Implicit: Checklist

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