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What impact does different values of FSTRESS in NLPARM card influence the results?

Viplava
It is a SOL106 problem and I tried NLPARM card with FSTRESS default value 0.2 and the convergence stopped at 70%. I tried ​FSTRESS with value 0.99 and it converged to 90%. Actually I wanted to use RIKS method( Arc length method), but since there were Enforced Displacements and it is not compatible with RIKS(NLPCI). I tried to apply equivalent forces instead of Enforced displacements with RIKS method which didn't work either.
So to get better deformation I just used NLPARM and tried different parameters within the NLPARM card. What impact does FSTRESS make in the solution process? And is there a way to get 100% convergence?
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    I assume you are doing a problem with plasticity, as that is where FSTRESS becomes more impacting.  In general, you wouldn't want FSTRESS to be 'large' as it is a measure of how much the stress state is changing within a single increment, and this impacts the material routines.  From the QRG:  "The number of subincrements in the material routines (elastoplastic and creep) is determined so that the subincrement size is approximately FSTRESS*equivalent stress. FSTRESS is also used to establish a tolerance for error correction in the elastoplastic material; i.e., error in yield function < FSTRESS*equivalent stress. If the limit is exceeded at the converging state, the program will exit with a fatal message. Otherwise, the stress state is adjusted to the current yield surface."
     
    In general, if you are having stability issues, the first thing to do is reduce the increment size so you are taking small steps of applied loading.  Also, it is common to use multiple subcases to work your way up to the full loading, using smaller incremental loads as you get closer to the instability.
     
    This is really an issue you should contact technical support for additional assistance, as there might be various approaches to get past the issue, and a more detailed review of the results is necessary.
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    I assume you are doing a problem with plasticity, as that is where FSTRESS becomes more impacting.  In general, you wouldn't want FSTRESS to be 'large' as it is a measure of how much the stress state is changing within a single increment, and this impacts the material routines.  From the QRG:  "The number of subincrements in the material routines (elastoplastic and creep) is determined so that the subincrement size is approximately FSTRESS*equivalent stress. FSTRESS is also used to establish a tolerance for error correction in the elastoplastic material; i.e., error in yield function < FSTRESS*equivalent stress. If the limit is exceeded at the converging state, the program will exit with a fatal message. Otherwise, the stress state is adjusted to the current yield surface."
     
    In general, if you are having stability issues, the first thing to do is reduce the increment size so you are taking small steps of applied loading.  Also, it is common to use multiple subcases to work your way up to the full loading, using smaller incremental loads as you get closer to the instability.
     
    This is really an issue you should contact technical support for additional assistance, as there might be various approaches to get past the issue, and a more detailed review of the results is necessary.
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