Fatigue Assessment Of Structures Research Articles

Numerical study of full penetration single- and double-sided U-rib welding in orthotropic bridge decks

Under the repeated action of residual stress and wheel load caused by welding, fatigue cracks easily occur on the top plate-U rib welding of an orthotropic bridge deck (OBD). These affect the service life. To study the influence of different full-penetration welding methods on the residual stress and fatigue of OBD, this study compared the residual stress, welding deformation, and fatigue performance of full-penetration single- and double-sided welding through finite element analysis. The welding temperature and residual stress fields were simulated based on Goldak's double ellipsoid heat source model using the thermal elastic-plastic method. The numerical results were verified using measured data from relevant literature, and the two were in good agreement. The results showed that the longitudinal residual stress of the double-sided welded U-rib was larger than that of the single-sided welded U-rib, primarily in the compressive stress range, and the maximum deviation reached 99.44 MPa. The longitudinal displacement of single-sided welding along the weld direction was 0.060 mm lesser than that of double-sided welding, and the maximum vertical displacement perpendicular to the weld direction was 2.457 mm lesser than that of double-sided welding. For the same frequency of fatigue load (wheel load), the fatigue strength of single-sided welding was larger than that of double-sided welding, and the fatigue life increased by 29.7%. In this study, analysis models of the full-penetration single-sided and full-penetration double-sided welding were established, which could provide a reference for structural fatigue assessment using the two full-penetration welding techniques under the influence of residual stress.

Simulation of non-Gaussian stochastic processes with prescribed rainflow cycle count using short-time Fourier transform

Random phenomena described by non-Gaussian processes are ubiquitous in many scientific fields. Properly understanding their characteristics is crucial in safety analysis, where the random occurrence of extreme values is often critical. In practice, the information available for fully characterizing a stochastic process is often limited by experimental constraints. Thus, prediction based on simulations comes as a valid alternative. Motivated by applications in structural fatigue assessment, this research presents a simulation method for non-Gaussian stochastic processes that is able to jointly reproduce a prescribed power spectral density and a rainflow cycle count. The main idea is to model non-Gaussian behavior by a frequency-dependent non-linearity in the short-time Fourier transform. As such, the power spectral density is directly imposed, while the non-linear model is optimized to match the rainflow cycle count. It is shown that the model is flexible enough to be interpolated between different operating points, so as to allow the simulation of stochastic processes at non-measured operating conditions. The full methodology is illustrated for assessing the fatigue assessment of a hydroelectric turbine runner.

Fatigue strength assessment of additively manufactured metallic structures considering bulk and surface layer characteristics

This paper extends a previously published fatigue strength estimation methodology for additively manufactured metallic bulk material by additionally accounting for effects of as-built surface layers. Interaction of intrinsic defects and surface texture convergently initiates fatigue failure. Holistic consideration of influencing factors significantly contributes to scientific fatigue assessment of structures fabricated by Laser-Powder Bed Fusion (L-PBF). Surface texture is highly dependent on the building parameters and performed post treatments. Three dimensional, optical topography scans form the basis of determining representative areal surface texture parameters. Areal notch valley depth Sv, alongside notch radii ρ evaluation, enables usage of a modified Peterson’s approach. Effects of notch-like roughness features are quantified by a reduction factor ks, as analogously published for bulk material imperfections kb. Superimposition of ex- and intrinsic material characteristics is empirically assessed by an interaction exponent derived from experimental fatigue data. Macroscopic, tensile residual stresses acting as mean stresses are considered by Smith-Watson-Topper’s approach. Unifying presented influencing factors derives a comprehensive model, conceived to estimate fatigue strength of additively manufactured metallic structures. Regardless of post processing condition, sound applicability of developed design approach is substantiated by averaging −7.1%, comparing estimated fatigue strength to experimental results.

Numerical simulation of residual stresses in aluminum alloy welded joints

The lightweight design makes thin-plate welded structures used widely, especially for the aluminum alloy thin-plate welded structure, it is very important to study the fatigue fracture properties of the structure considering the influence of residual stress. A solid model of a butt joint of aluminum alloy with a double-pass weld is established. Both the welding temperature fields and residual stress fields are simulated through the thermal elastic–plastic method. The welding experiment is also performed with the corresponding joint, which is welded by means of metal inert-gas (MIG) welding. Consequently, the computational and experimental results of residual stress are in good agreement. Moreover, the thermal elastic–plastic method is employed to calculate the residual stress fields of typical welded joints. Based on the inherent strain theory, the transverse and longitudinal inherent strain of the joints are decomposed from the total strains and applied to both solid and shell models of the three types of joints to calculate the residual stress fields of them. The results of the solid and shell models using the two methods are practically consistent, which indicates that the inherent strain method can predict the residual stress on the shell model efficiently and accurately. Thus, the method of predicting the welding residual stress of thin-plate structures based on the inherent strain is proposed, which provides a reference for the structural fatigue assessment under the influence of residual stress.

Criteria for Fatigue Failure of Materials: Application in Fatigue Assessment of Structures

Presently in rules for fatigue assessment of structures subjected to intensive alternating service loading the Stress-Life (S-N) criteria are recommended in versions of the Nominal stress approach, Hot-spot stress and Notch-stress approach based on using the stress range a representative of the current damage. The criteria and approaches provide assessment of fatigue properties of structures accompanied with a series of approximations and uncertainties. A physically and mechanically more correct procedures might be provided by the Strain-life and Inelastic strain energy criteria for fatigue failure and approaches, although specific with intrinsic sources of approximations. The nature of approximations in the approaches is briefly commented and feasible means of improvement the fatigue assessment procedures and applications are presented.

S-N-критерии усталости материалов для расчетов ресурса конструкций: проблемы применения

S-N-критерии усталости материалов для расчетов ресурса конструкций: проблемы применения

An overview of fatigue in aircraft structures

AbstractFrom safe‐life to fail‐safe and damage‐tolerance approaches, the last one emerged as the main design philosophy for aerostructures, allowing weight savings and at the same time increasing reliability and structural integrity in the presence of damages which may have occurred during the manufacturing process or during service. The application of damage‐tolerance philosophy requires extensive know‐how of the fatigue and fracture properties, corrosion strength, potential failure modes and non‐destructive inspection techniques, particularly minimum detectable defect and inspection intervals.To face scatter in material properties, conservative approaches considering the worst scenario or statistical methods dealing with the variability of material have been employed in the fatigue assessment of structures. The fatigue life estimation can display substantial variability, illustrating the need for a probabilistic assessment in practical applications. As an example, the variation in the fatigue life for the common scenario of a through crack in an Al alloy plate was evaluated taking into account scatter of properties.The 2010 FAA rule establishing a limit of validity (LOV) puts a bound in the indefinite operational life allowed for by earlier regulations. This requirement, together with the diminishing role of aluminium in airframes, will certainly shape the directions of fatigue, fracture and damage mechanics research in years to come, expanding the knowledge base upon which substantiation of LOV values is made, and ensuring safety under sustainable conditions.

Numerical simulation of fatigue behavior for cable-stayed orthotropic steel deck bridges using mixed-dimensional coupling method

Effective methods and technologies in fatigue behavior and assessment for cable-stayed orthotropic steel deck bridges are critical to ensure their safety and serviceability. In this study, a mixed-dimensional finite element coupling method is used for structural fatigue assessment. A general framework of the Mix-dimensional Coupling (MDC) method is constructed on the basis of a compromise between simplicity and efficiency as compared to conventional sub-modeling or substructure method. Fatigue details and performance at welded joints have been investigated through the MDC method. Efficiency of the MDC method is demonstrated by a comparison with the simplified Bridge-deck-system (BDS) method. Besides the benefits of the saving time, the numerical simulation also indicated that the MDC method can effectively capture the global behavior for better fatigue prediction, that be ignored in the conventional BDS method as expected. Findings suggest that the MDC method is a cost-effective alternative for fatigue behavior and fatigue assessment of large-span orthotropic steel deck bridges.

Structural fatigue assessment of a fixed mono-pile platform

Fatigue assessment of fixed offshore platforms is essentially important to ensure their structural safety. Because of the small-scale leg diameter, these structures are often drag dominated, and linearized wave force is often utilized for the fatigue estimation. To evaluate the influence of nonlinear effect on fatigue damage, the nonlinear drag force is directly incorporated into the computation of the wave force spectrum based on autocorrelation function. The features and the contributions of nonlinear wave force are first investigated in the frequency domain. Then, structural fatigue damage associated with the linear and nonlinear wave forces is compared using a time-domain fatigue assessment method. Investigation indicates that the influence of nonlinear drag force is closely related to the characteristic diameter of the structure. The nonlinear drag force can contribute up to 10% of the total wave force and lead to about 10% structural fatigue life reduction when the characteristic diameter is small.

An innovative block partition and equivalence method of the wave scatter diagram for offshore structural fatigue assessment

The fatigue assessment plays an increasing role for the offshore structural safety. Many fatigue assessment methods have been developed for this purpose. Among those methods, the time domain method is regarded as the most accurate method but less adopted in practice due to time consuming. In order to improve the efficiency of the time domain method, an innovative block partition and equivalence method of the wave scatter diagram is developed for offshore structural fatigue assessment. After the wave scatter diagram is partitioned into several blocks, the newly developed method, involves determination of the equivalent wave height, wave period and occurrence probability of the representative sea states based on modified energy equivalent principle. The equivalent wave period of the representative sea state is calculated via the spectral moment formula in which the equivalent spectral moments of zero and second order are obtained based on the weighted averaging principle. Combining with the determined wave period, the equivalent significant wave height can be determined by reversing the wave spectrum integral formula, where the equivalent wave energy of a divided block of the wave scatter diagram is modified by introducing a factor to compensate the effect of low- and high-amplitude cycles fatigue damage. The equivalent occurrence probability is equal to the summation of the original sea states’ occurrence probability within the divided block. The developed method has the advantage of preserving the stochastic characteristics of the short term sea states within the divided block during determining the representative sea state. At the same time the newly developed method has no limitation on block partition and can be applied on different offshore structure. Two structural models, a fixed mono-pile platform and a floating semi-submersible platform, are demonstrated in the numerical examples. Results indicate that the newly developed method is robust, computationally affordable, and accurate within engineering expectations.

Damage identity in fatigue assessment of structures

Damage identity in fatigue assessment of structures

Measurement, Data Interpretation, and Uncertainty Propagation for Fatigue Assessments of Structures

The real behavior of existing structures is usually associated with large uncertainty that is often covered by the use of conservative models and code practices for the evaluation of remaining fatigue lives. To make better decisions related to retrofit and replacement of existing bridges, new techniques that can quantify fatigue reserve capacity are required. This paper presents a population-based prognosis methodology that takes advantage of in-service behavior measurements using model-based data interpretation. This approach is combined with advanced traffic and fatigue models to refine remaining fatigue-life predictions. Study of a full-scale bridge revealed that this methodology provides less conservative estimations of remaining fatigue lives. In addition, this approach propagates uncertainties associated with finite-element, traffic, and fatigue-damage models to quantify their effects on fatigue-damage assessments and shows that traffic models and structural model parameters are the most influential sources of uncertainty.

Experimental Study on Local Plastic Collapse in a Plate Weakened by Two Collinear Cracks

Fracture and fatigue assessment of structures weakened by multiple site damage (MSD) currently represents a challenging problem. Here, we communicate the outcomes of an experimental study conducted on a plate weakened by two closely-spaced collinear cracks, which represents the simplest form of MSD. The experimental results are used to verify the predictions of a recently developed theoretical approach.

Establishment and Application of the Wind and Structural Health Monitoring System for the Runyang Yangtze River Bridge

Structural health monitoring can provide a practical platform for detecting the evolution of structural damage or performance deterioration of engineering structures. The final objective is to provide reasonable suggestions for structural maintenance and management and therefore ensure the structural safety according to the real-time recorded data. In this paper, the establishment of the wind and structural health monitoring system (WSHMS) implemented on the Runyang Yangtze River Bridge (RYRB) in China is introduced. The composition and functions of the WSHMS are presented. Thereinto, the sensory subsystem utilized to measure the input actions and structural output responses is introduced. And the core functions of the data management and analysis subsystem (DMAS) including model updating, structural condition identification, and structural condition assessment are illustrated in detail. A three-stage strategy is applied into the FE model updating of RYRB, and a two-phase strategy is proposed to adapt to structural health diagnosis and damage identification. Considering the structural integral security and the fatigue characteristic of steel material, the condition assessment of RYRB is divided into structural reliability assessment and structural fatigue assessment, which are equipped with specific and elaborate module for effective operation. This research can provide references for the establishment of the similar structural health monitoring systems on other cable-supported bridges.

Advanced methods of fatigue assessment by DieterRadaj and MichaelVormwald (Springer Verlag, Berlin, 2013), a new, fundamental reference for people working on fatigue assessment of structures and components

Advanced methods of fatigue assessment by DieterRadaj and MichaelVormwald (Springer Verlag, Berlin, 2013), a new, fundamental reference for people working on fatigue assessment of structures and components

Assessment of the interaction between two collinear cracks in plates of arbitrary thickness using a plasticity‐induced crack closure model

ABSTRACTFracture and fatigue assessment of structures weakened by multiple site damage, such as two or more interacting cracks, is currently a very challenging problem. The main objective of this paper is to develop a mathematical model and an approach to investigate fatigue crack closure behaviour of two through‐the‐thickness collinear cracks of equal length in a plate of arbitrary thickness under remote tensile cyclic loading. The developed mathematical model of the problem under consideration is based on the Dugdale strip yield model and plasticity‐induced crack closure concept. The approach utilises the fundamental solution for an edge dislocation in a plate of finite thickness and the distributed dislocation technique to obtain an effective and accurate solution to the system of governing equations. The obtained results show a very good agreement with the previously published analytical solutions for limiting cases. In particular, the new results confirm that the crack closure behaviour and the opening stress variation in the case of two collinear cracks are significantly dependent on the separation gap between two cracks as well as the plate thickness.

Fatigue assessment of structures on the Docklands Light Railway, London, UK

The Docklands Light Railway in London, UK has been in service for some 25 years and has now been upgraded for the second time to enable it to use longer trains at higher frequencies. The structures had to be assessed and, when necessary, strengthened. The assessment would normally have been done using a similar approach to that which would be used for a new design. However, this showed up a significant need for strengthening purely for fatigue. It was therefore decided to strain gauge some structures and the results showed significantly smaller ranges than predicted. Rather than use the results directly, they were used to develop a more realistic assessment approach. This was partly because of the need to apply results for a limited number of structures subjected to two-car trains, to a larger number of structures subjected to three-car trains. Comparison with the strain gauge readings suggested the adopted approach was still very conservative but major savings were still made compared with the strengthening that would have been required by the conventional approach. Two of the structures had more specific and localised problems. Here it was possible to strain gauge all the critical elements and more direct use of the results was possible.

A fracture mechanics interpretation of rolling bearing fatigue

In fatigue assessment of structures and components, it is now commonplace to use a defect-tolerant approach to account for the inevitable presence of surface and internal flaws, both detectable and undetectable. This approach has been applied to rolling bearings in an attempt to present contact stress limits as a function of defect size below which fatigue failure would not be expected. In essence, it is an extension to the existing rating life methods developed by Ioannides and Harris that incorporate a fatigue limit stress in the estimation of bearing life, a concept similar to that of the endurance limit in structural fatigue. Such a fracture mechanics approach to sub-surface initiated fatigue in rolling bearing steels containing non-metallic inclusions, based upon the work of Murakami, has led to an allowable contact stress limit as a function of maximum inclusion size (as estimated by extreme value analysis). An alternative, less conservative fracture mechanics based stress limit is the prevention of propagation of cracks formed on inclusions (termed ‘butterflies’) by shear (Mode II) loading. The approach has been developed by examination of butterflies formed in service in rolling element bearings manufactured in bainitic steel with maximum inclusion sizes in excess of 100 µm. The observed lack of micro-crack formation on inclusions and non-propagation of butterflies support the concept of a fatigue endurance limit that is related to the cleanliness of the bearing steel.

Structural fatigue assessment and management of large-scale port logistics equipments

With the advances of port enterprises, much intensive research has been gradually involved in the structural fatigue assessment and management of port logistics equipments. However, relevant work on large-scale port logistics equipments is still lacking due to their complex structures. In addition, a single technique could not effectively deal with complicated structures. As such, a hybrid fatigue assessment method was investigated using the S-N curve and fracture mechanics methods for crack formation and propagation life estimations, respectively. Consequently, the fatigue assessment of a gantry crane was used as an example to illustrate the practicality and efficiency of this approach.

Hot Spot Stress S-N Curve for Fatigue Analysis of Plated Structures

Abstract Finite element analysis is being used by designers for fatigue assessment of structures. It is therefore important that a proper link between calculated hot spot stress and fatigue capacity is established. The fatigue capacity is expressed as a hot spot stress S-N curve. This paper presents a derivation of a hot spot stress S-N curve to be used when the hot spot stress is derived from finite element analysis of plated structures. The hot spot S-N curve is linked to the methodology used for finite element analysis of plated structures that is being included in design recommendations for fatigue assessment of welded structures.