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Microstructural Characterisation of Structural Bolt Assemblies in Fire

Johnson, Lucy (2014) Microstructural Characterisation of Structural Bolt Assemblies in Fire. PhD thesis, University of Sheffield.

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Abstract

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.

Item Type: Thesis (PhD)
Academic Units: The University of Sheffield > Faculty of Engineering (Sheffield) > Materials Science and Engineering (Sheffield)
Identification Number/EthosID: uk.bl.ethos.634328
Depositing User: Miss Lucy Johnson
Date Deposited: 29 Jan 2015 15:51
Last Modified: 03 Oct 2016 12:09
URI: http://etheses.whiterose.ac.uk/id/eprint/7730

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