Accurate Fatigue Life Prediction Research Articles

Wind-induced fatigue collapse of real slender structures

This paper analyzes the fatigue collapse of two slender structures due to wind-induced vibrations. The analysis is based on a complete procedure for the wind-induced fatigue analysis proposed by the authors. The application of the proposed method, compared with the real cases examined, leads to accurate fatigue life prediction; moreover, it gives a complete picture of the stress state and damage of the structures, furnishing useful information on the structural behavior and a key for the interpretation of collapses and damage causes. In particular, the analysis highlights the critical aspects of the aerodynamic behavior of the considered structures and suggests some remarks on the current state-of-the art on design against wind-induced actions.

Fatigue Indicator Sensor의 형상에 따른 균열진전 특성의 비교 연구

It is difficult to predict the accurate fatigue life of the ship structure because of load uncertainty and load redistribution at the ship structure members. As one of studies for accurate evaluation and prediction of fatigue life, it is a promising way to detect the crack previously by attaching the Fatigue Indicator Sensor (FIS) at the crack prediction region. In order to predict the fatigue life of the ship structure by using FIS, it is required to know previously the crack propagation characteristics according to pre-notch shapes. In this study, we obtained the stress distribution phase, stress concentration factors and stress intensity factor of various pre-notch shapes through FEA. Additionally, we conducted the fatigue test and obtained the characteristics of crack propagation according to the pre-notch shapes through comparison between the fatigue test and the FEA. Consequently, we classified the pre-notch shape into 3 categories: Long, Medium, and Short life type. On the basis of the numerical and experimental results, the FIS can be developed.

Using Artificial Neural Networks to Predict the Fatigue Life of Different Composite Materials Including the Stress Ratio Effect

Artificial Neural Networks (ANN) have been successfully used in predicting the fatigue behavior of fiber-reinforced composite materials. In most cases, the predictions were obtained for the same material used in training subjected to different loading conditions. The method would be of greater value if one could predict the failure of materials other than those used for training the network. In a recent paper, ANN trained using the experimental fatigue data obtained for composites subjected to a constant stress ratio $$ \left( {{\hbox{R}} = {\sigma_{{ \min }}}/{\sigma_{{ \max }}}} \right) $$ was successfully used to predict the cyclic behavior of a composite made of a different material. In this work, this method is extended to include the stress ratio effect. The results show that ANN can provide accurate fatigue life prediction for different materials under different values of the stress ratio. These results can allow for the development of a materials smart database that can be used for various engineering applications.

A three-segment anisomorphic constant life diagram for the fatigue of symmetric angle-ply carbon/epoxy laminates at room temperature

The anisomorphic constant fatigue life (CFL) diagram approach that was developed in an earlier study is further tested for applicability to the matrix-dominated fatigue failure in symmetric angle-ply carbon/epoxy laminates. An extension of the CFL diagram approach is also attempted to improve the accuracy of fatigue life prediction. The original anisomorphic CFL diagram approach can be used for approximately predicting the CFL diagrams for the [±30] 3S and [±45] 3S laminates, while it fails to accurately predict the CFL diagram for the [±60] 3S laminate due to its significant local distortion. For accommodating the anisomorphic CFL diagram approach to the local distortion in CFL curves due to a significant change in mean stress sensitivity in fatigue, a transitional segment is inserted between the tension–tension and compression–compression dominated segments. It is demonstrated that the three-segment anisomorphic CFL diagram approach allows improved predictions of the CFL diagrams and S– N relationships for the angle-ply laminates.

An Energy Based Fatigue Life Prediction Framework for In-Service Structural Components

An energy based fatigue life prediction framework has been developed for calculation of remaining fatigue life of in service gas turbine materials. The purpose of the life prediction framework is to account aging effect caused by cyclic loadings on fatigue strength of gas turbine engines structural components which are usually designed for very long life. Previous studies indicate the total strain energy dissipated during a monotonic fracture process and a cyclic process is a material property that can be determined by measuring the area underneath the monotonic true stress-strain curve and the sum of the area within each hysteresis loop in the cyclic process, respectively. The energy-based fatigue life prediction framework consists of the following entities: (1) development of a testing procedure to achieve plastic energy dissipation per life cycle and (2) incorporation of an energy-based fatigue life calculation scheme to determine the remaining fatigue life of in-service gas turbine materials. The accuracy of the remaining fatigue life prediction method was verified by comparison between model approximation and experimental results of Aluminum 6061-T6. The comparison shows promising agreement, thus validating the capability of the framework to produce accurate fatigue life prediction.

Predicting the Fatigue Life of Different Composite Materials Using Artificial Neural Networks

Artificial Neural Networks (ANN) have been recently used in modeling the mechanical behavior of fiber-reinforced composite materials including fatigue behavior. The use of ANN in predicting fatigue failure in composites would be of great value if one could predict the failure of materials other than those used for training the network. This would allow developers of new materials to estimate in advance the fatigue properties of their material. In this work, experimental fatigue data obtained for certain fiber-reinforced composite materials is used to predict the cyclic behavior of a composite made of a different material. The effect of the neural network architecture and the training function used were also investigated. In general, ANN provided accurate fatigue life prediction for materials not used in training the network when compared to experimentally measured results.

Application of a fatigue equivalent static load methodology for the numerical durability assessment of heavy vehicle structures

It is common practice in the automotive industry to employ multi-parameter strain-life methods, in combination with dynamic finite element analyses, based on very extensive measurement exercises, to conduct analytical fatigue life assessments, which is then verified through intensive durability testing. The expense and complexity of this approach makes its application impractical for low volume “special” vehicles. In this paper, a fatigue equivalent static load (FESL) methodology for the numerical durability assessment of heavy vehicle structures is presented, where fatigue load requirements are derived from measurements as quasi-static g-loads, the responses to which are considered as stress ranges applied a said number of times during the lifetime of the structure. The application of the method is demonstrated using two case studies, namely a road tanker and a load haul dumper. In both cases, it was possible to obtain adequately accurate fatigue life prediction results, using simplified loading, static finite element analyses and a stress-life approach to fatigue damage calculations, with material properties available in design codes.

Comparative study of multiaxial fatigue damage models for ductile structural steels and brittle materials

Fatigue damage prediction under a general multiaxial service loading consists of three main steps: multiaxial cycle counting, damage evaluation for an identified cycle (or reversal), and damage accumulations. The accuracy of fatigue life predictions depends on all the above steps. This paper reviews the evolutions of various multiaxial fatigue damage models, a comparative study is conducted about the physical basis, the computational efficiency, and the application range of the approaches. Based on the comparative studies, a new procedure is proposed to evaluate fatigue damage under general multiaxial random loading, which uses the Wang and Brown´s multiaxial cycle counting method for identifying cycles (or reversals), the modified procedure of the minimum circumscribed ellipse (MCE) approach for fatigue damage evaluation for an identified cycle (or reversal), and the Miner´s linear damage law for fatigue damage accumulations. By comparisons of the predicted life results with experimental results and with other approaches, it is shown that the proposed procedure is very efficient and suitable for computer aided structural optimization against fatigue.

Use of Small Samples to Predict Fatigue Lives of Field Cores

Fatigue cracking is one of the major distresses in asphalt pavements. Accurate prediction of fatigue life of asphalt pavements can be extremely important both during the design stage and for prediction of remaining service life of in-service pavements. Traditional fatigue life predictions based on bending beam tests can be costly and time-consuming. The uniaxial push–pull (tension–compression) tests run on cylindrical samples have been a novel alternative. However, the traditional sample size for the push–pull tests may prevent its use for thin in-service pavements. This paper presents the results of a study investigating the possibility of using smaller sample sizes for push–pull tests. The viscoelastic continuum damage (VECD) characteristics of regular and small-size samples are compared, and the difference is observed to be negligible. In addition, a practical fatigue life formulation is derived on the basis of VECD theory. Uniqueness of the derived fatigue life ( Nf) equation, differing from previously derived VECD-based Nf equations, stems from the fact that it does not force a certain form of equation to fit the damage characteristic curve. Finally, the differences in fatigue lives of different layers of the field sections at FHWA's accelerated loading facility are investigated.

J0301-1-4 軸・捩り複合荷重下の疲労寿命評価(規格における疲労関連課題と方向性(1))

Fatigue failure is one of the essential failure modes to be considered in the design of light water reactor plants. Treatment of stress multiaxiality in the fatigue assessment is of practical importance for many components subjected to multiaxial cyclic loading during operation. Based on the results of multiaxial fatigue tests on hollow and solid bar specimens of SUS316NG and SGV410 under combined axial and torsional loading, accuracy of fatigue life prediction based on the conventional parameters as well as a newly proposed two parameter approach proposed here was studied. The conventional parameters tended to provide conservative life prediction for the case of shear-dominated cases especially for 316NG while the new approach was able to give more realistic prediction.

Study on Applicability of Multiaxial Fatigue Parameters to Nuclear Piping Materials

Treatment of stress multiaxiality in the fatigue assessment is of practical importance for many components subjected to multiaxial cyclic loading during operation. Based on the results of multiaxial fatigue tests on hollow and solid bar specimens of SUS316NG and SGV410, accuracy of fatigue life prediction based on the conventional parameters as well as a two parameter approach proposed here was studied. The conventional parameters tended to provide conservative life prediction for the case of shear-dominated cases especially for 316NG while the new approach was able to give more realistic prediction

Evaluation of spectrum fatigue crack growth using variable amplitude data

The ability to understand and predict fatigue life remains a key technical factor in maintaining aircraft fleets which are required to safely operate up to their design lives, and sometimes beyond. In particular military aircraft are often required to achieve long lives under demanding operational conditions. The load spectra applied to these aircraft are complex and highly variable, and experience has shown that traditional fatigue prediction tools do not always perform well in calculating the lives of modern, highly optimised airframes. This paper summarises a recent semi-empirical model that appears to be capable of producing more accurate fatigue life predictions using flight load spectra based on realistic in-service usage. The model relies on experimentally derived variable amplitude crack growth data. The model itself is founded on the observation that, as a first approximation, for lead cracks in many materials the log of the crack depth is proportional to the number of cycles or spectrum loading blocks. This relationship can also be used to define the effective crack-like size of the initiating defect or discontinuity. The implications of this exponential growth, including the crack depth dependency of the crack growth rate, are discussed leading to a reconciliation of a Paris-type growth model with that of the earlier Frost and Dugdale stress-cubed growth relationship.

Elasto-Viscoplastic Behavior of the Ferritic Stainless Steel AISI 441-EN 1.4509 from Room Temperature to 850°C

Abstract In order to predict the thermomechanical fatigue (TMF) damage of stainless steel parts submitted to thermal loading, the elasto-viscoplastic behavior has to be described accurately in a wide range of temperature, strain, and strain rate levels. The study is done on a stabilized ferritic stainless steel grade equivalent to AISI 441 or EN 1.4509 standard which is used more and more in high temperature application up to 950°C, for example, in exhaust manifolds, heat exchanger, burners, or water-boiler device. The elasto-viscoplastic model proposed by Chaboche is chosen and identified on the basis of isothermal cyclic tests including relaxation and graduated cyclic strain tests performed under air at 300, 650 and 850°C. The validation procedure was performed afterward by comparison with stabilized behavior under nonisothermal conditions, especially in-phase, out-of-phase, and diamond TMF cycles. Results of simulation show very good fitting with the experimental curves which would lead to a more accurate fatigue life prediction.

Development of an Improved High Cycle Fatigue Criterion

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.

The effect of model building on the accuracy of fatigue life predictions in electronic packages

Empirical fatigue life models such as the Coffin–Manson rule and its variants are commonly used at the present time to predict the reliability of microelectronic packages. While there have been reports of substantial error in empirical correlations relative to the experiments, this has not been accompanied by a rigorous understanding of the sources of the error. In this paper we systematically explore the various modeling errors in the fatigue life prediction. These errors include those in geometry representation, material behavior, load history and boundary condition application, and in the numerical solution procedure. As part of the study, experimentally validated correlations between temperature cycling and power cycling are developed for a TI 144 chip-scale package. The accuracy of the predicted life under power cycling conditions compared to the experimentally determined life is used as the basis for judging the model accuracy. The criticality of spatial refinement, temporal refinement, and accurate boundary conditions, including the often ignored natural convection boundary conditions, and their effect on predicted life is described in detail. It is shown that model errors can be a significant part of both the constitutive life models and the application models that use the constitutive life models to predict the fatigue life under a given environmental condition. It is also shown that with careful model building, solutions accurate to within 3% can be obtained.

Fatigue of clamped connections with application to a stem-handlebar assembly for off-road bicycles

The objectives of this study were: (i) to determine the stresses as a result of assembly and the stress concentration of a clamped connection, (ii) to demonstrate the importance of the effects of assembly in fatigue life predictions and (iii) to verify the accuracy of fatigue life predictions by measuring fatigue life experimentally. A stem-handlebar assembly used in off-road bicycles was chosen for the study because it is a critical assembly for which structural reliability must be insured to prevent serious injury. Assembly stresses in the handlebar exceeded 200 MPa and stress concentration because of the attached stem increased the applied stress by 40%. A stress-life prediction indicated that assembly effects would reduce predicted fatigue life of the handlebar by a factor of approximately 20. The measured fatigue lifetime of the handlebar was longer than that predicted by using the stress-life approach, but was much shorter than that predicted when assembly effects were ignored. Therefore, assembly effects have a significant effect on fatigue life predictions and should be included in lifetime assessments for clamped mechanical assemblies.

Neuro-Fuzzy Modeling of Fatigue Life Prediction of Unidirectional Glass Fiber/Epoxy Composite Laminates

Fatigue behavior of unidirectional glass fiber/epoxy composites under tension–tension and tension–compression loading is important in the design of composite structures. Adaptive neuro-fuzzy modeling was successfully used to model the relationship between the input/output variables of fatigue behavior of unidirectional glass fiber/epoxy composites. The experimental input variables were the maximum stress, fiber orientation, and stress ratio, while the output variable was the number of cycles to failure. In comparison with previous results obtained using neural networks only, the proposed hybrid neuro-fuzzy method gave more accurate fatigue life predictions.

Prediction of degree of bending in tubular X and DT joints

Analysis of the large scale fatigue testing results of offshore welded tubular joints showed that the fatigue life is not dependent on the hot spot stress alone, but is also significantly influenced by the through thickness stress distribution which can be characterised by degree of bending (DoB). Accurate prediction of DoB data in tubular joints is crucial for improving the accuracy of fatigue life prediction using stress-life (S–N) curve and particularly for predicting fatigue crack growth using the fracture mechanics method. However, there is no set of DoB parametric equations available for tubular X and DT joints. For this reason, comprehensive thin shell finite element (FE) analyses were carried out for 330 tubular X and DT joints typical of those used in offshore structures, under six different modes of loading. The results of these FE analyses were used to derive a set of parametric equations to predict the DoBs at the critical positions on both brace and chord toes in tubular X and DT-joints under each mode of loading.

The influence of dynamic strain ageing on stress response and strain-life relationship in low cycle fatigue of 316L(N) stainless steel

It has been recognized that the essential prerequisite for accurate prediction of low cycle fatigue life (LCF) at elevated temperatures is the determination of rate controlling time dependent damage process that influences the cyclic deformation and fracture behavior of alloys. At intermediate temperatures (0.25 < T/T{sub m} < 0.55, where T{sub m} is melting point in Kelvin), where the effects of time dependent processes such as creep and oxidation are found to be minimal, the drastic reduction in low cycle fatigue life observed with increasing temperature and decreasing strain rate in alloys such as type 04 stainless steel, Nimonic PE 16 superalloy and Haynes 188 superalloy has been ascribed primarily to the deleterious effects of dynamic strain ageing (DSA). These alloys displayed anomalous cyclic hardening and an increase of maximum cyclic stress with increasing temperature or decreasing strain rate due to DSA. The nitrogen alloyed 316L stainless steel is currently used for a variety of components in the liquid metal cooled fast breeder reactor (LMFBR). Although the temperature range of application of 316L(N) in LMFBRs encompass the domain in which DSA is active, to-date there has been little information available on the effects of DSA on cyclic behavior of 316L(N)more » stainless steel (SS). The present investigation is therefore focused on assessing the effects of DSA on LCF life and cyclic stress response of 316L(N) SS. The influence of DSA on cyclic strain-life relationship has also been examined in the regime where other time dependent mechanisms such as creep and oxidation are virtually absent.« less

Damping Measurements Using Operational Data

We have measured modal damping using strain-gauge data from an operating wind turbine. This new technique for measuring modal damping is easier and less expensive than previously used methods. Auto-correlation and cross-correlation functions of the strain-gauge data have been shown to consist of decaying sinusoids which correspond to the modal frequencies and damping ratios of the wind turbine. We have verified the method by extracting damping values from an analytically generated data set. Actual operating response data from the DOE/Sandia 34-m Test Bed has been used to calculate modal damping ratios as a function of rotor rotation rate. This capability will allow more accurate fatigue life prediction and control.