Temperature-Dependent Constitutive Model of Austenitic High-Strength A4L-80 Bolts after Furnace Fire

High-strength carbon steel bolts were fractured by the tension, shear, or combined failure mode in semirigid and flexible beam-to-column connections as observed from natural fire incidents and full-scale fire tests. This phenomenon is attributed to the temperature-sensitivity of quenching and tempering procedures on these bolts. However, provided that these bolts are replaced by stainless steel bolts in these connections, the connection performance of the latter can be better improved compared to that of the former. Accordingly, this paper documents an experimental investigation of low-carbon austenitic high-strength A4L-80 bolts after fire exposure to determine the temperature-based stress-strain curves. The residual properties of Young's modulus, yield and ultimate strengths, and ultimate strain obtained from the experimentally measured stress-strain curves were compared with those of base materials regarding A4L-80 and carbon steel bolts including Grade 8.8, 10.9, and 12.9. Considering the limitations of prediction accuracy of reduction models of base materials in the existing standards and design manuals, reduction models after fire exposure were proposed for five mechanical parameters (Young's modulus, yield and ultimate strengths, ultimate strain, and strain-hardening exponent in the Ramberg-Osgood model). In combination with the currently proposed reduction models, the temperature-dependent constituent model of A4L-80 was formulated using five mechanical parameters at ambient temperature. It is concluded that austenitic high-strength bolts after fire exposure are capable of providing a more pronounced enhancement to the fire resistance of semirigid connections than carbon steel bolts, and the formulated material model showed good consistency with stress-strain curves acquired from experimental tests. Finally, a finite element model (FEM) with this material model was established based on the previous experimental study of web angle cleat connections with A4L-80 after fire, according to which the moment-rotation curves obtained from FEM can correspond well to those done from tests in previous study. This confirms the further validation and prediction accuracy of the formulated material model.