When defining a User Defined Soil Model in the material database, all the parameters describing the interface behaviour are defined in the tabsheet for Interfaces. One parameter that cannot be controlled this way is the tension cut-off value for the interfaces, even if the User Defined Soil Model (UDSM) uses a tension cut-off option.
This tension cut-off for UDSM interfaces is usually1) initialized at zero stress, which means the the tensile (effective) stress in the interface is cut off at zero stress.
This applies to all User Defined Soil Models, including the user defined soil model implementations of UBC Sand, Barcelona Basic Model and Hypoplastic models.
If one still wants to allow tension in the interface, or limit it to a different value, it is possible by assigning a separate material data set for the interfaces alone. In that case an appropriate Plaxis soil model needs to be chosen in which the tension cut-off can be defined (e.g. Mohr-Coulomb, Hardening Soil, or HSsmall) and shows similar behaviour for undrained behaviour, (stress-dependent) stiffness and strength compared to the user defined soil model.
1)see related issue for interface tension cut-off: Tension cut-off for Soft Soil (Creep) and UDSM interfaces