On the use of Bernstain-Bézier functions for modelling the post-fire stress-strain relationship of ultra-high strength steel (Grade 1200)

• Published in: Engineering structures 2018-11, Vol.175, p.605-616
• Main Authors: Azhari, Fatemeh, Heidarpour, Amin, Zhao, Xiao-Ling
• Format: Article
• Language: English
• Subjects: Axial loads; Axial stress; Bernstain-Bézier functions; Cooling; Creep; Creep (materials); Fire; High strength steel; High strength steels; High temperature; Load; Mathematical models; Metals creep; Steel; Strain; Stress-strain curves; Stress-strain relationships; Sustained axial load; Temperature effects; Ultra-high strength steel
• ISSN: 0141-0296; 1873-7323
• DOI: 10.1016/j.engstruct.2018.08.088

•Bernstain-Bézier functions are used to develop a material model for UHSS cooled from fire.•The model considers the effect of fire temperature and sustained axial load ratio.•The model is calibrated and validated against experimental tests results.•The model is capable of extrapolating the results out of the range of tests data.•The creep strain of UHSS under a transient fire and a constant stress is derived. Ultra-high strength steels (UHSS) have significant potential applications in the engineering fields due to their unique specifications. In recent years, it has been experimentally shown that the post-fire stress-strain response of this material is highly dependent on the maximum steel temperature and the sustained load applied to it during fire. This paper employs the Bernstain-Bézier functions to present the relationship between the stress, strain, the maximum fire temperature and the sustained axial load ratio (β) for Grade 1200 UHSS cooled for fire temperatures to room temperature. The experimental results are used to verify and validate the proposed model throughout the paper. The model showed to be capable of not only interpolating the stress-strain curves, but also extrapolating them out of the range of the available experimental tests data. Also, taking advantage of the stress-strain-temperature response of the UHSS tested at elevated temperatures, the instantaneous stress-induced strain and consequently the creep strain of UHSS subjected to different constant sustained load values during transient fire are obtained.