Assessment of Fatigue Strength of Welded Connections in Thick Plates

2017 ◽  
Author(s):  
Kashif Kamran Toor ◽  
Inge Lotsberg
Keyword(s):  
Wind Turbine ◽  
Alternative Assessment ◽  
Hot Spot ◽  
Limit State ◽  
Design Procedure ◽  
Assessment Method ◽  
Hot Spot Stress ◽  
Notch Stress ◽  
Standard Design ◽  
Reliable Design

Fatigue is a governing limit state for design of wind turbine structures. This implies use of thicker plates in these structures than that used as basis for derivation of standard design S-N curves. Due to a significant number of wind turbine support structures designed in a similar way it is important to use a reliable design procedure that does not require use of unnecessary steel costs or fabrication costs. There are also questions related to calculation of structural stresses to be entered the different S-N curves for calculation of fatigue lives. Therefore an alternative assessment method based on notch stress and very fine element meshes has been used to assess target hot spot stress values for calculation of best estimates of fatigue lives.

Fatigue strength assessment of ship structures accounting for a coating life and corrosion degradation

2016 ◽  
Vol 7 (2) ◽  
Author(s):  
Yordan Garbatov
Keyword(s):  
Finite Element ◽  
Fatigue Strength ◽  
Fatigue Damage ◽  
Hot Spot ◽  
Limit State ◽  
State Function ◽  
Fatigue Reliability ◽  
Hot Spot Stress ◽  
Notch Stress ◽  
Fatigue Limit State

Purpose Fatigue strength and reliability assessment of complex double hull oil tanker structures, based on different local structural finite element approaches, is performed accounting for the uncertainties originating from load, nominal stresses, hot spot stress calculations, weld quality estimations and misalignments and fatigue S-N parameters including the correlation between load cases and the coating life and corrosion degradation. Design/methodology/approach Ship hull wave-induced vertical and horizontal bending moments and pressure are considered in the analysis. Stress analyses are performed based on the nominal, local hot spot and notch stress approaches. A linear elastic finite element analysis is used to determine the stress distribution around the welded details and to estimate structural stresses of all critical locations. Fatigue damage is estimated by employing the Palmgren-Miner approach. The importance of the contribution of each random variable to the uncertainty of the fatigue limit state function is also estimated. The probability of fatigue damage of hot spots is evaluated taking into account random coating life and corrosion wastage. Fatigue reliability, during the service life, is modelled as a system of correlated events. Findings The fatigue analysis showed that the fatigue damage at the hotspot, located at the flange of the stiffener close to the cut-out, is always highest in the cases of the structural hot spot stress and effective notch stress approaches, except for the one of the nominal stress approach. The sensitivities of the fatigue limit state function with respect to changes in the random variables were demonstrated showing that the uncertainty in the fatigue stress estimation and fatigue damage are the most important. Fatigue reliability, modelled as a parallel system of structural hot spots and as a serial system of correlated events (load cases) was evaluated based on the Ditlevsen bounds. As a result of the performed analysis, reliability and Beta reliability indexes of lower and upper bounds were estimated, which are very similar to the ones adopted for ultimate strength collapse as reported in literature. Originality/value This paper develops a very complex fatigue strength and reliability assessment model for analysing a double hull oil tanker structure using different local structural finite element approaches accounting for the associated uncertainties and the correlation between load cases and the coating life and corrosion degradation. The developed model is flexible enough to be applied for analysing different structural failure modes.

Applicability Evaluation of SS (Structural Stress) Method on Actual Project

2008 ◽  
Vol 580-582 ◽  
pp. 633-636 ◽  
Author(s):  
Kwang Seok Kim ◽  
Joong Kyoo Kang ◽  
Joo Ho Heo ◽  
Sung Geun Lee
Keyword(s):  
Fatigue Life ◽  
Hot Spot ◽  
Fatigue Analysis ◽  
Correlation Factor ◽  
Structural Stress ◽  
Hot Spot Stress ◽  
Notch Stress ◽  
Loading Case ◽  
Application Data ◽  
Applicability Evaluation

The structural stress (SS) method developed by BATTELLE has been studied based on small or mid-size scale specimens. In order to apply the new method, such as SS, on an actual project, it should have application results on actual project. However, SS method didn’t have a lot of application data compared to class procedure using hot spot stress (HSS). In order to find out whether the SS method, for the evaluation of fatigue life, can give reasonable results when it is applied under the same loading suggested by classification societies, it was compared with fatigue lives derived by class. ABS & DNV’s simplified fatigue analysis method were adopted to check the validity of SS method. Before applying complicated loading of class, static loading case was applied, since the class method has their own correlation factor for wave loading. And then, simplified fatigue analysis was performed with more complicated loading cases. From the results of fatigue life calculation, it can be said that SS shows reasonable fatigue lives with respect to HSS or notch stress based fatigue lives.

Design of Mooring Lines of Floating Offshore Wind Turbine in Jeju Offshore Area

2014 ◽  
Author(s):  
Hyungjun Kim ◽  
Joonmo Choung ◽  
Gi-Young Jeon
Keyword(s):  
Wind Turbine ◽  
Limit State ◽  
Design Procedure ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Ultimate Limit State ◽  
Mooring Lines ◽  
Offshore Area ◽  
Floating Offshore Wind Turbine ◽  
Wind Turbine System

This paper presents a mooring design procedure of a floating offshore wind turbine. The offshore environment data of Jeju south sea collected from Korea Hydrographic and Oceanographic Administration (KHOA) are used as environmental conditions for hydrodynamic analysis. A semi-submersible floating wind turbine system is considered based on Offshore Code Comparison Collaborative Continuation (OC4) DeepCWind platform and the National Renewable Energy Laboratory (NREL) 5MW class wind turbine. Catenary mooring with studless chain is chosen as the mooring system. Important design decisions such as how large the nominal sizes are, how long the mooring lines are, how far the anchor points are located, are demonstrated in detail. Considering ultimate limit state and fatigue limit state based on 100-year return period and 50 year design life, respectively, long-term predictions of breaking strength and fatigue are performed.

Fatigue Analysis of a Thin Cover Plate Structure in a Light Craft

2015 ◽  
Author(s):  
Till Köder ◽  
Berend Bohlmann
Keyword(s):  
Hot Spot ◽  
Gas Metal Arc Welding ◽  
Plate Thickness ◽  
Fatigue Analysis ◽  
Fatigue Tests ◽  
Cover Plate ◽  
Element Analysis ◽  
Hot Spot Stress ◽  
Notch Stress ◽  
Metal Arc Welding

Experimental fatigue analysis of a fillet-welded cover plate detail (‘floating frame’) of small and light craft was carried out at Kiel University of Applied Sciences. The structural detail is an intersection of longitudinal deck stiffener and transverse web frame with a plate thickness of 3.5mm and a doubling length of 100mm. Manual gas metal arc welding was used for the production of the 46mm long transverse fillet welds. The load-controlled constant amplitude fatigue tests at stress ratio R = 0 were supported by 3D finite-element analysis based on laser scans of the weld seams. Structural hot-spot stress, stress linearisation and Xiao and Yamada’s 1mm geometrical stress approaches were applied to the specimens as well as the notch stress concept with reference radii rref = 0.05mm and 1.00mm.

Qualification of the Notch Stress Approach for the Fatigue Assessment of Welded Pressure Equipment and Power Plant Components

2017 ◽  
Author(s):  
Jürgen Rudolph ◽  
Ralf Trieglaff ◽  
René Stößlein ◽  
Fabian Hauser
Keyword(s):  
Test Data ◽  
Welded Joints ◽  
Low Cycle Fatigue ◽  
Hot Spot ◽  
Pressure Vessels ◽  
Assessment Procedure ◽  
Element Analysis ◽  
Hot Spot Stress ◽  
Fatigue Assessment ◽  
Notch Stress

The fatigue assessment of welded joints in different engineering disciplines is usually based on nominal, structural or notch stresses on one hand (elastic concept using component fatigue curves of load controlled test data) and local strains on the other hand (elasto-plastic concept using material fatigue curves of strain-controlled push-pull test data of un-notched and polished standard specimens). The concepts of the first mentioned group are implemented in widespread standards and recommendations such as [1] to [3]. The fatigue assessment procedure of the European standard for unfired pressure vessels (EN 13445-3, Clause 17 & 18 and related annexes) [4] is currently under revision with one focus on the elaboration of user friendly fatigue assessment options for welded components [5]. The current state of the art focuses on the application of an adapted structural hot spot stress approach to the fatigue assessment of welded pressure equipment [5]. Although this is a significant step forward, the implementation of a notch stress approach can furtherly increase the fatigue assessment options by detailed weld seam analysis. The paper focuses on respective methodological proposals and application examples of typical welded joints. The finite element analysis as part of the procedure has to be harmonized with the requirements of the assessment procedure. Of course, the compatibility of the hot spot stress approach and a notch stress approach has to be guaranteed for individual examples. The direct comparison of the different approaches allows for a qualitative evaluation of methods. The application of an appropriate master fatigue curve FAT100 and the limitations with regard of stress/strain ranges in the low cycle fatigue (LCF) regime as well as the fatigue assessment of welded joints with mild weld toe notches is the subject of special considerations. The latest recommendations of German Welding Society (DVS) [6] constitute a reference for the last two subjects raised.

Support Structure Optimization for Offshore Wind Turbines With a Genetic Algorithm

2014 ◽  
Author(s):  
Lucía Bárcena Pasamontes ◽  
Fernando Gómez Torres ◽  
Daniel Zwick ◽  
Sebastian Schafhirt ◽  
Michael Muskulus
Keyword(s):  
Genetic Algorithm ◽  
Wind Turbine ◽  
Wind Turbines ◽  
Hot Spot ◽  
Offshore Wind ◽  
Support Structures ◽  
Hot Spot Stress ◽  
Offshore Wind Turbines ◽  
Structural Design Optimization ◽  
The Time Domain

This study considers the use of a genetic algorithm for the structural design optimization of support structures for offshore wind turbines. Member diameters, thicknesses and locations of nodes are jointly optimized. Analysis of each design is performed with a complete wind turbine simulation, for a load case in the time domain. Structural assessment is in terms of fatigue damage, evaluated for each joint using the hot-spot stress approach. This defines performance constraints. Designs are optimized with respect to their weight. The approach has been tested with the modified 4-legged UpWind jacket from the OC4 project. The weight is quickly reduced, convergence slows after about 100 iterations, and few changes occur after 250 iterations. Interestingly, the fatigue constraint is not active for any member, and it is the validity of stress concentration factors that determines the best design, which utilizes less than 90 percent of the available fatigue lifetime. These results of the preliminary study using the genetic algorithm demonstrate that automatic optimization of wind turbine support structures is feasible under consideration of the simplified load approach. Even for complex, multi-member structures such as the considered jacket a weight reduction was achieved.

Applicability Evaluation of SS (Structural Stress) Method on Actual Project

2007 ◽  
Author(s):  
Kwang-Seok Kim ◽  
Joong-Kyoo Kang ◽  
Joo-Ho Heo ◽  
Sung-Geun Lee
Keyword(s):  
Fatigue Life ◽  
Hot Spot ◽  
Fatigue Analysis ◽  
Correlation Factor ◽  
New Method ◽  
Structural Stress ◽  
Hot Spot Stress ◽  
Notch Stress ◽  
Loading Case ◽  
Application Data

The structural stress (SS) method developed by BATTELLE has been studied based on small or mid size scale specimens. In order to apply new method on actual project, the new method such as SS should have application results on actual project. However, SS method didn’t have a lot of application data compared to class procedure using hot spot stress (HSS). In order to find that the SS method for the evaluation of fatigue life can give reasonable results when it is applied under the same loading suggested by classification societies, fatigue lives derived by class and SS method were compared. ABS & DNV’s simplified fatigue analysis method were adopted to check the validity of SS method. Before applying complicated loading of class, static loading case was applied since the class method has their own correlation factor for wave loading. And then simplified fatigue analysis was performed with more complicated loading cases. From the results of fatigue life calculation, it can be said that SS shows reasonable fatigue lives with respect to HSS or notch stress based fatigue lives.

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

2018 ◽  
Vol 26 ◽  
pp. 1-8
Author(s):  
Sergei Petinov ◽  
Ruslan Guchinsky
Keyword(s):  
Fatigue Failure ◽  
Strain Energy ◽  
Hot Spot ◽  
Inelastic Strain ◽  
Fatigue Properties ◽  
Hot Spot Stress ◽  
Fatigue Assessment ◽  
Notch Stress ◽  
Assessment Procedures ◽  
Nominal Stress Approach

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.

scholarly journals NUMERICAL METHODS FOR THE FATIGUE ASSESSMENT OF WELDED JOINTS: INFLUENCE OF MISALIGNMENT AND GEOMETRIC WELD IMPERFECTIONS

2017 ◽  
Vol 9 (1) ◽  
pp. 9-24 ◽  
Author(s):  
Andreas TARAS ◽  
Harald UNTERWEGER
Keyword(s):  
Numerical Methods ◽  
Welded Joints ◽  
Numerical Models ◽  
Hot Spot ◽  
Nominal Stress ◽  
Geometric Imperfections ◽  
Hot Spot Stress ◽  
Notch Stress ◽  
Effective Notch Stress ◽  
Nominal Stress Approach

The fatigue design life of welded joints in steel structures is increasingly assessed by using numerical models and methods, such as the structural (hot-spot) stress method and the effective notch stress method. When compared to the classical design approach using nominal stress S-N design curves, these methods offer the advantage of flexibility and a wider scope of application. However, a number of questions arise when these methods are used to assess geometrically "imperfect" welded joints, such as joints with plate misalignments or excessive weld convexity or concavity. In these cases, the classical S-N curves are known to cover imperfections up to the common tolerance classes for fatigue-prone welded joints (e.g. in accordance with ISO 5817 class B). For the numerical methods, differing and conflicting recommendations exist on how to account for the geometric imperfections in the welded joints, with little or no background to these recommendations available. In this paper, a study is presented in which two standard welded joints (butt welds between plates of equal and unequal thickness; T-joints with fillet welds) are analysed with the help of the structural (hot-spot) stress and the effective notch stress approach, considering various levels of geometric imperfection up to the tolerance limits, and the resulting fatigue life predictions are compared to test results from the literature and the nominal stress approach predictions. Since the nominal stress approach curves are based on reliable statistical data and desired survival probabilities for these known, standard cases, this methodology allows one to determine the correct application of the numerical methods to cases with geometric imperfections. This information may be used for a pertinent refinement of design recommendations for these methods, as well as for cases where these methods are applied to fitness-for-purpose assessments - e.g. because the nominal stress approach is not applicable.

Fatigue Strength Characteristics of Cast Steel Weld Joints

2011 ◽  
Author(s):  
Ho Jung Kim ◽  
Seong Min Kim ◽  
Jae Myung Lee ◽  
Myung Hyun Kim
Keyword(s):  
Fatigue Strength ◽  
Hot Spot ◽  
Cast Steel ◽  
Geometrical Parameters ◽  
Steel Weld ◽  
Hot Spot Stress ◽  
Butt Weld ◽  
Weld Joints ◽  
Fatigue Design ◽  
Notch Stress

The aim of the present paper is to investigate and compare the fatigue characteristics of weld joints in terms of S-N curve between cast steel and conventional mild steel. Two different joints such as thick butt weld joints and fillet weld joints are considered in this study. Fatigue strength of cast steel weld joints with various geometric designs are tested and compared. The fatigue life and the fatigue crack propagation characteristics are investigated by experimental and numerical methods in a systematic manner. The conventional hot spot stress, structural stress and notch stress approaches are employed for the precise interpretation of the fatigue test result of cast steel weld joints. Finally, the estimation of the fatigue limit of cast steel weld joints are attempted based on material properties and geometrical parameters. The results can be used for the efficient fatigue design of various cast steel welded joints.

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