Constitutive Model for Aluminum Alloys Exposed to Fire Conditions

An existing constitutive model for creep, developed by Dorn and Harmathy, is modified in order to be used for fire-exposed aluminum alloys. Two alloys, 5083-O/H111 and 6060-T66, are selected for the development of this constitutive model because of their different behavior at elevated temperature and their frequent application in structures for which fire design is relevant. The material parameters in the model are calibrated with the experimental results of creep tests, carried out with constant load and temperature in time. The model is validated with so-called transient state tests, with a constant load in time (stresses ranging from 20 to 150 N/mm2) and with an increasing temperature (with heating rates ranging from 1.6 °C/min to 11 °C/min and critical temperatures ranging from 170 °C to 380 °C). These tests are considered as representative for fire-exposed, insulated aluminum members. The existing constitutive model of Dorn and Harmathy provides good agreement with the transient state tests carried out for the 5xxx series alloy, but appeared to be not suited for the 6xxx series alloy. This is attributed to the early development of tertiary creep in case of 6xxx series alloys. The existing model was modified to incorporate this first stage of tertiary creep, to arrive at a good agreement between the tests and the modified model for 6xxx series alloys