Swelling originated from needs in the nuclear industry, although it is also often used for other applications. For example, in cure simulations of composites, the volume shrinkage can be expressed with user-defined swelling. In electronics, moisture analysis can model the hygroscopic strains via swelling.

User-defined swelling can be programmed via the usersw subroutine with material parameters input with the TB,SWELL command. The implementation of usersw is quite straightforward, although there are several points to keep in mind:

  • Available for legacy elements only
  • No temperature-dependent input (TBTEMP not supported)
  • Isotropic swelling strain assumed
  • Treated as nonlinear materials being active
  • Fluence body load (BF,,FLUE) must always increase and > 0
  • TB,SWELL material parameters C67, C68 need to be nonzero (e.g., C67=10, C72=10)
  • Temperature body load (BF,,TEMP) must be > 0 (if temperatures are not used, just define TOFFST > 0)

Usually, the equivalent “fluence loading” (e.g., degree of cure in or “wetness” in moisture analysis) either tends to increase or decrease. For the case of decreasing values, one must define the “fluence” loading with an inverse relationship since the fluence body load values must always increase.