Wrinkling and splitting are two kinds of common defects in sheet metal forming processes. An overall review is summarized
on various tensile
instability theories for prediction of splitting and the plastic buckling theories for prediction of wrinkling, and the shortcomings of these various theories are pointed out meanwhile.Based on the general variational principle method for the bound analysis, the sheet metal during plastic wrinkling is supposed to satisfy all equations, including differential equations of equilibrium, strain compatibility equations, yield criteria and flow rules. This problem comes as to solution of the maximum value of a variational function so that an approximately analytical solution to the plastic wrinkling in sheet metal processes can be conveniently obtained by the direct method. It is indicated that it is necessary to develop a universal
formability theory as a general rule for prediction of splitting and wrinkling in metal sheet forming. Some research topics are given concerning this universal formability theory.