4. Integrating the element into Code_Aster

Element QUAS4 has two families of Gauss points. The first family consists of a point located in the center and whose weight is equal to four. The elementary calculations are done naturally with the first family. The options calculated are the same as those calculated by QUAD4. Therefore, the internal constraints and variables, as well as the tangent behavior, are calculated at the center of the element. In the context of post-processing, each element has constant fields.

The second family is identical to that of the classic QUAD4 and includes 4 Gauss points. It is used to calculate the stabilization matrix. To store the stabilization forces, 6 components are added to the stress field at the Gauss point.

This element is activated by choosing the “C_ PLAN_SI “or “D_ PLAN_SI “models in AFFE_MODELE for QUAD4 meshes. Only calculations in small deformations are possible. There is still an important limitation: in version 7.2, two types of stabilization are programmed: ASBQI and ASOI (1/2), but are not accessible by a keyword in the command file. To activate them, you need to change the PROJ parameter in the NMAS2D routine.