NEVER NEVER
NEVER use a mesh significantly comprised of 1st order tetrahedral elements.
The reasons are as follows
- All of the warnings about triangular shell elements hold, plus many more.
- They are grossly over-stiff, and will result in shockingly bad answers.
Exception
The only
situation in which I would ever use them is in a 1st order
hexahedral/pentahedral mesh that needs one in a transition zone (but then, if
it's a complex mesh I'll use 2nd order tets).
