The shell solution should be used when the plastic deformations are caused mostly by bending (versus drawing or stretching). The shell solution adds strains related to curvature. These strains go into the balance of energy, which affects the results everywhere. However, the strain solution does not take into account in history that during the drawing some points of the blank could have passed over the curved areas of the die or punch and then ended up on the flat. It does not add the curvature-related strains on those flat areas. Therefore, it should not be used for deep drawing.