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Implicit

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hmtoggle_plus1Is it possible to use a negative slope in material curve (like in LAW36) for RADIOSS implicit analysis?

“ ** ERROR : RADIOSS IMPLICIT STOPPED DUE TO MODELLING DATA ** “

No. Not like explicit solution, stiffness matrix needed to be inversed during the implicit solution. Generally the stiffness matrix should be positive and definite (should be inversed), that’s why negative slope is forbidden (which leads to non-positive of stiffness matrix) with implicit solution.
hmtoggle_plus1Is Tstop of imposed boundary condition (like /IMPDISP) available in implicit analysis?

Yes. Tstop should work with implicit solution, just watch that large time steps are used in implicit, stop time might not be taken exactly like Tstop the input, excepting that one of the following workaround are used:

1.Using RADIOSS restart with the second run begins at Tstop. This is recommended, since the Tstop of /IMPDISP is similar to the unloading but too brutal to get convergence difficult.
2.Using /IMPL/DT/FIXPOINT to forcing RADIOSS to do Tstop (by defining the one of the fixing point with the Tstop).
hmtoggle_plus1Why does it shows mass added in implicit analysis?

faq_implicit

In computation mass, change is due to kinematic conditions in the model (constraints like interface TYPE2 or RBE3) which are independent to explicit or implicit solutions.

In these cases, the mass change does not mean that supplementary mass has been added, just the change between slave parts to master ones.