Residual mechanical properties of aluminum alloys AA5083-H116 and AA6061-T651 after fire

Aluminum alloys are increasingly being used in a broad spectrum of load-bearing applications such as light rail and marine crafts. Post-fire evaluation of structural integrity and assessment of the need for structural member replacement requires understanding of the residual (post-fire) mechanical behavior. Aluminum alloys are strengthened by either strain hardening (cold/hot rolling) or precipitation hardening (heat treatment). Prevalent structural alloys strengthened by each method were investigated in this research: AA5083-H116 (strain hardened) and AA6061-T651 (precipitation hardened). An experimental study was conducted to quantify the residual mechanical behavior of aluminum alloys following a fire exposure. Heating of the aluminum alloys results in evolution of material microstructure, which governs mechanical behavior. Microstructural evolution can be predicted as a kinetically-driven process. As a result, the effects of exposure temperature and heating rate on the residual mechanical properties were quantified. Monotonic, uniaxial tension tests were performed to measure the residual stress–strain relations, which were used to quantify the residual Young’s modulus, yield strength, ultimate strength, and work hardening behavior. AA6061-T651 was determined to have a more significant decrease in strength after fire exposure as compared to AA5083-H116. The heating rate affected the temperatures at which property degradation initiated as well as the residual property magnitude at a given temperature. This is most prevalent in temperature regions with significant microstructural changes, such as recrystallization and precipitate growth. The knowledge elucidated in this study was used to develop empirical evolution models to estimate residual yield strength after linear (ramp) heating and isothermal exposure. Utilizing these models, residual yield strength evolution after realistic fire exposure, which includes combinations of linear and isothermal heating, may be estimated and understood.