Fatigue Analysis of Welded Structures Using the Finite Element

  • View
    217

  • Download
    1

Embed Size (px)

Text of Fatigue Analysis of Welded Structures Using the Finite Element

  • Fatigue Analysis of Welded Structures Using the Finite Element Method MUSTAFA AYGL Department of Civil and Environmental Engineering Division of Structural Engineering, Steel and Timber Structures CHALMERS UNIVERSITY OF TECHNOLOGY Gothenburg, Sweden 2012

  • THESIS FOR THE DEGREE OF LICENTIATE OF ENGINEERING

    Fatigue Analysis of Welded Structures Using the Finite Element Method

    MUSTAFA AYGL

    Department of Civil and Environmental Engineering

    Division of Structural Engineering, Steel and Timber Structures CHALMERS UNIVERSITY OF TECHNOLOGY

    Gothenburg, Sweden 2012

  • Fatigue Analysis of Welded Structures Using the Finite Element Method

    MUSTAFA AYGL

    MUSTAFA AYGL, 2012

    ISSN no. 1652-9146 Lic. No: 2012:4 Department of Civil and Environmental Engineering Division of Structural Engineering Steel and Timber Structures Chalmers University of Technology SE-412 96 Gothenburg Sweden Telephone: + 46 (0)31-772 1000 Cover: Finite element models for the determination of fatigue design stresses created by the author Chalmers Repro Service / Department of Civil and Environmental Engineering Gothenburg, Sweden, 2012

  • I

    Fatigue Analysis of Welded Structures Using the Finite Element Method

    MUSTAFA AYGL

    Department of Civil and Environmental Engineering Division of Structural Engineering Steel and Timber Structures Chalmers University of Technology

    ABSTRACT Fatigue design and analysis of steel and composite bridges is generally based on the notion of the nominal stress using the classified S-N curves with corresponding fatigue classes for typical details. Such an approach can yield unrealistic the estimation of the load effects for structure components because of an ever increasing number of structural details and loading situations resulting in a limited number of possible treatable design cases. The hot spot stress method has been developed to enable an accurate estimation of the load effects for the fatigue strength of welded steel structures, in cases where the nominal stress is hard to estimate because of geometric and loading complexities or in cases where there is no classified detail that is suitable to be compared with. Although this method has been used in fatigue design and analysis in tubular structures for several decades, the method has not been applied on a larger scale on steel and composite bridges.

    The essential of adopting the finite element method for determining the design stresses for fatigue life calculations has been increased recently especially when utilizing the advanced fatigue assessment methods for welded steel structures. However, the result from finite element analysis can be highly sensitive to modelling technique since the stresses obtaining from these advanced methods such as the structural hot spot stress method and the effective notch stress method are often in an area of high strain gradients, i.e. stress singularities. The resulting stresses may differ substantially depending on the type and size of elements. In this study, this was recognized by evaluating the hot spot and effective notch stresses obtained from the finite element analyses with the fatigue test data collected from the literature.

    Orthotropic steel bridges have both complex geometry and loading conditions producing complex bridge deck behaviour which is hard to estimate and analyse using the traditional fatigue assessment methods. The effects of the loading and geometrical conditions, i.e. decks components which are working in a group, need to be considered accurately in the stress calculations. Assuming overall elastic behaviour in such complex structure systems, in which the stress raising sources that have decisive effects on the fatigue strength capacity are partly included, can yield over/underestimated stress values to be evaluated in fatigue design. The application of the advanced life assessment methods using the finite element method studied in this thesis produce more accurate stress results, including stress raising effects at welded details that are prone to fatigue.

    Keywords: Hot spot stress, effective notch stress, orthotropic bridge deck, welded details, the finite element method

  • II

    Utmattningsanalys av svetsade konstruktioner med finita elementmetoden

    MUSTAFA AYGL

    Institutionen fr bygg- och miljteknik

    Konstruktionsteknik, Stl- och trbyggnad

    Chalmers tekniska hgskola

    SAMMANFATTNING

    Dimensionering av utmattningshllfastheten fr stl- och samverkansbroar baseras generellt p tillmpning av metoden med nominella spnningar i samband med typdetaljer med angiven utmattningshllfasthet. Dylika konventionella dimensioneringsmetoder kan orealistiskt utvrdera utmattningsbelastade komponenter p grund av att det stndigt kande antal frbandsdetaljer och lastkombinationer leder till begrnsade antal mjliga designfall som r behandlingsbara. Hot spot spnningsmetoden har utvecklats fr att underltta en mer noggrann utvrdering av utmattningshllfastheten hos svetsade stlkonstruktioner, srskilt tillmpliga r fallen dr den nominella spnningen r svr att definieras korrekt p grund av komplexiteten i geometrin och lastfrhllandena eller de fall dr det saknas ett klassificerat frband som r lmpligt att jmfras med. ven om denna metod under flera decennier har anvnts fr utmattningsdimensionering i rrformiga konstruktioner, har denna dock inte tillmpats i strre skala p stl- och samverkansbroar.

    Tillmpning av finita elementmetoden p spnningsanalyser innebr dremot mer noggranna spnningsberkningar som inkluderar bde globala och lokala effekter p svetsade detaljer. Resultaten frn sdana analyser kan dock vara mycket knsliga fr modelleringstekniken eftersom spnningarna som anvnds p de avancerade metoderna ssom hot spot spnningsmetoden och effektiv notch spnningsmetoden ofta r i ett omrde med stora hga spnnigsgradienter, dvs. singulariteter dr spnningarna gr mot ondligheten. Beroende av elements typ och intensitet kan de dimensionerande spnningarna skilja sig avsevrt.

    Broar med ortotrop platta har geometrier och belastningsfrhllanden som r komplexa. Denna komplexitet resulterar i invecklade beteenden av plattan vilket kan frsvra att analysera med hjlp av traditionella utmattningsdimensioneringsmetoder. Lasteffekter och geometrisk komplexitet, dvs. komponenter som arbetar i grupp, mste beaktas noggrant i spnningsberkningarna. Genom att anta en global linjrelastisk balkteori berr sdana komplexa detaljer, dr spnningshjande effekterna som har avgrande inverkan p utmattningshllfastheten bara delvis ingr, kan leda till ver- eller underskattade spnningsvrden som i sin tur ger orealistisk uppskattning p utmattningskapaciteten. Vid tillmpning av de avancerade metoderna med finita elementmetoden som studierats i detta arbete, erhlls mer plitliga spnningsvrden som inkluderar spnningshjande effekter p svetsdetaljerna.

    Nyckelord: Hot spot spnningsmetoden, effektiv notch spnningsmetoden, ortotrop platta, svetsade frband, finita elementmetoden

  • III

    LIST OF PUBLICATIONS

    This thesis is based on the work contained in the following papers, referred to by Roman numerals in the text.

    I. Aygl, M., M. Al-Emrani and S. Urushadze, (2011). Modelling and fatigue life assessment of orthotropic bridge deck details using FEM. International Journal of Fatigue (Article in Press)

    II. Aygl, M., Bokesj, M., Heshmati, M. and Al-Emrani, M., (2012). A comparative study on different fatigue failure assessments of welded bridge details. International Journal of Fatigue (Submitted article)

    ADDITIONAL PUBLICATIONS BY THE AUTHOR

    Conference Papers I. Aygl, M., Al-Emrani, M., Frba, L. and Urushadze, S., (2010). Evaluation of

    the fatigue strength of an orthotropic bridge deck detail using hot spot stress approach. 63rd Annual Assembly & International Conference of the International Institute of Welding. AWST-10/63, Istanbul, Turkey. 261-268

    .

  • IV

  • V

    Contents ABSTRACT I

    SAMMANFATTNING II

    CONTENTS V

    PREFACE VII

    NOTATIONS VIII

    1 INTRODUCTION 1

    1.1 Background 1

    1.2 Aim and scope 2

    1.3 Scientific approach 2

    1.4 Limitations 3

    1.5 Outline of the thesis 3

    2 FATIGUE LIFE ASSESSMENT METHODS 5

    2.1 Nominal stress approach 7

    2.2 Hot spot stress approach 8

    2.3 Effective notch stress approach 13

    3 WELD MODELLING TECHNIQUES 18

    3.1 Weld modelling using oblique shell elements 18

    3.2 Weld modelling using rigid links 19

    3.3 Weld modelling using increased thickness 20

    3.4 Weld ends modelling 21

    3.5 Weld modelling using solid elements 22

    4 APPLICATION OF THE FATIGUE ASSESSMENT METHODS 24

    4.1 Modelling and fatigue life assessment of orthotropic bridge deck details using FEM Paper I 24

    4.2 A comparative study on different fatigue failure assessments of welded details Paper II 30

    5 CONCLUSIONS 35

    5.1 General conclusions 35

    5.2 Outlook for future research 37

    6 REFERENCES 38

  • VI

    APPENDED PAPERS

    PAPER I

    PAPER II

  • VII

    Preface The work presented in this licentiate thesis was conducted between January 2009 and

    November 2011 at Chalmers University of technology, Department of Civil and Environmental Engineering, Division of Structural Engineering, Steel and Timber Structures.

    The work has been carried out within the part of BriFaG research project "Bridge Fatigue Guidance Meeting Sustainable Design and Assessment". The project was financed by European Commission Research Fund for Coal and Steel and the Swedish transport administration, Trafikverket, all of which are gratefully acknowledged for making it