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

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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. Page 9.14
Oasys LS-DYNA Environment: User Guide (Version 8.1) It can be difficult to detect very rapid penetrations of this type since they may only occur over a few time-steps. It is for this reason that the summary contact forces reported to .XTF file are averaged over the time between data points: spikes at inter-data points are detected, albeit with attenuated force. Contact forces reported to the .CTF file are instantaneous and will almost always miss rapid spikes of force. Shell Edge Contacts In complex contact situations (e.g. crash analysis) it is possible for the edges of shell elements to come into contact with other Shell elements. This often leads to energy jumps and unrealistic local deformation. Where the likelihood of edge-on contact is great, it is better to use a nodeson-surface contact type. *CONTACT AUTOMATIC-GENERAL is better able to deal with these situations. Page 9.15
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Oasys LS-DYNA Environment 8.1 VOLUME 3 ... - Oasys Software

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