Personal tools

Implicit: Checklist

  1. Activate implicit by setting IMFLAG=1 on *CONTROL_IMPLICIT_GENERAL
    • Set DT0 to a reasonable initial time step
  2. Check possible singularities in an eigenvalue analysis by requesting NEIG eigenvalues on *CONTROL_IMPLICIT_EIGENVALUE
    • Find a way to constrain spurious modes
  3. Initially use dynamic analysis if rigid body modes are present in a static problem by setting IMASS=1 on *CONTROL_IMPLICIT_DYNAMICS
    • Use TDYBIR, TDYDTH and TDYBUR
    • Use numerical damping by putting GAMMA=0.6 and BETA=0.38
  4. Only activate geometric stiffness contribution as a last resort, except for arc length methods
  5. Use nonlinear solver type 12 by putting NSOLVR=12 on *CONTROL_IMPLICIT_SOLUTION
  6. Use default BFGS parameters ILIMIT and MAXREF on *CONTROL_IMPLICIT_SOLUTION
    • Increase or decrease ILIMIT based on convergence characteristics
    • Use full Newton (ILIMIT=1) for hard problems
    • Keep MAXREF to a reasonably low number
  7. Use default convergence tolerances DCTOL, ECTOL and RCTOL on *CONTROL_IMPLICIT_SOLUTION
    • ABSTOL may be set to 1.e-20 to prevent premature convergence
    • Relaxing DCTOL may not necessarily result in better convergence
    • Future strategy may be to focus on residual
  8. Activate automatic time stepping on *CONTROL_IMPLICIT_AUTO
    • Set DTMAX to a number that resolves features of interest
      • Contacts
      • Material nonlinearities
      • Frequencies
  9. Use default line search method on *CONTROL_IMPLICIT_SOLUTION
    • Don’t use LSMTD=1 for nonlinear solver 12
    • Switch to LSMTD=5 if negative volumes is an issue (typically rubbers)
    • Don’t change LSTOL
  10. Use at least NLPRINT=2 on *CONTROL_IMPLICIT_SOLUTION to get convergence diagnostics into log files
    • Use NLPRINT=3 to thoroughly track the residual norm and monitor line search behavior if debugging model is necessary
  11. Don’t use *CONTROL_IMPLICIT_LINEAR
  12. Set D3ITCTL=1 on *CONTROL_IMPLICIT_SOLUTION to get a database with Newton iterates when debugging a model
    • Complement this with RESPLOT=1 on *DATABASE_EXTENT_BINARY to get the possibility to fringe the residual force vector
  13. For forming Mortar contact, make sure
    • Tools are oriented towards the blank
    • Contact on top and bottom of blank are separarated among contact interfaces
  14. Use part (set) definitions of slave and master in Mortar contact
  15. Always use weak part as slave in a Mortar contact definition to get best possible convergence
  16. Set SST to a reasonable characteristic length for slave side consisting of solid elements
    • With this option, separate solids and shells into different contact interfaces in order to not manipulate the contact thickness for shells
  17. If penetrations are large, activate penetration diagnostics on *CONTROL_CONTACT
  18. To avoid release of contact pairs, either
    • Increase stiffness scaling factor SFS
    • Increase IGAP for progressive stiffness increase
    • Increase SST for solids while at the same time increasing SFS
  19. Use IGNORE appropriately to deal with initial penetrations
    • Check initial penetrations in message file
  20. Don’t use contact damping

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TB 2015