Which type of mesh is better for my case? C3D8, C3D8R, C3D8I, C3D20, or C3D20R?

[hangv]

I haven't used abaqus for a long time, so my case is relatively simple, I think. I try to state my problem as clearly as possible. Thank you guys.
Question Background:
I have simulated the behavior of a concrete plate placed on a base, with ABAQUS. The interaction and concrete and base is 'hard contact' and separation is allowed. A friction of 0.15 is set between plate and base. In step 'initial', the temperature is 0 at both top and bottom surfaces of plate. In step 'apply load', the temperature remains 0 at bottom surface while that drops to -20 at top. The plate also undergoes the gravity force. The base is fixed. No other boundaries or constraints for the plate. The result is shown in the following picture.

Question:
When I used different mesh types, I got different results for the displacement of the node at middle plate edge. C3D8R(2.60mm), C3D8(no converge), C3D8I(no converge), C3D20R(1.05mm), C3D20(1.05mm). It means C3D20 and C3D20R get less than half the result of C3D8R. But which one is the right result? Or how to judge which mesh type is better for my case? How to make it a converge for C3D8 and C3D8I?

 

[ShadowWarrior]

Generally speaking, accuracy wise -
C3D20R > C3D8R, both are reduced integrated
C3D20 > C3D8, both are fully integrated
C3D8I > C3D8, both are fully integrated, C3D8I have additional DoF's to capture bending better.

Fully integrated elements will always give a better result than reduced integrated, but at a higher computational cost.

If you are using ABAQUS/Standard, then use fully integrated element. Personally, I'd accept the result given by C3D20 and call it a day.

 

[Dave442]

Each element type has advantages/disadvantages. For a more thorough discussion check the documentation:

Abaqus Analysis User's Guide -> Elements -> Continuum Elements -> Sold (continuum) elements