Showing papers on "Vibration fatigue published in 1997"

Life Prediction Methodology for Composite Structures. Part II—Spectrum Fatigue:

Abstract: A previously developed model for predicting the life of composite structures under constant amplitude and two-stress level fatigue loadings is extended and applied to structures subjected to randomly-ordered loading spectra. The model is phenomenological and a limited amount of experimental data is required for its characterization. For uniaxially loaded laminates, this consists of static tension and compression strength distributions, S-N curves based on constant amplitude fatigue life distributions for two-to-three stress ratios, and a limited amount of two-stress level fatigue test results. The model is verified by comparing predicted fatigue life distributions to experimentally observed fatigue life data for a variety of laminates and load spectrums. Good correlation between theory and experiment is obtained for all loadings and laminates studied.

Fatigue Life Predictions for Thermally Loaded Solder Joints Using a Volume-Weighted Averaging Technique

Abstract: Fatigue lives of thermally loaded solder joints are predicted using the finite element method. An appropriate constitutive relation to model the time-dependent inelastic deformation of the near-eutectic solder is implemented into a commercial finite element code, and the stress-strain responses of different electronic assemblies under the applied temperature cycles are calculated. The finite element analysis results are coupled with a newly developed approach for fatigue life predictions by using a volume-weighted averaging technique instead of an approach based on the maximum stress and strain locations in the solder joint. Volume-weighted average stress and strain results of three electronic assemblies are related to the corresponding experimental fatigue data through least-squares curve-fitting analyses for determination of the empirical coefficients of two fatigue life prediction criteria. The coefficients thus determined predict the mean cycles-to-failure value of the solder joints. Among the two prediction criteria, the strain range criterion uses the inelastic shear strain range and the total strain energy criterion uses the total inelastic strain energy calculated over a stabilized loading cycle. The obtained coefficients of the two fatigue criteria are applied to the finite element analysis results of two additional cases obtained from the literature. Good predictions are achieved using the total strain energy criterion, however, the strain range criterion underestimated the fatigue life. It is concluded that the strain information alone is not sufficient to model the fatigue behavior but a combination of stress and strain information is required, as in the case of total inelastic strain energy. The superiority of the volume-weighted averaging technique over the maximum stress and strain location approach is discussed.

Semi-Analytical Modeling of Crack Initiation Dominant Contact Fatigue Life for Roller Bearings

Abstract: The fatigue test of a needle roller bearing suggests that the dominant failure mechanism is subsurface crack initiation and propagation. Therefore, a new semi-analytical contact fatigue model is derived from a micromechanics based crack initiation model. The analysis indicates that in the life calculation the selection of the critical stress, such as the maximum orthogonal shear stress, maximum shear stress, octahedral shear stress, or von Mises equivalent stress, becomes arbitrary under the nonfrictional Hertzian line contact condition. The fatigue life of roller bearings under the pure rolling condition can be predicted by simply knowing the Hertzian contact pressure and the contact width, which avoids complicated calculation of the subsurface stresses. The film thickness, roughness, and the material hardness effects on contact fatigue are also included in the new model. The comparisons with different models and the experimental data indicate that the new model makes similar life predictions as the loannides-Harris model, but the new model is much simpler to use. The Lundberg-Palmgren model does not fit with the experiment data.

Prediction of Fatigue Life of a Conical Shaped Joint System for Fiber Reinforced Plastics under Arbitrary Frequency, Load Ratio and Temperature

Abstract: A prediction method for the fatigue life of polymercomposites under arbitrary frequency, load ratio andtemperature was extended to that of polymer compositestructures. The method is based upon fourhypotheses: (a) the same mechanism applies to static,creep and fatigue failure, (b) the same time-temperaturesuperposition principle holds for all failure loads, (c) thelinear cumulative damage law applies to monotone loading, and(d) there exists a linear dependence of fatigue failure load uponload ratio. The tensile tests of a conically shapedjoint system for fiber reinforced plastics (FRP joint)for static, creep and fatigue loadings were conductedat various temperatures. The validity of theproposed method and the applicability of thehypotheses for this FRP joint are discussed.

Comparison of 2 and 4 point fatigue tests and healing in 4 point dynamic bending test based on the dissipated energy concept

Abstract: At the 4th Eurobitume Symposium a fatigue life definition was introduced, which was based on the dissipated energy concept (Hopman et al, 1989). This new definition is based on a change in a material related aspect (dissipated energy per cycle) instead of a specimen property. It was shown that, using the dissipated energy law and this new fatigue life definition, the results were exchangeable for strain and stress controlled fatigue tests even at different frequencies. These experiments were only carried out in a 4 point dynamic bending test. In this paper the results are presented of a comparison of fatigue lives measured in 2 and 4 point dynamic bending tests. In spite of the new fatigue life definition, the results are not comparable for both tests. This may be due to the composition of the mix at issue and the small dimensions of the specimen in the 2 point bending test. Also the healing phenomenon is studied in the 4 point bending test. Rest periods between loadings ought to enlarge the fatigue life considerably. After a resting period the stiffness modulus is largely restored. If instead of resting periods (with zero loading), load blocks are used with a smaller load amplitude, it is shown that during these pseudo rest periods the stiffness modulus will increase. Depending on the duration of the pseudo rest period and the amplitudes of the loadings, this increase will be followed by a decrease in stiffness modulus. These findings confirm the opinion that healing already takes place during the fatigue process and should be taken into account in the interpretation of the fatigue measurements and the characterization of the fatigue properties. Based on the dissipated energy concept a model is proposed taking into account the healing phenomenon. This model should enable a more fundamental description of the asphalt fatigue properties including healing.

Effect of Some Parameters on the Plastic Fatigue Behavior with Micromechanical Approach

Abstract: The fatigue life and the micro-damage heterogeneity for FCC polycrystalline metals are qualitatively evaluated employing a coupled phenomenological micro-mechanical model of the early plastic fatigue damage initiation. This model is based on the slip theory and on the localization homogenization technique associated with the self-consistent scheme. For representing the micro-damage on each slip system, an internal damage variable dS is introduced. The influence of the main parameters of this model (aggregate composition, damage interaction matrix Drs, and loading path) on the related plastic fatigue phenomena as the fatigue lives and the micro-damage heterogeneity is studied. Hence, various fatigue simulations are performed using several types of aggregates and different loading paths (simple and complex). It is shown that such parameters have great influences on the fatigue behavior. Moreover, the distributions of the local damage heterogeneity are studied under the effect of each determined parameter.

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 recession 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.

Effects of Hold-Time on Thermal Fatigue Life of Solder Joints.

Abstract: The fatigue life estimation of solder joints is one of the most critical requirements in the development of reliable electronic components. Since the electronic components are subject to temperature variations under field conditions with a trapezoidal wave, it is important to obtain quantitative knowledge regarding the effects of hold-time on fatigue life. In this study, we first carried out low cycle fatigue tests using thin-walled cylindrical specimens. The specimens were subjected to strain controlled loading conditions with a trapezoidal wave and a variable strain rate wave in consideration of stress relaxation and elastic follow-up phenomena. It was confirmed that the fatigue life of the 63Sn-37Pb solder can be uniformly evaluated using an inelastic strain range and Coffin-Manson equations when the strain rate is low. Second, elastic-creep stress analysis was carried out for solder joints in a TSOP (thin small outline package). It was shown that the increase in creep strain during hold-time has a noticeable effect on the fatigue life under field conditions. Thus, careful consideration of creep deformation is essential, and consequently stress analysis based on the elastic-creep model can serve as a fatigue life estimation method in the reliability design.

Fatigue Assessment of an Automotive Suspension Component using Deterministic and Probabilistic Approaches

Abstract: This paper contains a case study on the fatigue assessment of a forged component for an automotive suspension. The fatigue strength reduction effects of the ‘as-forged’ surface resulting from the surface roughness and the presence of residual stresses were investigated. Test results under constant and variable amplitude bending loads are presented. In addition, test data on hourglass specimens specially prepared from the component material (a low alloy, medium carbon steel) under two different surface conditions, were also obtained to compliment the fatigue studies for the suspension arm. A range of analytical techniques were used to predict the effects of the surface condition on the fatigue behaviour, including a probabilistic approach in which the surface roughness and residual stresses were treated as random variables.

A strength-based wearout model for predicting the life of composite structures

Abstract: A model to predict the residual strength and life of polymeric composite structures subjected to spectrum fatigue loadings is described. The model is based on the fundamental assumptions that the structure undergoes proportional loading, that the residual strength is a monotonically decreasing function of the number of fatigue cycles, and that both the life distribution due to continuous constant amplitude cycling and the residual strength distribution after an arbitrary load history may be represented by two parameter Weibull functions. The model also incorporates a cycle mix factor to account for the drastic reduction of fatigue life that may be caused by a large number of changes in the stress amplitude of the loading. The model`s predictions are compared to experimentally determined fatigue life distributions for uniaxial loadings of a number of laminates comprised of different materials and layups. Constant-amplitude, two-stress level, and spectrum fatigue loadings, including the FALSTAFF (Fighter Aircraft Loading Standard for Fatigue) spectrum, are considered. The theoretical fatigue life distributions are shown to correlate well with the experimental results. Moreover, excellent correlation of theory and experiment is obtained for an average fatigue life that is based on the 63.2% probability of failure.

Consideration of loads for fatigue assessment of ship structures

Abstract: Technical issues associated with loads obtained for the purpose of fatigue assessment are discussed in this paper. The topic of discussion is focused specifically on spectral fatigue analysis and linear elastic fracture mechanics based crack-growth analysis. In order for the frequency domain formulation and the associated probability analysis to be valid, load analysis presented in this paper is largely limited to be linear. Some nonlinear features that should also be taken into consideration in the process of fatigue assessment are also discussed. This includes nonlinear roll motion and nonlinearity brought forth by intermittent application of loads, such as wetting of side shell in the splash zone by waves. The discussion emphasizes the dominant loads relevant to particular regions of the ship structure, as well as the eventual constitution of the set of loading. A simple, linear rule for load combination is proposed.

Complex fatigue of soldered joints-comparison of fatigue models

Abstract: The application of rainflow analysis to solder joint fatigue analysis is new, Rainflow analysis applies models developed from simple fatigue tests to predict complex fatigue life. In the work reported here, simple and complex fatigue testing was performed on single, eutectic tin-lead solder joints using a custom micromechanical tester. Simple, sinusoidal wave fatigue data were fitted to four models. From these models (two empirical models, a Coffin-Manson-based plasticity model, and a hysteresis energy model) fatigue life was predicted for complex wave conditions using rainflow analysis with linear damage accumulation. The correlation between the test data and complex fatigue model results was good, with correlation coefficients ranging from 0.956 to 0.977.

An Analytical Solution of Equivalent Stress for Structure Fatigue Life Prediction under Broad Band Random Loading

Abstract: An analytical solution of an equivalent stress-range calculation, based on the power-spectral density of stress in critical points of structures, and a statistical theory for the peak distribution of a stationary Gaussian random process are presented in this paper for fatigue life assessment under broad-band random loading. This model has more advantages than similar existing models.

Validation of a Deterministic Vibroacoustic Response Prediction Model

Abstract: This report documents the recently completed effort involving validation of a deterministic theory for the random vibration problem of predicting the response of launch pad structures in the low-frequency range (0 to 50 hertz). Use of the Statistical Energy Analysis (SEA) methods is not suitable in this range. Measurements of launch-induced acoustic loads and subsequent structural response were made on a cantilever beam structure placed in close proximity (200 feet) to the launch pad. Innovative ways of characterizing random, nonstationary, non-Gaussian acoustics are used for the development of a structure's excitation model. Extremely good correlation was obtained between analytically computed responses and those measured on the cantilever beam. Additional tests are recommended to bound the problem to account for variations in launch trajectory and inclination.

Correlation of a Sinusoidal Sweep Test To Field Random Vibration

Abstract: This paper presents a method for transferring the measured field random vibration into a sinusoidal sweep test by using the damage equivalence technique. This requires the fatigue damage generated in the sinusoidal sweep to be equivalent to the damage during the desired lifetime in the field operation. Based on this approach, a correlated lab-test specification, including the vibration level and test duration, can be determined according to the field random load input, the desired product life goal of a product, and the material/structural properties. If a generic test specification is required without knowing the material/structural properties, an approximate approach is proposed based on some engineering assumptions. In this paper, the development of a test specification for an automotive bracket is demonstrated as an application example.