Microstructural Characterisation of Structural Bolt Assemblies in Fire

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.