damage parameter The stress values necessary for calculations were found analytically on the basis of shell theories For an analysis of material inelastic behavior, some tension/compression fatigue tests were conducted at a frequency of 15 Hz The experimental stress and strain values were used to determine the energy dissipation The prediction has shown that the designed pressure vessel cannot serve 10 years without fatigue crack initiation Such neglected factors as temperature, corrosion environment, welded joints, and a spread of material characteristics result in additional service life reduction For the solution of the problem it is necessary to replace the steel and also to change the geometry of the vessel

Prediction of Transmitted Force between Components under Operating Condition for Reduction of Vibration and Noise.

Abstract: Complex structure assembled by many components need to reduce vibration and noise caused by the vibration considering not one part but all part of components. This paper pays attention to structural modification for the noise and the vibration reduction based on transmitted force between components under operating condition. First, frequency response functions of main component, which includes vibration source, and sub-components, which do not include vibration source are measured by modal analysis. Each component is connected at a few points and some sub-components are assumed to cause problem of the noise and the vibration. Second, a location where the transmitted force from the main component to the sub-component decreases is investigated by using mass struc-tural modification, furthermore it is confirmed that this method can reduce not only the vibration but also the noise caused by the vibration.

Stochastic fatigue of welded steel joints

Abstract: Fatigue tests were conducted on high strength welded steel cruciform shaped specimens subjected to random amplitude loadings. Results of these tests are used to experimentally investigate the effects of loading intensity, non-normality and frequency bandwidth on the rate of fatigue damage accumulation. Test results are compared with predictions made using Rayleigh approximation and rainflow analysis in terms of cycles and times to failure. Results indicate non-normality can significantly increase the rate of fatigue damage accumulation and result in non-conservative fatigue life estimates if its effect is not accounted for. Likewise, frequency content was also found to influence the rate of fatigue damage accumulation, but to a lesser extent than non-normality. When higher frequency components were included, shorter fatigue lives were observed. Fatigue life predictions using rainflow analysis produced generally good agreement with experimental results.

Fatigue Reliability Calculation For Structure Members of Offshore Fixed Platform

Abstract: In order to evaluate the Safety of offshore fixed platforms in service, the fatigue reliability calculation for structure members is necessary. The paper analyzes the alternating stress due to sea wave and ice force, and provides new pre-processing programs of hot spot stress calculation. Then the paper presents the fatigue life calculations for structure members under low temperature and ice loading and sea wave loading. At last, considering the three random coefficients: stress calculation B, fatigue life experiment K, fatigue damage calculation {Delta}{sub c}, the Fatigue reliabilities of structure members are calculated.