Abstract
In this paper preliminary results are reported on the strength characteristics and forming limits of various aluminum-alloy sheet, when subjected to hydraulic pressure over a circular area at temperatures up to 500 F. The strain distributions were analyzed by means of a photo-grid, after these bulges failed. The bulges exhibited a practically balanced biaxial strain state over most of the contour with the exception of a very narrow range of local deformation in the vicinity of the fractures. The maximum meridional strain outside the necked area observed at the pole of a bulge, in spite of its biaxial nature, generally exceeded the uniform elongation known from tensile tests. The forming limits for all annealed conditions increased considerably with increasing temperature if the temperature exceeded a certain critical value, between 250 and 400 F, depending upon the alloy. The heat-treated conditions of the alloys 24S and 61S were only slightly affected by the testing temperature, while the forming limits (uniform stretch) of 75S-T, R301W, and R301T were materially increased at elevated temperatures. The reduction of thickness, which is a measure of the ductility of an alloy, followed approximately the same trend as the forming limit.
