Incompatible Mode Elements 1

[mizzjoey]

Hello everyone.

Simple question: what are incompatible mode elements and how are they used? I'd like to get a general idea of the difference between this type of elements to the others (reduced, fully-integrated etc) and why are they so powerful for bending analyses - without going into too much maths.

Google search for 'incompatible mode elements' have produced unsatisfactory results. At least, none that are not riddled with too much technical/mathematical details.

Thanks y'all.
jo

 

[BlasMolero]

Dear Jo,
This is related to the use of low-order elements. In NX Nastran, when using CHEXA 8-nodes & CPENTA 6-nodes solid elements with no midside nodes, reduced shear integration with bubble functions is the default. This is recommended because it minimizes shear locking and Poisson’s ratio locking and does not cause modes of deformation that lead to no strain energy.

The effects of using non default values are as follows:
• "IN = 3" produces an overly stiff element.
• If "IN = 2" and the element has midside nodes, modes of deformation may occur that lead to no strain energy.
• Standard isoparametric integration (ISOP = “FULL” or 1 and IN = 2 or 3) produces an element overly stiff in shear. This type of integration is more suited to nonstructural problems.

For geometric nonlinear analysis, it is recommended not to use incompatible modes by setting IN=2 or 3. If bending behavior is significant, it is highly recommended to use elements with midside nodes.

Best regards,
Blas.

 

[EdR]

Incompatible mode elements are formulated in such a way that bending is better represented. Consider a 2-D linear plane element which in general has 2 dof/node (8 dof/element for a quad). If you formulate an incompatible mode element then 2 additional internal dof are included which have a parabolic displacement interpolation function. These internal dof are then condensed out of the final element formulation and the resulting element will provide a better representation of a constant bending situation than the original formulation.

Use of incompatible mode elements can provide significant reduction of run times and results improvement in both 2 and 3-D where bending is present..

Hope this gives you a bit more of what you were looking for....

Ed.R.