This library has a lot of Fortran Routines for modeling implicit material constitutive behavior. There are implementations of Neo-Hooken, Odgen and Hencky potential for hyperelastic behavior. Also the viscoplastic Perzyna potential is implemented. All these routines were used in my master thesis, which I implemented custom hyperelastic-viscoplastic constitutive model in Ansys using the User Programmable Feature, usermat.f. This is a very useful library if you intend to implement a constitutive model in Ansys Mechanical. Then you can modify your usermat.f in other to incorpared behaviors and functions implemented in this library. It contains methods to deviatoric and volumetic decomposition, eigenvalue and eingen vector decompostion and others linear algebra operations. There are some stress conversion as well, for instance the Cauchy stress to 1st Piola-Kirshoff stress, Kirshoff stress to 2nd Piola Kirshoof stress and so on. This is very useful because the model may be created with any stress measure and then converted to the formulation of the Ansys solver, which is a rotated Rotated Cauchy Stress.