Linear vs Constant Pressure in Element Type

[Techsan123]

I am modeling a Fully incompressible 3D cylinder in Abaqus 6.8.3. I have chosen to use a tet element type. Once I am in the element type I then choose my geometric order as quadratic. I realize the hybrid box needs to be checked and it states "A 10-node quadratic tetrahedron, hybrid, constant pressure" when only this box is checked. When I also check the box named Modified Formulation as well (so they are both checked) the description reads "A 10-node modified quadratic tetrahedron, hybrid with linear pressure". I don't understand the difference between the constant and linear pressure. Can someone please help!

 

[mrgoldthorpe]

It's the way that the mean stress (minus the pressure stress) is represented in the element in order to avoid 'mesh-locking' when solving with nearly incompressible materials. For constant pressure the pressure stress is simply modelled as a constant throughout the element. With linear pressure the pressure or mean stress is represented as a (tri-linear) variation throughout the element. The latter element is of course more accurate; at the expense of more computational time.

 

[Techsan123]

ok thanks a bunch for the info!