Showing papers on "Vibration fatigue published in 1993"

Failure mechanism models for cyclic fatigue

Abstract: This work illustrates design situations where mechanical fatigue under cyclic loading, of one or more components, can compromise system performance. In this failure mechanism, damage accumulates with each load cycle, thereby causing a physical wearout failure mechanism. Phenomenological continuum length-scale models, based on micromechanical considerations, are presented to predict the onset (or initiation) of fatigue cracking in ductile materials. Fatigue crack propagation is modeled with continuum fracture mechanics principles. The number of load cycles required to cause failure is predicted based on these models. Approaches for modeling creep fatigue interactions are briefly discussed. Analytic physics-of-failure method and examples are presented for designing against wearout failure due to cyclic fatigue. These models can be implemented in an engineering design environment. The associated stress analysis requires numerical finite element techniques in many cases. The associated material property characterization techniques have matured since the 1950s and are specified in engineering handbooks. >

Curvilinear fatigue crack reliability analysis by stochastic boundary element method

Abstract: In this paper, the stochastic boundary element method, which combines the mixed boundary integral equations method explored in Reference 1 with the first-order reliability method, is developed to study probabilistic fatigue crack growth. Due to the high degree of complexity and non-linearity of the response, direct differentiation coupied with the response-surface method is employed to determine the response gradient. Three random processes, the mode I and mode II. stress intensity factors and the crack direction angle, are included in the expression of the response gradient. The sensitivity of these random processes is determined using a first-order response model. An iteration scheme based on the HL-RF method2 is applied to locate the most probable failure point on the limit-state surface. The accuracy and efficiency of the stochastic boundary element method are evaluated by comparing the cumulative distribution function of the fatigue life obtained with Monte Carlo simulation. The reliability index and the corresponding probability of failure are calculated for a fatigue crack growth problem with randomness in the crack geometry, defect geometry, fatigue parameters and external loads. The response sensitivity of each primary random variable at the design point is determined to show its role in the fatigue failure. The variation of each primary random variable at the design point with the change of probability of failure is also presented in numerical examples.

Fatigue load spectra for a steel girder bridge

Abstract: The objective is to present an approach for the evaluation of fatigue performance. The proposed method is a combination analytical and experimental approach. The steps include identification of critical components and details, determination of the load distribution factors and stress range, instrumentation of the bridge, measurement of strains under a control vehicle to calibrate the equipment and determine the load distribution factors, measurement of strain under normal traffic, verification of load distribution factors, verification of load range, and evaluation of fatigue performance. Critical components and details are identified on the basis of analysis and past experience. The analysis provides theoretical values of load distribution factors and stress ranges. Measurement results provide a basis for verification of the analytical values. The number of load cycles is determined from the stress record under normal traffic. For a given number of load cycles, the effective stress range is calculated and compared with the critical level. Then the remaining fatigue life is estimated as a percent of the remaining number of cycles to failure. This serves as a basis for the evaluation of fatigue performance. The proposed approach is applied to an existing steel girder bridge. The superstructure consists of four plate girders and transverse floor beams. The bridge is instrumented, and the results of measurements are presented and discussed. For the considered components and details, the measured stress range and estimated number of load cycles are not critical. Some minor cracking may be expected, but in general the bridge may be considered as adequate with regard to fatigue.

Characterization of Random Fatigue Loads

Abstract: The general approach in fatigue life prediction is to relate the fatigue life of a construction, subjected to a random load, to laboratory fatigue experiments of simple specimens subjected to constant amplitude load, so called S-N data. Therefore, it is necessary to define amplitudes of equivalent “load cycles” S k , which are functions of the sequence of maxima and minima in the load, and assume a damage rule, i.e. a method to measure the damage caused by each simple cycle.

A mixed panel-stick hydrodynamic model applied to fatigue life assessment of semi-submersibles

Abstract: The fatigue life of semi-submersibles is sensitive to short waves where hydrodynamic interactions between structural members are important. The way of taking the interactions into account is to perform a diffraction-radiation analysis. A panel model representing columns and pontoons is made to evaluate wave loadings by numerical program. In addition, a stick model is used to compute the complementary contribution from bracings by using Morison's formulae in which the fluid kinematics are obtained from the panel model. The fluid pressure is evaluated on each panel and integrated to give the member loads on the corresponding beam of the structural model by means of an interface program. The structural calculation and then the fatigue analysis are carried out. The results from this mixed panel-stick hydrodynamic model and those from a pure stick model are compared. It is shown that the stresses and fatigue life of some structural nodes are quite influenced by hydrodynamic interactions.

Development of the microstructure based stochastic life prediction models

Abstract: : The goal of this program was to develop a methodology for predicting fatigue lives of structural alloys based on their microstructural characteristics and mechanical properties. Such methodology has been successfully implemented and verified by predicting fatigue lives of the four different variants of the aluminum airframe 7050-T7451 plate alloy and, with modifications, of the butt welds of the ship hull HSLA-80 steels. The key features of the method is the assumption that the incipient fatigue crack size distribution is related to the size distribution of the bulk material flaws through the statistics of extreme. When combined with the Monte-Carlo (MC) crack growth model the extreme value estimates of the initial crack sizes gave excellent predictions of the fatigue lives of the 7050-T7451 alloy for samples both with and without stress concentrators. The specially for this purpose developed MC model utilized initial crack size distribution, crack location, crystallographic texture on the crack path and crack deflections as the random variables. A modified, closed-form three-parameter version of the model has been proposed for the butt welds of the HSLA-80 steels. This version, with parameters obtained based on the constant amplitude data, showed outstanding predictive capabilities for the samples with welds subjected to the variable amplitude loading conditions. Both versions of the model represent very useful and economical alternative to the lengthy fatigue testing programs. They allow for rapid differentiation between fatigue qualities of different material variants and on the parametric studies of the effects of the microstructural variables on fatigue lives. The methodology should instrumental in aiding alloy designers and process engineers in optimizing alloy microstructures for fatigue performance.

Full Scale Dynamic Monitoring of Highway Bridges

Abstract: Sensitivity of identified modal properties by physical testing is critical for automated monitoring of bridge structural condition. It is investigated here considering flexure modes and fatigue cracking. Variation in bridge modal parameters by impact testing due to random environmental conditions is obtained on a full-scale bridge. Extent of changes in the same parameters caused by simulated fatigue cracks is determined analytically. Preliminary results indicate that the sensitivity of the experimental modal analysis techniques is too low to detect initial and possibly well visible fatigue cracks for steel bridge members, and changes in frequencies are insensitive to the relative location of multiple fatigue cracks.

A fatigue failure criterion for concrete based on deformation

Abstract: A hypothesis is presented for fatigue failure of concrete structures. It is based on a deformation formulation, and utilizes the monotonic F - delta curve and the fatigue creep curve. The hypothesis is applied to flexural fatigue tests on notched beams of a plain high performance concrete. The experimental part comprises monotonic loading in deformation control and constant amplitude loading at three different load levels in flexural tension. The hypothesis is in all essentials consistent with the experimental findings. Furthermore, it provides a deformation formulation for accumulated damage estimation and remaining service life prediction, which takes account for the nonlinear nature of damage development in contrast to the linear Palmgren-Miner hypothesis.

Marine Component Fatigue Reliability

Abstract: A method is presented for evaluating the fatigue of offshore structural steel members based on structural component reliability procedures coupled with spectral analysis methods. The approach is based on formulating a nonlinear performance function that expresses the cumulative fatigure damage in the form of Miner's rule. Uncertainties in the cumulative fatigue damage rule and stress calculation are handled by the treatment of the parameters in question as random variables. As a measure of reliability against fatigue, the Hasofer‐Lind reliability index of the performance function is calculated using the first‐order marginal distribution method. The approach is invariant, accounts for correlation among random variables, and permits the ordering of the random variables and deterministic parameters to be readily performed. A numerical application on a simple idealized platform is used to demonstrate the method and study the fatigue reliability sensitivity. Results of the analysis indicate that the reliabilit...

Load-history sensitive cyclic curve concept in random load fatigue life predictions

Abstract: The results of two-level strain control tests of cyclic hardening aluminium alloy were used to derive the load-sensitive cyclic curve model (LHSCC). It incorporates effects of material cyclic strain hardening and of loading sequence. A computer algorithm for generating random load sequences is developed. It takes account of the statistical distribution of amplitudes and of autocorrelation properties of the process. A numerical example of a fatigue damage calculation based on the LHSCC concept is presented.

Fatigue Damage Prediction for Combined Random and Static Mean Stresses

Abstract: Closed form analytical expressions have been derived and are proposed for use to predict accumulated fatigue damage and fatigue life of structural elements subjected to a combination of fully reversed narrow-band Gaussian random and static mean stresses Such mean stresses can significantly alter fatigue life The proposed method of combining random alternating and mean stresses shows excellent agreement with published experimental data for a steel alloy Reasonable agreement is maintained, surprisingly, even for static tensile stress values up to near the material's yield stress where the failure mode shifts from that of typical brittle fatigue to that of stress rupture (ic,creep) Numerical examples are provided to illustratc the application

An orthotropic damage model with damage field mobility (dfm) method for fatigue crack propagation

Abstract: In a fatigue process, not only fatigue damage due to cyclic loads, but also plastic damage due to monotonic loads, should be considered. In this paper, an orthotropic damage model of fatigue and plastic deformation is established. A new method known as Damage Field Mobility (DFM) method is developed for fatigue crack propagation analysis. This method takes into account the effect of damage gradient, similar to the conventional stress gradient, on fatigue crack propagation. A general purpose finite element program was modified to calculate fatigue crack propagation rate. A series of experiments have been conducted to verify the proposed model and the DFM method.

Fatigue under variable amplitude loading: a new approach

Abstract: The prediction of fatigue life under random loading is usually based on Palmgren-Miner damage and a cycle counting method. The calculation is expensive and various fast fatigue assessment methods have been proposed. A brief comparison is given of some of these methods. A new damage model is proposed which does not depend on the Palmgren-Miner hypothesis or involve cycle counting. This Damage Integral approach predicts the same crack growth rate as the (approximate) Rayleigh model; on restriction to constant amplitude loading, however, the fatigue laws for the new model have an R-ration dependence which is absent from the Rayleigh model.

Stochastic linearization as a tool for fatigue damage estimation

Abstract: The paper discusses the problem, of linearization for the purpose of simplified calculation of long-term fatigue damage in offshore structures. Specific linearization procedures are proposed that may be better suited to deal with the problem of fatigue life calculation than the standard mean square stochastic linearization (MSSL). These are based on minimizing higher order moments of the linearization error, and it is indicated how the optimal order is linked to the street exponent of the S-N curve. It is shown by specific example studies that the proposed method may lead to substantial improvement over MSSL in estimating fatigue damage.

The Effect of Nonlinear Drag PSD on Fatigue of Compliant Offshore Structures

Abstract: Structural fatigue damage induced by random ocean wave loading on a compliant type of offshore structure is considered. The nonlinear wave force is modeled by the Morison equation and its effect on fatigue is investigated using an appropriate PSD in the frequency domain. It is shown that the nonlinear drag effect on PSD is very significant for compliant offshore structures. Fatigue damage predictions using nonlinear and linearized wave force PSD's are compared with time domain simulation.

A Damage Integral Based Analysis and Simulation of the Thermal Fatigue of Diebonds

Abstract: Microelectronic packages undergo substantial thermal excursions during processing and often in service. Differences in the thermal expansion of the various components may therefore lead to fatigue of the interconnects. A computer simulation has been developed which is based on a damage integral approach for the modeling of damage and failure in the various bonding layers found throughout such a package. This code should apply equally well to solder die bonds and polymeric or epoxy adhesive layers, assuming that the corresponding crack growth parameters and constitutive relations are known. Using literature values for these properties, predictions have been compared to experimental thermal cycling data for solder die bonds. Consequences for the extrapolation of accelerated test results to service conditions are discussed.

An Engineering Approach to Determine the Fatigue Life of Sheet Structures Under Cyclic Loading

Abstract: The structural integrity and the reliability of sheet structures, especially those of high risk like pressure vessels and pipes, is a relevant topic which has centered the attention of many researchers. This paper offers an engineering approach to determine the fatigue life of sheet structures under cyclic loading. The theoretical development presents a general method for calculating, by using a micro-computer, the initiation and fracture loci and the two-parameter crack growth paths as well as the integral value and therefore the fatigue life of the structure. This permits an evaluation of the influence of the different factors from the structural integrity point of view, and particularly of the material. The proposed method is applied to the computation of the fatigue life of a pressure vessel under cyclic loading produced by the thermal oscillations of the environment, allowing a study of leak before break.

Rationally-based fatigue design of tankers

Abstract: The authors have developed a practical rationally-based fatigue design method which is sufficiently rapid and efficient to be part of preliminary structural design. The method consists of five basic steps: specifying a realistic wave environment, generating a hydrodynamic ship-wave interaction model, computing cyclic nominal stress due to waves and ship motions, using S-N data to predict fatigue life, and using structural optimization to re-size the scantlings such that the desired fatigue life is obtained. The paper describes the method and explains the similarities and the differences with other approaches. Example calculations involving a TAPS tanker are presented which validate the method, and which also demonstrate the importance of both local and overall structure response. The examples show that the heretofore unexplained fatigue cracking of midheight side longitudinals is caused by the local pulsating pressure due to waves and ship motions. This demonstrates clearly that fatigue analysis and design must include response to local panel pressure. Other fatigue analysis methods do not properly account for this effect.

A Thermal Fatigue Model for Probabilistic Lifetime Strength of Propulsion System Components

Abstract: This paper describes the development of methodology for a probabilistic material strength degradation model, that provides for quantification of uncertainty in the lifetime material strength of structural components of aerospace propulsion systems subjected to a number of diverse random effects. The model has most recently been extended to include thermal fatigue. The discussion of thermal fatigue, in the context of probabilistic material strength degradation, is the central feature of this paper. The methodology, for all effects, is embodied in two computer programs, PROMISS and PROMISC. These programs form a “material resistance” model that may be used in the aerospace structural reliability program, NESSUS or in other applications. A probabilistic material strength degradation model for thermal fatigue and other relevant effects, in the form of a postulated randomized multifactor interaction equation, is used to quantify lifetime material strength. Each multiplicative term in the model has the property that if the current value of an effect equals the ultimate value, then the lifetime strength will be zero. Also, if the current value of an effect equals the reference value, the term equals one and lifetime strength is not affected by that particular effect. Presently, the model includes up to four effects that typically reduce lifetime strength: high temperature, mechanical fatigue, creep and thermal fatigue. Statistical analysis of experimental data for Inconel 718 obtained from the open literature and laboratory reports is also included in the paper. The statistical analysis provided regression parameters for use as the model’s empirical material constants, thus calibrating the model specifically for Inconel 718. Model calibration was carried out for four variables, namely, high temperature, mechanical fatigue, creep and thermal fatigue. Finally, using the PROMISS computer program, a sensitivity study was performed with the calibrated random model to illustrate the effects of mechanical fatigue, creep and thermal fatigue, at about 1000 °F, upon random lifetime strength.Copyright © 1993 by ASME