Friday, October 31, 2008

Choosing Elements

Jason asks..
Would you please instruct us how to choose the type of solid elements in large deformation analysis? For example,linear or quadratic, reduced integration or not ?

Answer :
The choice of the elements, depend on the type of problem and the material. I will try to give brief guidelines.

1. In General, the Quadratic meshes give more accurate results than the Linear meshes. But the quadratic elements should NOT be used for the faces coming in contact. This will produce unrealistic jump in Contact Pressure values. In that case, better to refine the mesh and use linear elements

2. Many non-linear problems use nearly incompressible materials. Usage of Fully Integrated Elements in this case will lead to volumetric locking and therefore excessive stiffness. Usage of Reduced Integration elements ( e.g. C3D8R) will relax it and provide more realistic results. But, one should be careful with the effect of Hour glassing when using Linear Reduced Integration Elements

3. Hour glassing is the phenomena of elements distorting in such a way that there is no change in Strain Energy. It behaves like a rigid body mode. One should be concerned with hour glassing effect only in Linear Reduced Integration elements. In Quadratic Reduced Integration Elements, Hour glassing doesn't propagate and therefore has no big effect. Most of the times, hour glassing can be controlled by using enhanced hour glass stiffness option. When using hour glass stiffness one must keep an eye on the artificial energy created in the system and make sure it is low.


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