Abstract: Accurate upper-bound solutions for several typical plastic forming problems are found, such as an obtuse wedge under the action of a one-sided uniform pressure, the extension of a strip with ideal cuts, and steady plastic flow, for example, drawing of a strip through a tapered die with smooth walls, forward extrusion of a sheet from a container with smooth walls through a slit, etc. The slip-line fields consist of uniform fields and central fan fields in these problems. By using of upper-bound technique, a general formula of the upper-bound solution with n being suitable for the plastic problems is deduced. Then , when , the accurate upper bound solutions to be identically equal to the slip-line solutions are obtained.

Key words: Accurate upper-bound solutions, Central fan fields, Reduction ratio, Uniform fields