Stopped due to Modeling Data.

If an implicit simulation stops with the error:

The error is caused by having a negative slope in the material stress-strain curve. Unlike an explicit solution, the stiffness matrix needed to be inverted (positive and definite) during the implicit solution and this is not possible when there is a negative slope in the material stress-strain curve.

Tstop in boundary conditions.

The Tstop of imposed boundary conditions (like /IMPDISP) can be used in an implicit analysis. Since large time steps are used in implicit, the stop time might not occur at exactly Tstop. There are two ways to avoid this limitation.
  1. Define the simulation end time in /RUN to equal Tstop and continue the simulation using the restart files and another Engine file. This is recommended since the Tstop of /IMPDISP is similar to unloading and convergence could be difficult after the imposed boundary condition is removed.
  2. Use /IMPL/DT/FIXPOINT to force Radioss to use the Tstop time by defining one of the fixed time points to equal Tstop.

Mass added in an implicit analysis.

Figure 1.

If there is mass added in an implicit simulation, the mass change is caused by TYPE2 or RBE3 kinematic conditions in the model.

In this case, the mass change does not mean that supplementary mass has been added; but instead, there is a change in mass between secondary parts and main ones.