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Inapplicability of limit design to structures made of some high strength aluminium alloys. Yu, Lawrence Kuang


The basic assumption of limit design, that moments are equalized by the formation of a mechanism has proven applicable to steel beams and certain types of frames. It is not known, however, if the theory applies to light metal alloys. Steel possesses a considerable amount of strain hardening which is essential to the formation of plastic hinges in the beam, but some aluminum alloys which have little strain hardening may not be suitable for limit design. Two beam tests were carried out on a twice statically indeterminate beam made of Alcan 65S-T6 aluminum alloy to determine whether the mechanism predicted by the theory of limit design is realized before failure occurs in the beam. Moments and deflections of the beam near failure are compared with theoretical predictions obtained from the theory of limit design and the theory of inelastic bending. The latter, developed by Dr. A. Hrennikoff in 1918, is more "exact" than the theory of limit design. Test results showed that the beam failed at one of the early plastic hinges, before the mechanism was fully developed. It demonstrated that limit design is not always applicable to beams made of aluminum alloys which have very little strain hardening.

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