AbstractWhen using numerical methods in soil mechanics, one often needs to define a yield surface in three‐dimensional principal‐stress space. A special class of yield surfaces, given byJ= (p+a)α(1−β sin 3ν)n, where ν is the Lode angle, is considered from the point of view of convexity and agreement with experimental data. Some recently proposed yield functions which belong to this class are compared. It is shown that the model withn= −0.229 is optimal as regards convexity, and can give reasonable agreement wi