Mechanical properties of ultra-high strength (Grade 1200) steel under cooling phase of a fire

This thesis mainly investigates the post-fire mechanical properties of ultra-high strength steel (UHSS). Tensile coupon tests are carried out on UHSS standard coupons under realistic fire conditions with and without sustained axial loads. A material model is developed which describes the post-fire stress-strain behaviour of UHSS. The mechanical response of UHSS under extreme cooling conditions is also evaluated. The steel grade effect is studied by conducting similar experiments on lower steel grades. The microstructures of the tested materials are evaluated using optical and scanning electron microscopy methods. Manufacturing process effect on the post-fire behaviour of UHSS tubes is also investigated. Using the results of this study, guidelines can be added to the standards to take into account the in-fire and post-fire stress-strain response of UHSS materials.