|
Negative Volumes in Foams (or other soft materials)
In materials that undergo extremely large
deformations, such as soft foams, an element may become so distorted
that the volume of the element is calculated as negative. This may
occur without the material reaching a failure criterion. There is
an inherent limit to how much deformation a Lagrangian mesh can
accommodate without some sort of mesh smoothing or remeshing taking
place. A negative volume calculation in LS-DYNA will cause the
calculation to terminate unless ERODE in *control_timestep is set to 1
in which case the offending element is deleted and the calculation
continues.
Some approaches that can help to overcome negative volumes include the following.
- Simply stiffen up the material stress-strain curve at large strains. This
approach can be quite effective.
- Sometimes tailoring the initial mesh to accomodate
a particular deformation field will prevent formation of negative
volumes. Again, negative volumes are generally only an issue for very
severe deformation problems and typically occur only in soft materials
like foam.
- Reduce the timestep scale factor. The default of 0.9 may not be sufficient to
prevent numerical instabilities.
- Avoid fully-integrated solids (formulations 2 and 3) which tend to be less stable
in situations involving large deformation or distortion.
Use the default element formulation (1 point solid) with type 5 or 6 hourglass
control. Perhaps as an even better alternatative, model the foam with tetrahedral
elements using solid element formulation 10.
- Increase the DAMP parameter (foam model 57) to the maximum recommended value of 0.5.
- Use optional card B of *contact to turn shooting node logic off for contacts involving foam.
- Use *contact_interior. This approach is generally not very effective.
*********************************************************** `
Version 970 will have improved contact_interior as
implemented by Dave Benson
*********************************************************** |
|