Implicit: Checklist

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  1. Activate implicit by setting IMFLAG=1 on *CONTROL_IMPLICIT_GENERAL
  • Set DT0 to a reasonable initial time step
  • Check possible singularities in an eigenvalue analysis by requesting NEIG eigenvalues on *CONTROL_IMPLICIT_EIGENVALUE
    • Find a way to constrain spurious modes
  • 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
  • Only activate geometric stiffness contribution as a last resort, except for arc length methods
  • Use nonlinear solver type 12 by putting NSOLVR=12 on *CONTROL_IMPLICIT_SOLUTION
  • 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
  • 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
  • Activate automatic time stepping on *CONTROL_IMPLICIT_AUTO
    • Set DTMAX to a number that resolves features of interest
    • Contacts
    • Material nonlinearities
    • Frequencies
  • 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
  • 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
  • Don’t use *CONTROL_IMPLICIT_LINEAR
  • 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
  • For forming Mortar contact, make sure
    • Tools are oriented towards the blank
    • Contact on top and bottom of blank are separarated among contact interfaces
  • Use part (set) definitions of slave and master in Mortar contact
  • Always use weak part as slave in a Mortar contact definition to get best possible convergence
  • 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
  • If penetrations are large, activate penetration diagnostics on *CONTROL_CONTACT
  • 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
  • Use IGNORE appropriately to deal with initial penetrations
    • Check initial penetrations in message file
  • Don’t use contact damping

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    TB 2015

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