Influence of material data input on accuracy of fatigue assessments for beam weldments

This numerical study investigates the effects of fatigue material data and finite element types on accuracy of residual life assessments under HCF conditions. The bending of cross-beam connections is simulated in ANSYS Workbench for three different combinations of beam profiles. The weldments are made of the high-strength steel grades C350LO and C450LO according to AS3678. The stress analysis of weldments is implemented with solid and shell elements using linear material and geometry consideration. The stress distributions are transferred to the embedded fatigue code nCode DesignLife. For both variants of FE-mesh, the nominal stress in the weld toes is extracted by splitting the total stress into membrane and bending components and filtering out non-linear component. Considering the effects of bending, size and mean stress, failure locations and fatigue life are predicted using the Volvo method and rules from ASME BPV Code. Three different pairs of experimental S-N curves (stiff and flexible) are used as material data input for fatigue analysis. The obtained numerical predictions are compared to the experimental results for shell FE-models. The predictions using the S-N curves for an equivalent steel demonstrate the best accuracy proving the fact that specific material data input is more effective than a generic data. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Diagnostics in Birnbaum–Saunders accelerated life models with an application to fatigue data

In industrial statistics, there is great interest in predicting with precision lifetimes of specimens that operate under stress. For example, a bad estimation of the lower percentiles of a life distribution can produce significant monetary losses to organizations due to an excessive amount of warranty claims. The Birnbaum–Saunders distribution is useful for modeling lifetime data. This is because such a distribution allows us to relate the total time until the failure occurs to some type of cumulative damage produced by stress. In this paper, we propose a methodology for detecting influence of atypical data in accelerated life models on the basis of the Birnbaum–Saunders distribution. The methodology developed in this study should be considered in the design of structures and in the prediction of warranty claims. We conclude this work with an application of the proposed methodology on the basis of real fatigue life data, which illustrates its importance in a warranty claim problem. Copyright © 2012 John Wiley & Sons, Ltd.     

Isotropic softening model for high-cycle fatigue

As known the high cycle loading of structural elements above the fatigue limit results in catastrophic propagation of macroscopic fatigue crack(s). Therefore it is in demand to find a method, which would allow for early prediction of the fatigue life-time of particular structural element. The series of force-driven tests carried on 30CrNiMo8 steel have shown that, although the stresses by high-cycle fatigue do not reach the tensile stress, the material strength (measured indirectly from the strain-stress loop) decreases from the begin of the test, hence the damage nucleates and propagates from the very begin of the loading history. In this study we propose a thermodynamically consistent, strain driven constitutive model for isotropic damage accumulation by high-cycle fatigue of a homogeneous isotropic material, which should be capable to account for the above-mentioned experimental results. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Multicycle fatigue resistance of brittle polymers

Conclusion A model of multicycle fatigue of polymers based on the hypothesis of self-simulation of the failure process was proposed and a method of predicting the durability was elaborated in the present study. It was found that in low-temperature conditions of self-heating, the crack is the basic cause of fatigue failure. A cyclic durability equation was obtained in the form of a modified time summation criterion for the stressed state in the failure zone in the vicinity of the apex of a fatigue crack. It was shown that in the case of brittle failure in multicycle fatigue, the maximum principal stresses which arise in the failure zone, one order of magnitude smaller than the size of the fatigue crack, are the criterion of failure.Translated from Mekhanika Kompozitnykh Materialov, No. 4, pp. 633–638, July–August, 1990.     

A constitutive and damage model for high cycle fatigue of tetragonal ferroelectrics

Reliability and life time of smart materials are crucial features for the development and design of actuator and sensor devices. Being widely used and exhibiting brittle failure characteristics, ceramic ferroelectrics are of particular interest in this field. Due to manifold interactions of the complex nonlinear constitutive behavior on the one hand and the damage evolution in terms of microcrack growth on the other, modeling and simulation are inevitable to investigate influence parameters on strength, reliability and life time. A condensed approach is used for the simulations considering just one characteristic point in the material, nonetheless accounting for polycrystalline grain interactions. On this basis, a model to predict the life time in terms of high cycle fatigue under electromechanical loading conditions is introduced. (© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Life Prediction of FRP Composites by a Direct Method

A direct computational approach for lifetime prediction of fibre-reinforced polymer (FRP) composites is presented. The approach is based on a direct method which allows predicting the fatigue life from the stabilised damage state. The classical direct method is generalised to the case of coupled plasticity with damage mechanics of the UD-FRP composite materials [1]. The constitutive model is based on a continuous damage meso-scale approach [2]. By analysing damage variables and thermodynamical forces associated with damage at the stabilised state, fatigue life prediction law is proposed as a power law function of stabilised thermodynamic forces. The obtained numerical results have been validated by experimental test results on standard glass-fibre/epoxy angle-ply and cross-ply laminate plates. The proposed approach could serve as a useful tool for the design of FRP composites. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

A FEM model for lifetime prediction of Ancorsteel1000C in rolling contact fatigue

Ancorsteel1000C is commonly used for important components including bearing rollers, gears, connecting rods, wheels and rails; most failures of these components are caused by rolling contact fatigue (RCF). In this study the fatigue damage indicator based on Fatemi-Socie critical plane were determined in a local hot spot area in finite element (FE) model in case of Hertzian stress with zero slip condition. The simulation results compared with respect to experimental data, which is in good agreement. Therefore, this approach can be implemented as a useful tool for lifetime calculations in RCF in the process of structure fatigue design. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Residual strength of GFRP at high-cycle fatigue

Degradation of the residual strength of a glass-fiber-reinforced polyester composite of a lay-up typical of the wind rotor blade material is studied at low-cycle fatigue. A gradual reduction of the residual strength is observed as expected for GRP, accompanied by an increasing scatter of strength. The residual strength model based on the strength-life equal rank assumption yields an accurate approximation of experimental data. The strength reduction at a stress level corresponding to high-cycle fatigue (N>10 ⁶ cycles) appears to correlate well with the test results at higher stress levels, which indicates that the strength degradation at the design stress level can be evaluated using low-cycle tests. Assuming that the parameters of the strength degradation model do not depend on the applied stress level, the residual strength data obtained in low stress level tests of comparatively short duration can be used to estimate the average fatigue life at the same stress thus reducing the total test time. Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 5, pp. 595–604, September–October, 1999.     

Structure analysis, fatigue testing, and lifetime prediction of composite steels

Composite steels prepared by technology of powder metallurgy are widely used as low cost parts with good resistance to wear, fracture, and corrosion. The development of powder composite steels is directly related to strength under vibration, fatigue stabilizing, and accurate lifetime prediction for actual composite topology. The fatigue behavior of powder steels was studied by experimental and numerical methods of composite mechanics and materials sciences. The chemical composition of composite steel is a pure iron powder as the base material and a handful of carbon, chromium, nickel, or phosphorus powders. The powder multi-component mixture is compacted by cold isostatic pressing to a rectangular form. The compactants are sintered in protective atmosphere. The microscale examination of the composite structure included an METAM-RV-21 metallographic optic microscope with a high-resolution ScanNexIIc scanner and an image processing package on the PC platform. The phase composition of powder steels has complex disordered topology with irregular ferrite/austenite grains, iron carbide inclusions, and pores. The microstructure images are treated according to the theory of stochastic processes as ergodic probability functions; statistical moments and a structural covariance function of the composite steels are given. The microscale stress-strain state of the composite steel is analyzed by finite element methods. The stiffness matrix of the composite steel is also presented together with stiffness matrices of ferrite/austenite grains, iron carbide inclusions, and pores as zero matrices. Endurance limits of the microstructural components are described by the Basquin or Coffin-Manson laws, respectively, as high and low cycle fatigue; cumulative microdamage in loading with a variable amplitude is taken from the Palmgren-Miner rule. Planar specimens were tested by console bending. Symmetric fatigue cycling was performed at a stable frequency of 20 Hz with endurance limits up to 5·10⁶ cycles. The probabilistic S-N curves were studied for various types of the composite steels. The fatigue properties of the structural components such as ferrite/austenite grains and carbide particles were defined by the microscale stress-strain modeling. Structural impact on the fatigue lifetime was computed; the probabilistic fatigue curves of the composite steels of various phase compositions are given. The prediction of cyclic lifetime and fatigue testing show good agreement for the powder composite steels studied.Presented at the 10th International Conference on the Mechanics of Composite Materials (Riga, April 20–23, 1998).Perm' State Technical University, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 34, No. 3, pp. 371–382, May–June, 1998.     

Growth of arterial cracks in polymers subject to static and fatigue loading

The growth kinetics of artificial and natural cracks (the former arising from notches) are studied in viscose, di- and triacetyl cellulose, nitrocellulose, polymethylmethacrylate, polystyrene, polypropylene, and carpon films by a micro-motion-picture method over a wide temperature range under both static and cyclic (fatigue) loading. In all the cellulose materials studied, the time required to form the nuclei of visible cracks is much shorter than the total life of the sample. The initial rate of crack growth depends exponentially on the applied stress and test temperature, so that the equation for the growth rate of an arterial crack is analogous to the general life equation. The different ways in which stress and test temperature affect the parameters of the equation describing the kinetics of crack growth enable us to distinguish the effect of local heating and that of the relaxation processes in the fatigue problem.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 2, pp. 253–265, March–April, 1970.     

Stochastic modeling of fatigue crack propagation

This paper presents a stochastic model of fatigue-induced crack propagation in metallic materials. The crack growth rate predicted by the model is guaranteed to be non-negative. The model structure is built upon the underlying principle of Karhunen–Loève expansion and does not require solutions of stochastic differential equations in either Wiener integral or Itô integral setting. As such this crack propagation model can be readily adapted to damage monitoring and remaining life prediction of stressed structures. The model results have been verified by comparison with experimental data of time-dependent fatigue crack statistics for 2024-T3 and 7075-T6 aluminum alloys.     

A more precise definition of the method of determining the fatigue life of polymer materials from the reheat temperature

Conclusions 1. The rate of increase in the reheat temperature on the linear segment remains constant (for a periodic loading with higher stress levels) until a critical damage is sustained for the stress in question.2. The magnitude of T_c for a given stress is independent of the loading history, provided the critical damage has not been sustained for this stress.3. With decreasing stress T_c decreases slightly. A method for the accelerated determination of T_c from a single specimen for different stress levels has been developed.4. A method of predicting the fatigue life from the results of short-term tests, which take into account the variation of T_c due to stress level, has been developed.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 5, pp. 906–913, September–October, 1977.     

Determination of the fatigue life of plastics from the self-heating temperature

The nondependence of the critical self-heating temperature of plastics on the variable load level and the linearity of polymer self-heating have been used to devise an accelerated method of determining fatigue life, which makes it possible to shorten testing times by a factor of up to 10 with satisfactory agreement between the calculated and actual fatigue lives.Mekhanika Polimerov, Vol. 3, No. 1, pp. 111–117, 1967     

Design of hybrid genetic- SVD ANFIS networks for fatigue life modelling of GRP composites

Genetic algorithm (GA) and singular value decomposition (SVD) are deployed for the optimal design of both Gaussian membership functions of antecedents and the vector of linear coefficients of consequents, respectively, of adaptive neurofuzzy inference systems (ANFIS) networks that are used for fatigue life modelling and prediction of unidirectional GRP Composites. The aim of such modelling is to show how the fatigue life varies with the variation of important parameters namely, maximum stress, stress ratio, fiber angle. It is demonstrated that SVD can be effectively used to optimally find the vector of linear coefficients of conclusion parts in ANFIS models and their Gaussian membership functions in premise parts are determined by GA.     

The dynamics of athletic performance,fitness and fatigue

We provide a mathematical dynamic model of athletic performance, fitness and fatigue based on the two well-known principles ‘train to failure’ and ‘use it or lose it’. The anabolic and catabolic processes are modelled with differential equations. Fitness is defined as muscle fitness. We model the work power of any muscle or set of muscles, and the muscle's maximum work power. Parameters are estimated and we present analytical and numerical results. The relationships between performance, fitness and fatigue are demonstrated for various activity scenarios. For example, the model quantifies the exact manner in which the optimal rest period can be determined to maximize the performance on a given day. The model provides realistic predictions, and constitutes a powerful tool which describes the processes by which performance, fitness and fatigue can be regulated and controlled.     

Effect of aggressive media on the fatigue life of plastics

The dependence of fatigue life on concentration has been obtained for PVC and polystyrene exposed to the action of acetone solutions. As the mechanical stress increases, the role of the aggressivity of the medium diminishes, and its corrosion activitya tends to a minimum (a=1). When a polymer is exposed to the action of a binary liquid, both of whose components are aggressive, a transition region is observed on the log 1/-log C curve. In this region the fracture rate increases sharply.Mekhanika Polimerov, Vol. 4, No. 3, pp. 489–493, 1968     

Estimating the parameters of fatigue curve of a composite material

By using the method of maximum likelihood, the parameters of two versions of a mathematical model for fatigue damage accumulation in a laminate are estimated. The models, which are founded on the Markov chain theory, are very simple: they do not take into account the specific structural features of a composite and therefore cannot provide numerical coincidence with experimental fatigue test data, but they can be used for a nonlinear regression analysis of fatigue curves. A simple method is offered for approximately estimating model parameters, some of which characterize the distribution of the local static strength. By using such models, we can predict the relative changes in fatigue curves from known relative variations in the parameters of static strength and also predict the distribution function of fatigue life in program fatigue tests.Translated from Mekhanika Kompozitnykh Materialov, Vol. 41, No. 1, pp. 109–120, January–February, 2005.     

Optimum design of compressive residual stress field caused by ultrasonic surface rolling with a mathematical model

Compressive residual stress fields induced by ultrasonic surface rolling process play a key role in determining the fatigue performance of machine parts. The present work is an analytical approach to conducting optimum design of this field to obtain an optimum fatigue resistance. Thus, a mathematical model was presented to predict residual stresses based on circular and elliptical Hertz contact areas. Moreover, to validate the proposed analytical approach, experimental verification was carried out on 18CrNiMo7-6 steel. Analytic solutions were derived from the mathematical model and optimum characteristic parameters were obtained by investigating the characteristic parameters in this field, such as surface residual stress, maximum residual stress and its depths. Results showed that increasing the total force, Hertz contact area and ratio of the radius of tool tip to that of target body could significantly enhance the peak of compressive residual stress and its depth.     

Acoustic fatigue of polymer materials

An experimental investigation of the fatigue properties of filled rubber under acoustic loading is briefly discussed. It is shown that two processes take place in the material—hardening and softening. A corresponding mathematical model is proposed. The form of the Wöhler fatigue curve in logarithmic coordinates is almost linear for high-frequency cyclic loading and may be assumed linear in the calculations with a risk of not more than 1%.Mekhanika Polimerov, Vol. 3, No. 3, pp. 467–475, 1967