Although Zencrack has a powerful feature set for crack growth predicition, the subject is a complex one. It is recognised that the complexity of some analyses means that they may benefit from specific control of certain analysis features during the analysis itself. In addition, non-standard crack growth data may sometimes need to be used.

The user subroutines available in Zencrack allow the user to have complete control over certain analysis features and to use proprietary crack growth models if required. The crack growth subroutines also enable investigation of complex elastic-plastic growth models.

User subroutines require that the user provides Fortran coding for the subroutine. Some "utility functions" are also provided. These utility functions can be called from within user subroutines to access or assign values to certain model data.

User subroutines can also be used to allow novel analyses to be carried out, as shown in the two examples below.

User subroutines are ASCII files and as such any text editor can be used to generate them. The subroutine file may have any name but the file extension should be .f or .for. A user subroutine source file can contain coding for one or more of the available user subroutines. The file is compiled and linked to create a new Zencrack executable using the -u command line option when a Zencrack job is executed. The analysis then uses the new executable.