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Application of limit design to high-strength aluminum alloy beams Allen, David Elliott


Although investigations have shown that the theory of limit design applies to beams and some frames made of mild steel, it is not certain whether it applies in the same way to the light alloys. Steel frames satisfy the limit design prediction of a failure mechanism not only because steel is very ductile but also because steel exhibits strain hardening. Light alloys such as high-strength aluminum alloy exhibit very little strain hardening. Two load tests were carried out on redundant beams made of the aluminum alloy to see if the mechanism condition of limit design was reached before failure took place in the beam. Measurements of beam deflections and moments are compared to the deflections and moments predicted by the theory of inelastic bending. The theory of inelastic bending is based on the stress-strain diagram and takes account of strain hardening and a failure strain. Tables of unit functions derived from the stress-strain diagram of the aluminum alloy are presented for use with the inelastic bending theory. In both tests, the mechanism condition of limit design was reached before failure took place. Shortly after the mechanism condition was reached, a fracture occurred in the flange on the tension side of the beam. Thus the type of failure indicates that not all structural configurations will achieve the mechanism condition. Beyond the limit of elastic deformation (17 kips load) and up to a load of about 27 kips, the beam moments were similar to those predicted by the inelastic bending theory. From 27 kips load to failure at 32 kips, the moments were distributed in the beam differently than the predicted moments due to the presence of high shear force. The load-deflection curves are the same as the curves from the theory, although measured deflections were always greater. The ultimate curvature at the section of failure was greater than predicted from the theory. There were some shortcomings of the tests. The tests were originally set up to be unfavourable towards the limit design theory. However, stiffeners were added at the plastic hinge locations to prevent web failure, and the presence of the stiffeners was helpful in allowing redistribution of moments to take place in much the same way as strain hardening does in steel beams. Also the presence of the stiffeners and stiffener holes made interpretation with the inelastic bending theory uncertain. Finally there were some errors in measuring the moments by means of strain gauges.

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