Lode angle:

https://etd.ohiolink.edu/rws_etd/document/get/akron1334113425/inline http://www.imbm.bas.bg/uploads/pdfs/papers/MDat/lecture8_2007-11-19.pdf http://homepages.engineering.auckland.ac.nz/~pkel015/SolidMechanicsBooks/Part_II/08_Plasticity/08_Plasticity_03_YieldCriteria.pdf