In ANSYS Release 11.0 and prior, user-defined failure criteria could be defined via usrfc1 through usrfc6 subroutines with the TB,FAIL command, although these were limited for use with older composite elements (SOLID46/95/191, SHELL91/99).

In ANSYS 12.0, a new user-defined failure criteria userfc is introduced which allows use with current-technology elements and FC commands.  Unfortunately, this new feature didn’t make it in the ANSYS 12.0 New Release documentation, although it is mentioned in the 12.0.1 Errata.

A unique feature of userfc is that it calculates up to 9 user-defined failure criteria on-the-fly.  This means that data is not stored in the ANSYS result file (.rst), thus saving on disk space.  However, the drawback is that user-defined failure-crteria cannot be used with Workbench Mechanical application’s “User Defined Results”, so postprocessing must be done in ANSYS (Mechanical APDL) or via APDL Commands, such as PLESOL,FAIL,USRx, to generate static images.

For users wishing to postprocess user-defined failure criteria for composite elements, userfc is a huge advantage over the older usrfc1 through usrfc6 subroutines.  Moreover, one can also use userfc for homogenous materials as well — for example, temperature-dependent safety factors can be plotted, which may be more flexible than SFACT and related APDL commands.