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  • The University of Sheffield

    Department of Materials Science and Engineering

    & Department of Civil and Structural Engineering

    Microstructural Characterisation of

    Structural Bolt Assemblies in Fire

    By Lucy Johnson

    A thesis submitted for the degree of Doctor of Philosophy

    September 2014

  • Page i

    Summary

    In structural fire engineering, the importance of bolt assemblies is often overlooked.

    Connection design uses the temperature-dependent bolt strength-reduction factors

    prescribed in Eurocode 3, despite the existence of two distinct failure modes under

    tension; bolt breakage, and thread-stripping. This thesis investigates the factors

    which influence failure modes at ambient and elevated temperatures and a range

    of strain-rates through microstructural characterisation, tensile testing and finite

    element modelling.

    Microstructural characterisation carried out on M20 galvanised bolt assemblies

    consisting of Grade 8.8 bolts and Property Class 10 nuts from a range of

    manufacturers has highlighted that, despite a specified tempered-martensite

    microstructure, microstructural variations existed between different manufacturers

    and within a single batch. These microstructural variations not only affected the

    flow behaviour of the bolt material but determined the failure modes of bolt

    assemblies at ambient temperatures. Tensile testing of turned-down bolts allowed

    the temperature and strain-rate dependent flow behaviour of bolt material to be

    investigated, eliminating the effect of thread deformation. The flow curves obtained

    were input to a finite element model to represent true bolt material behaviour,

    which was validated against force-displacement curves obtained from uniaxial

    tensile testing of bolt assemblies from the same batch.

    Both experimental and finite element modelling work have highlighted the

    importance of using a tight thread tolerance class combination and a suitably tall

    nut to ensure ductile bolt breakage failures and avoid thread-stripping.

  • Page ii

    Page Intentionally Left Blank

  • Page iii

    Contents Summary .................................................................................................................... i

    Contents ................................................................................................................... iii

    List of Figures ............................................................................................................ v

    List of Tables ............................................................................................................ ix

    Symbols .................................................................................................................... xi

    Acknowledgements ................................................................................................ xiii

    Declaration.............................................................................................................. xiv

    1 Introduction .......................................................................................................1

    2 Background .......................................................................................................5

    2.1 History of Bolts .............................................................................................5

    2.2 Structural Behaviour in Fire ..........................................................................6

    2.2.1 Structural Response to Fire ...............................................................7

    2.2.2 Connection Behaviour ..................................................................... 11

    2.2.3 Bolts ................................................................................................ 13

    2.3 Bolting Standards ...................................................................................... 33

    2.3.1 Property Class Designation ............................................................ 34

    2.3.2 Mechanical Properties .................................................................... 35

    2.3.3 Thread Tolerance ............................................................................ 36

    2.3.4 External Geometry .......................................................................... 41

    2.4 Manufacture ............................................................................................... 42

    2.5 Chemical Composition and Heat Treatment ............................................. 43

    2.5.1 Pearlite ............................................................................................ 46

    2.5.2 Martensite ....................................................................................... 47

    2.5.3 Bainite ............................................................................................. 48

    2.5.4 Continuous Cooling Transformation (CCT) Diagrams .................... 48

    2.5.5 Hardenability ................................................................................... 49

    2.5.6 Tempering ....................................................................................... 51

    2.5.7 Galvanising ..................................................................................... 52

    2.6 Summary ................................................................................................... 53

    3 Microstructural Characterisation .................................................................... 55

    3.1 Chemical Composition ............................................................................... 57

    3.2 Vickers Hardness Testing .......................................................................... 58

    3.3 Optical Microscopy .................................................................................... 63

    3.4 Scanning Electron Microscopy .................................................................. 66

    3.5 Prior-austenite Grain Size ......................................................................... 69

    3.6 Continuous Cooling Transformation (CCT) Curve Calculation ................. 70

    3.7 Uniaxial Tensile Testing of Turned down Bolts ......................................... 73

    3.7.1 Experimental Methods .................................................................... 73

    3.7.2 Experimental Results ...................................................................... 81

    3.8 Summary ................................................................................................... 87

    4 Mechanical Testing of Bolt Assemblies ......................................................... 91

    4.1 Experimental Methods ............................................................................... 91

    4.1.1 Thread Tolerance Measurement ..................................................... 93

  • Contents

    Page iv

    4.2 Results ....................................................................................................... 94

    4.2.1 Effect of strain-rate and temperature .............................................. 94

    4.2.2 Effect of Thread Clearance ............................................................. 99

    4.2.3 Thread Deformation ...................................................................... 102

    4.3 Summary ................................................................................................. 104

    5 Finite Element Modelling .............................................................................. 107

    5.1 Input Parameters ..................................................................................... 107

    5.1.1 Geometry ...................................................................................... 107

    5.1.2 Material Properties ......................................................................... 113

    5.1.3 Interactions ................................................................................... 120

    5.1.4 Constraints .................................................................................... 120

    5.1.5 Boundary Conditions .................................................................... 120

    5.1.6 Predefined fields ........................................................................... 121

    5.1.7 Verification of whether axisymmetric model accurately represents

    3D behaviour ................................................................................................ 122

    5.1.8 Mesh ............................................................................................. 123

    5.2 Validation of the model ............................................................................ 126

    5.3 Study of the effects of different variables on failure mode and strength . 129

    5.3.1 Influence of number of threads below nut .................................... 130

    5.3.2 Influence of nut height .................................................................. 131

    5.3.3 Influence of Thread Clearance ..................................................... 134

    5.3.4 Influence of Nut Height and Thread Clearance ............................ 139

    5.4 Summary ................................................................................................. 143

    6 Discussion .................................................................................................... 147

    6.1 Microstructural Variations ...................