Post-fire modeling of aluminum structures using a kinetic driven approach

Abstract

Aluminum alloys undergo permanent material property degradation that results in a reduced post-fire capacity of a structure. This degradation occurs at temperatures as low as 200 °C and is a function of both exposure temperature and duration. Material models have been previously developed that account for both temperature and duration but previous modeling efforts for aluminum structures have only considered maximum temperature. This research utilized an Arrhenius kinetics model to predict the material property degradation for AA6061 caused by the thermal exposure. The model was calibrated using post-fire yield strength data at different linear heating rates. The method was validated by modeling a set of small-scale experiment of an aluminum beam using thermal and mechanical finite element analyses. The beams were subjected to nominal 50 kW/m2 and 70 kW/m2 radiant heating from underneath for up to 20 min, water quenched, and loaded in 4-point bending. Beam temperatures were predicted within 25 °C during the transient response and 10 °C during the steady-state response. The predicted peak bending loads for all tested exposure levels and durations were within 8% of the experimentally measured values. Continued degradation of beam strength observed in the experiments during thermal steady-state was also captured by the kinetics-based model.