Oasys LS-DYNA Environment 8.1 VOLUME 3 ... - Oasys Software

Oasys LS-DYNA Environment: User Guide (Version 8.1)
is still a wedge of no-man's land through which node B can lose contact with the surface and
escape.
This error occurs only where contact forces are very large or the stiffness of the contact surfaces
is very low.
The symptom is that nodes appear to escape through the contact surface after crossing from one
segment to another.
Escape can also occur during contact with very soft or very thin shell elements, or with very fast
impact speeds, even if the surfaces are not concave. In this case, the node simply penetrates too
far and is set free.
The cure is to increase the stiffness of the contact surface. One easy way to achieve this with
group C surface types is to set the SOFT flag to 1 in the *CONTACT cards. This flag causes the
“soft constraint” contact formulation to be used, which is a penalty method where the maximum
stable contact stiffness is chosen for each penetrating node.
For type 13 contacts the 2.5% margin can be increased (MAXPAR on *CONTACT).
Precision
Figure 9.4 Escaped Node
If the equilibrium penetration of one surface into another (i.e. the force divided by the stiffness)
is too small to be detected, difficulties can arise. A typical symptoms is very noisy contact force.
In this case the stiffness of the contact surface should be reduced.
Over-Fast Impact
In one timestep a node penetrates so far into the surface that the contact force is very large and
shoots the node out again. If this occurs, the stiffness of the contact surface should be reduced.
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