Abstract Many aircraft parts consisting of a flat web and a concave flange are produced by rubber-die-forming on a hydropress. The forming action consists of bending the flange around the die radius and stretching the outer fiber of the flange from its original length to that which is achieved when it contacts the die. Several types of failures and defects are encountered, the most important being a radially directed fracture initiated at the outer fiber of the flange. When this fracture occurs without necking, the maximum per cent stretch that may be produced approximates the per cent local ductility in the tension test. If, however, fracture is preceded by necking, the permissible per cent stretch is intermediate between the uniform and local per cent strain in tension. Increasing the forming temperature permits greater per cent stretches for some of the more-ductile alloys.