Fatigue Life Estimation Research Articles

Vibration fatigue analysis of circumferentially notched specimens under coupled multiaxial random vibration environments

AbstractThe time domain critical plane‐based Carpinteri et al. criterion and the Theory of the Critical Distance are here combined together to propose a novel procedure for vibration fatigue analysis of circumferentially notched specimens subjected to coupled multiaxial random vibration environments. A novel metallic testing equipment is specially devised for notched specimens; meanwhile, a biaxial random vibration environment fatigue test is specially implemented on notched specimens in order to investigate how both coherence and phase shift between the auto‐spectral density functions in different directions influence the specimen fatigue life. The location of the verification point where to perform the fatigue damage estimation is determined by applying both the point method and the Carpinteri et al. criterion (for damage evaluation). The obtained results prove that the proposed procedure can provide satisfactory fatigue life estimations.

Crack propagation characterization and statistical evaluation of fatigue life for locally corroded bridge steel based on metal magnetic memory method

Corrosion has a great endangerment for the fatigue performance of bridge steel members. As a newly developed self-magnetic leakage (SMFL) nondestructive testing (NDT) technique, metal magnetic memory (MMM) is regarded as a potential and feasible approach for examining fatigue damage of ferromagnetic material. Thus, the tension-tension high cycle fatigue (HCF) tests for the Q345qD bridge steel sheet specimens with local corrosion were carried out in this paper, and the normal component HSF(y) of SMFL field on the surface of specimens were investigated innovatively via MMM method. The experimental results show that the mutational crest (or trough) and the crest-trough (or trough-crest) behavior on the curves of HSF(y) signal and its gradient K can accurately indicate the range of corroded region and locate fatigue crack position. Then, the significant experimental correlations between magnetic characteristic parameters and crack size were demonstrated. The maximum value (or the minimum value) Km and the crest area Ka of the gradient curve can be utilized for the comprehensive characterization of crack initiation and propagation in the corroded region. Comparing with the experimental correlations proved that the established improved magnetic dipole models were reliable to simulate the variation of HSF(y) signal at the corroded region before and after cracking. Finally, based on the naive Bayesian model, the statistical evaluation of normalized fatigue life for corroded bridge steel was realized by classification via magnetic characteristic parameter. This research will serve as a base for future studies of fatigue crack detection and fatigue life estimation on the corroded bridge steel component based on MMM method.

Fatigue reliability evaluation of aging prestressed concrete bridge accounting for stochastic traffic loading and resistance degradation

Fatigue damage accumulation is a critical factor resulting in the failure of prestressed concrete (PC) bridges. The fatigue damage is usually caused by the coupled effect of cyclic vehicle loading and environmental corrosion. This study investigated probabilistic fatigue damage on aging PC bridges considering both stochastic traffic loading and corrosion. A stochastic traffic model was derived based on long-term monitoring data aiming to simulate fatigue stress spectra of critical rebar. The effect of cracks on the fatigue stress spectra was investigated in order to model the fatigue stress state more realistically. A three-stage traffic growth model was established based on traffic volume histories of three highways in China. A fatigue limit state function considering traffic growth and corrosion effect was deduced for fatigue reliability assessment of PC bridges. Numerical results show that the stress amplitude of rebar considering cracks is 1.53 times greater than the rebar with no-cracks, resulting in a decrease of fatigue life by 68 years. In addition, the three-stage traffic growth models lead to 25 years shorter fatigue life than the one considering a linear traffic growth model. Finally, the corrosion effect results in a fatigue life of 44 years. The numerical results provide a theoretical basis for fatigue life estimation and maintenance of aging PC bridges.

Polar damage sum concept for constant amplitude proportional and nonproportional multiaxial fatigue analysis

This paper proposes a new concept for the evaluation of fatigue damage parameters. It is suggested that evaluation of fatigue damage may not necessarily be confined to a single plane, i.e., a critical plane. Rather, it is suggested here that the culmination of fatigue crack on a particular plane(s) is due to the contributions of damage on all planes. Mathematically, the polar damage sum concept is calculated as the area enclosed by the polar curve of the damage parameter. The proposed concept is examined using multiaxial experimental data for six different alloys. A total of 241 samples are included in this study. Using the proposed concept, fatigue life estimations are improved compared to those obtained from the original critical plane based on Fatemi-Socie and Smith-Watson-Topper models.

A Novel Multiaxial Strain-Based Criterion Considering Additional Cyclic Hardening.

The present paper is dedicated to the theoretical evaluation of a loading feature, that may have a significant influence on fatigue phenomenon: non-proportionality. As a matter of fact, considerable interactions between dislocations, leading to the formation of dislocation cells, cause additional cyclic hardening of material. Such a phenomenon is experimentally observed for materials sensitive to non-proportionality. In such a context, the present paper is aimed to propose a novel multiaxial strain-based criterion, the refined equivalent deformation (RED) criterion, which allows to take into account, in fatigue life estimation, both strain amplitude and additional cyclic hardening. The accuracy of the novel criterion is evaluated by considering experimental tests, performed on Ti-6Al-4V specimens, subjected to multiaxial LCF loading.

Fatigue life estimation of the weld joint in K-node of the offshore jacket structure using stochastic finite element analysis

Several methods for finite element modelling and analysis of tubular welded joints are described in various design codes. These codes provide specific recommendations for modelling of the welded joints, using simple weld geometries. In this paper, experimental strain range results from two full-scale semi-automatically welded K-nodes tests are compared to corresponding finite element models. Three dimensional (3D) scans of the weld surfaces have been developed for the automatically welded K-joints. These 3D scans are included in the finite element (FE) models to determine the accuracy of the FE models. Some discrepancies are observed between finite element predictions and test results. Finite element model updating has been applied to improve the accuracy of the developed finite element models. The results are compared to a FE model with a simple weld geometry based on common offshore design codes and a FE model without any modelled weld. The results show that the FE model with 3D scanned welds are more accurate than the other FE models. The precise location of weld toe in the 3D scanned weld is difficult to locate in the FE model, thus stochastic finite element modelling is performed to analyse the probabilistic hot-spot stresses. The results show large variations in the hot-spot stresses. It can be concluded that it is necessary to evaluate the probabilistic nature of stresses for calculating fatigue lives of welded tubular joints in offshore structures.

Geometrical effects on the degree of bending (DoB) of multi-planar tubular KK-joints in jacket substructure of offshore wind turbines

The degree of bending (DoB) characterizes the member through-the-thickness stress distribution which has a profound effect on the fatigue behavior of tubular joints commonly found in steel offshore structures and the determination of DoB values is essential for improving the accuracy of fatigue life estimation. Multi-planar joints are an intrinsic feature of offshore tubular structures and the multi-planarity might have a considerable effect on the DoB values at the brace-to-chord intersection. Nevertheless, for multi-planar joints which cover the majority of practical applications, much fewer investigations have been reported on the DoB due to the complexity and high cost involved. In the present research, data extracted from the stress analysis of 243 finite element (FE) models, verified based on available parametric equations, was used to study the effects of geometrical parameters on the DoB values in multi-planar tubular KK-joints which are among the most common joint types in jacket substructure of offshore wind turbines (OWTs). Parametric FE study was followed by a set of nonlinear regression analyses to develop three new DoB parametric equations for the fatigue analysis and design of axially loaded multi-planar KK-joints.

Fatigue life evaluation of an electrically driven shuttle frame using finite element analysis

Abstract This article provides the fatigue life estimation of the frame of a new electrically powered shuttle used in airports and resorts. The frame and components of the electrically powered shuttle were designed by expert employers of the OSCAR Co. using a CATIA computer program. Accelerometers were mounted under and above the prototype electric vehicle frame and external load path interaction data were collected by road tests. By means of five varied degrees of road roughness and three different conditions for vehicle loading, the raw road data were recorded via accelerometers on the vehicle frame. The recorded raw data was made available by processing with the FDesign program. After the external load was determined, a 3D drawing of the frame of the vehicle was transferred to the ANSYS program as a step file. In the chassis model transferred to the ANSYS program; material assignment, meshing of the frame structure, defining the boundary conditions of the structure and static analysis were determined according to the most critical road load, following which fatigue analyses were performed for five different roads. The fatigue life and damage amount of each road on the vehicle frame were calculated separately by considering Miner’s rule according to the S-N high cycle method. Experiments showed that no fatigue damage occurred within the predicted 200,000 km.

Modeling of Stiffness Anisotropy in Simulation of Self-Piercing Riveted Components

The so-called substitute models based on shell elements can be used to design the self-piercing riveted components economically and with sufficient accuracy. In this study, the SPR3 (Self-Piercing Rivet) model with anisotropic stiffness parameters implemented in commercial simulation software LS-DYNA is used to describe the stiffness of self-piercing riveted joints subjected to different loading conditions. The model provides the basis for the subsequent fatigue life estimation of self-piercing riveted joints under cyclic loading. By accurate prediction of the stiffness of self-piercing riveted joints subjected to cyclic loading, the accuracy of the fatigue life estimation can be improved. To identify the stiffness parameters, the self-piercing riveted joints are subjected to loading conditions: axial tension, shear tension, and bending. To validate the model, the specimens are simulated under different loading conditions and the results are compared to the experiments. It is shown that the model with anisotropic stiffness parameters predicts the stiffness of specimens more accurately compared to the model with isotropic stiffness parameter.

Fatigue life estimation of structures under statistically and spectrally similar variable amplitude loading

A new fatigue life prediction framework provides an improved life prediction under statistically and spectrally similar irregular variable-amplitude loading for a notched beam model. It enables the cumulative damage rule to account for the load sequence effects by modifying the probability density function of the stress-amplitude history through (1) identification of overloads based on the rainflow-counting algorithm; (2) analytical characterization of the overload retardation effects; and (3) correction to the damage rule using overload amplitude rate characterization. The fatigue lives estimated from experimentally acquired and synthetically generated load-time histories are compared to the ones generated from simulations that qualitatively reproduce the fatigue lives in physical experiments. The notable improvement in prediction accuracy outperforms the Palmgren–Miner’s rule and power-spectrum-based life estimation. The demonstrated application to the field acceleration data substantiates its use for in-service structural health monitoring and damage prognosis. This framework does not require a priory knowledge of the applied load, and it can be applied to other engineered structures with known structural and defect properties.

Study on the accuracy of axle load spectra used for pavement design

ABSTRACT Weigh-in-Motion (WIM) systems are used in order to reduce the number of overloaded vehicles. Data collected from WIM provide characteristics of vehicle axle loads that are crucial for pavement design as well as for the development of pavement distress prediction models. The inaccuracy of WIM data lead to erroneous estimation of traffic loads and in consequence inaccurate prediction of pavement distress process. The objective of the paper is to present a new methodology of heavy traffic axle load spectra (ALS) correction due to weighing errors (systematic and random) that occur in WIM systems. The theoretical solution which is proposed in the paper was validated successfully. The method enables correction of erroneous data to make traffic load statistics used for pavement design more reliable and precise, with no necessity to remove high number of records, as it is used in other methods. The practical meaning of the newly developed method was emphasised by analysis of the effect of relative and random error of WIM data on pavement fatigue life estimation, as well as on the estimated percentage of overloaded vehicles. Mechanistic-empirical approach (M-EPDG) was used for this purpose.

Fatigue life estimation of pre-corroded 42CrMo4 subjected to accelerated pitting corrosion method

One of the main causes of failure of the components and structures used by Oil&Gas and offshore industries is associated with pitting corrosion. In this work, two main objectives have been defined. Firstly, the main variables to perform accelerated pitting corrosion on 42CrMo4 steel under laboratory conditions are defined. These accelerated corrosion tests have been performed to obtain pitting defects as similar as possible to the real ones observed in offshore environments. Secondly, fracture mechanics has been applied to predict the fatigue propagation of pitting defects until the final fracture, and a sensitivity analysis has been performed to determine the influence of geometric parameters on the fatigue failure predictions. Finally, a series of fatigue tests were carried out to obtain the S-N curves of the pre-corroded steel specimens and the results have been compared with the fatigue predictions. The main conclusion of the paper is that fracture mechanic approaches can be used to predict the fatigue life of 42CrMo4 subjected to corrosive environments in a reasonably conservative way.

The effect of kinematic hardening on the fatigue behaviour of bent high strength steel

The integration of forming in the fatigue modelling of cold-formed components significantly improves the predictive accuracy of the estimated life. The current study investigated the fatigue behaviour of a bent specimen made from a 5 mm thick, S900MC high strength steel plate. Because of its superior static and dynamic strength, this grade is progressively used for hollow cold-formed sections in mobile applications. However, it exhibits a strong stress saturation as well as limited formability. In this regard, a finite element modelling framework was adopted from previous research and further developed to integrate bending in the fatigue modelling and life estimation procedure. However, this framework currently ignores the possible influence of kinematic hardening and associated Bauschinger effect. For this reason, a numerical study was performed that compares isotropic with kinematic hardening for this specific application. First, the characteristic behaviour of these models was verified in a virtual tension-compression test. Subsequently, they were implemented in forming simulation followed by fatigue loading. Herein, the stress-strain evolution was investigated and a multi-axial fatigue criteria was used to map the sensitivity of the estimated life to the type of hardening. In general, the stress that entered the fatigue calculation was at least 21% lower for the kinematic model. As a result, a significant increase of 65% was observed for the estimated fatigue life, yielding a better comparison with experimental data.

Fatigue Life Evaluation of Delaminated GFRP Laminates Using Artificial Neural Networks

Delamination between any two plies is an important damage commonly seen in composite structures. It may initiate and grow in the composite laminates for different loading conditions, and it may finally lead to failure of the component under the cyclic loading. Therefore, fatigue life prediction in these structures is important to avoid the effects of delaminations in economic and safety considerations. The usage of artificial neural network (ANN) in estimating fatigue failure in composites with delaminations would be high. In this work, experimental fatigue data were obtained for glass fiber-reinforced composite beams with and without delaminations. The experimental results were used to train and test the neural network. Finally, ANN was found to be accurate tool for fatigue life estimation for composite materials with delaminations as it produces reasonably good fatigue life prediction for delaminated composites.

Application of Spectral Method for Vibration-Induced High-Cycle Fatigue Evaluation of an High-Pressure Turbine Blade

Abstract A method of fluid–structure interaction coupling is implemented for a forced-response, vibration-induced fatigue life estimation of a high-pressure turbine blade. Two simulations approaches; a two-way (fully coupled) and one-way (uncoupled) methods are implemented to investigate the influence of fluid–solid coupling on a turbine blade structural response. The fatigue analysis is performed using the frequency domain spectral moments estimated from the response power spectral density (PSD) of the two simulation cases. The method is demonstrated in relation to the time-domain method of the rainflow cycle counting method with mean stress correction. Correspondingly, the mean stress and multi-axiality effects are also accounted for in the frequency domain spectral approach. In this case, a multiplication coefficient is derived based on the Morrow equation, while the case of multi-axiality is based on a criterion which reduces the triaxial stress state to an equivalent uniaxial stress using the critical plane assumption. The analyses show that while the vibration-induced stress histories of both simulation approaches are stationary, they violate the assumption of normality of the frequency domain approaches. The stress history profile of both processes can be described as platykurtic with the distributions having less mass near its mean and in the tail region, as compared to a Gaussian distribution with an equal standard deviation. The fully coupled method is right leaning with positive skewness while the uncoupled approach is left leaning with negative skewness. The directional orientation of the principal axes was also analyzed based on the Euler angle estimation. Although noticeable differences were found in the peak distribution of the normal stresses for both methods, the predicted Euler angle orientations were consistent in both cases, depicting a similar orientation of the critical plane during a crack initiation and propagation process. Finally, the fatigue life estimation was shown to be conservative in the fully coupled solution approach.

Phase-field modelling for fatigue crack growth under laser shock peening-induced residual stresses

For the fatigue life of thin-walled components, not only fatigue crack initiation, but also crack growth is decisive. The phase-field method for fracture is a powerful tool to simulate arbitrary crack phenomena. Recently, it has been applied to fatigue fracture. Those models pose an alternative to classical fracture-mechanical approaches for fatigue life estimation. In the first part of this paper, the parameters of a phase-field fatigue model are calibrated and its predictions are compared to results of fatigue crack growth experiments of aluminium sheet material. In the second part, compressive residual stresses are introduced into the components with the help of laser shock peening. It is shown that those residual stresses influence the crack growth rate by retarding and accelerating the crack. In order to study these fatigue mechanisms numerically, a simple strategy to incorporate residual stresses in the phase-field fatigue model is presented and tested with experiments. The study shows that the approach can reproduce the effects of the residual stresses on the crack growth rate.

Comparative fatigue properties estimation of composite structural nodes

PurposeThe purpose of this paper is to present the concept of the author’s method of fatigue properties assessment of polymer composite structures, especially structures having nodes of concentrated force introduction (NCFI) using fatigue test data of coupons of similar composites and the ratio of their structural stress rate factors.Design/methodology/approachBasing on fatigue properties of pure composite shells coupons subjected to cyclic loads, and basing on the static strength difference between pure composite shells and shells having the structure affected by NCFI – (considered here as not only a manner of load introduction but also a kind of structural discontinuity), a method of relative fatigue properties reduction (RFPR) was developed. In the RFPR evaluation process, the author used the results of experiments on a special type of an NCFI named “a labyrinth non-adhesive node of concentrated force introduction” (LNA-NCFI) applied in certain composite gliders for fitting glider wings with the fuselage and also referred to design directives relating to primary structure of composite gliders, which are presented in the form of lightness factors linking stress with a structural mass.FindingsThe result of RFPR method application matched well with the results of fatigue tests of the LNA-NCFI type of a NCFI. The RFPR method may significantly facilitate the estimation of fatigue life of a structure with a structural discontinuity or an NCFI.Practical implicationsThe RFPR method may significantly facilitate the estimation of fatigue life of a structure with a structural discontinuity or an NCFI.Originality/valueThe paper presents a proposal of a novel simplified method for fatigue life estimation of composite structures having a kind of structural discontinuity or an NCFI.

A multiaxial fatigue life prediction method for metallic material under combined random vibration loading and mean stress loading in the frequency domain

A spectral method for the determination of the critical plane is proposed for metallic material under random vibration loadings based on the criterion of maximum shear stress amplitude, where the direction of critical plane is obtained by solving the maximum value of a function using mathematical optimization method and the fatigue critical location of the structure can be determined. A multiaxial fatigue damage parameter is proposed for combined random vibration loading and mean stress loading cases based on Susmel’s criterion, which is a Power Spectral Density (PSD) of the equivalent shear stress and can be adopted for fatigue life evaluation. The fatigue life estimation of structures under multiaxial random vibration loadings with mean stresses in the frequency domain could be carried out based on this damage parameter constructed at the fatigue critical location with the spectral method for random vibration fatigue life. The multiaxial random vibration fatigue experiment with mean stress loadings is conducted on the notched plate specimens made of 2297-T87 aviation grade aluminum alloy, and the initiation sites of the fatigue cracks as well as the experimental fatigue lives are compared with those calculated by the proposed models, where satisfactory prediction capabilities on both the fatigue crack initiation locations and fatigue lives of the models are demonstrated.

Mapping of scatter in fatigue life assessment of welded structures\u2014a round-robin study

A round-robin study has been carried out within a national project in Sweden with the addition of an international participant, where several industrial partners and universities are participating. The project aims to identify variation and sources of variation in welding production, map scatter in fatigue life estimation, and define and develop concepts to reduce these, in all steps of product development. The participating organisations were asked to carry out fatigue life assessment of welded box structures, which is a component in load-carrying structures. The estimations of fatigue life have also been compared with fatigue test results. Detailed drawings, loads and material data were also given to the participants. The participants were supposed to use assessment methods based on global and local stresses using the design codes or recommendations they currently use in-house. Differences were identified between both methods and participants using the same codes/recommendations. Applicability and conditions from the cases in the codes were also identified to be differently evaluated between the participants. It could be concluded that for the applied cases the nominal stress method often overestimated the fatigue life and had a high scatter in the estimations by different participants. The effective notch method is conservative in comparison to the life of tested components with little scatter between the results derived by the participants.

Multiaxial fatigue assessment of complex steel connections: A case study of a vertical-lift gussetless truss bridge

Civil engineering structures grow more complex as engineers push innovation boundaries. Load complexity, costs, construction and maintenance efficiency, among other things, can lead to unique designs that are often excluded in current design codes and evaluation standards. Fatigue-induced damage is currently one of the most common damage types experienced by ordinary and complex civil engineering structures subjected to cyclic loading such as bridges. However, current fatigue evaluation and design standards provide limited guidance on how to approach complex welded connections. Uniaxial and multiaxial methods for the evaluation of fatigue life using strains histories are compared for the assessment of a welded gussetless truss connection of the Memorial Bridge, a vertical-lift bridge connecting Portsmouth, NH and Kittery, ME. As expected, using both approaches result in sufficient remaining fatigue life. However, it is demonstrated that commonly used uniaxial fatigue analysis methods can be insufficient in complex structures that experience variable amplitude, multiaxial, and non-proportional loading. Multiaxial non-proportional stresses experienced by a complex connection can cause a decrease in remaining fatigue life estimates. This paper presents assessment tools and a method for the evaluation of fatigue life of complex welded connections using strain time histories.