Mixed finite elements

[dbeurle]

In order to avoid locking problems for incompressible models (such as the rubber models implemented), the weak form needs to be adjusted. This can be done through the Q1P0 family of elements, where the additional pressure variable is condensed out on the element level. However this is only valid for the discontinuous pressure field and higher order shape functions need to include the entire (u, p) function set and solve a partitioned system.

This is a tracking issue of the progress.

[dbeurle]

This needs to happen before Issue #33 for large deformations.

[dbeurle]

Tasks:

• Extend mesh to perform under integration of volumetric tangent (H1 and T2 elements only)
• Partition constitutive models into incompressible and compressible models