Constitutive Model for Aluminum Alloys Exposed to Fire Conditions
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ALUMINUM alloys in series 5xxx and 6xxx are applied in structures such as living quarters and helicopter decks on drill platforms, fast ferries, and yachts. In case a fire breaks out in such a structure, people have to be able to safely escape before the structure collapses. National laws and regulations apply on the time that the structure has to keep its load-bearing function before it collapses. This time is called the fire resistance. Typical fire resistances specified are 30, 60, 90, or 120 minutes. Because of their relatively low melting temperature, low density, and high thermal conductivity, load-bearing structures composed of aluminum alloys need to be protected (insulated) in almost all cases in order to fulfill these fire resistances.[1] In many cases, the temperature of fire-exposed insulated aluminum members increases approximately linearly with time. To be able to determine the amount of insulation required, knowledge on the constitutive properties of the applied alloys when exposed to fire is required. The constitutive relations at elevated temperature are usually determined with one of the following types of tests. J. MALJAARS, Researcher, is with Netherlands Institute of Metals Research, P.O. Box 5008, 2600 GA Delft, The Netherlands. J. MALJAARS and F. SOETENS, Professor, are with Architecture, Building and Planning, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands and Civil Infrastructure, TNO, P.O. Box 49, 2600 AA Delft, The Netherlands. Contact e-mail: [email protected] L. KATGERMAN, Professor, is with Mechanical, Maritime and Materials Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands. Manuscript submitted July 23, 2007. Article published onlined February 12, 2008 778—VOLUME 39A, APRIL 2008
(a) Steady-state tests: specimens are subjected to a constant temperature and a certain strain rate. The force (stress) is measured. (b) Transient state tests: specimens are subjected to a (linearly) increasing temperature and a certain stress in time. The strain is measured. (c) Creep tests: specimens are subjected to a constant temperature and a constant stress in time. The strain is measured. Based on steady-state tests, data are given on the 0.2 pct proof stress of some aluminum alloys at elevated temperature in Reference 2. These data are incorporated in the design code for fire-exposed aluminum structures Eurocode 9, Part 1–2.[3] The Ramberg–Osgood relation is often used in engineering practice to describe the stress-strain relationship.[4] Values for the hardening factor of the Ramberg–Osgood relation for aluminum alloys at elevated temperature are determined and used in the fire design of frames.[5] However, the temperature and load history in steadystate test are not representative for a fire. Transient state tests are usually considered as appropriate tests for simulation of fire conditions. Due to viscoplastic behavior (or creep), overaging, and annealing, the strength resulting from steady-state tests differs in general from the
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