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Subject: TET and HEX elements / Some Comments
Author: Jan Christian Anker
Date: 1998-12-15 15:47:00Additional comments on TETs vs HEXs:
1) In (higher order) HEX elements, you may see that
the implementation involves 2 X 2 X 2 integration
for some stresses, 3 X 3 X 3 integration for others,
and not all integration points used. In ANSYS,
14 integration points are utilized (SOLID95) as well
as 2 X 2 X 2 (e.g. SOLID45).
2) If you want to be sure that you use the right element
with the right options, run a little test model and check
the results against known solutions.
3) In some respects, you may find lower order elements
to be more "intuitive". Example to demonstrate what
I have in mind: Uniformly distributed pressure on a
face of a higher order solid element represented as
concentrated nodal forces ( 4 * f/3 - (4*f/12) = f,
f = pres * area) is not intuitive to me. (This might be
my personal problem, sorry for that!)
4) What elements to chose for a given job is not easy
in all cases (nor is correct modelling!).
5) Personnally, I prefer lower order solid elements if
the model can be easily meshed. If the geometry
is complex, I use 10 noded TETs (if they can do the
job).
Jan Christian
ANKER - ZEMER Engineering A/S
e-mail: jc.anker@a...
Phone: +47 22 13 95 80 FAX : +47 22 13 95 95
Address: Grindbakken 1, N-0764 OSLO, Norway
> -----Original Message-----
> From: W. S. Zemitis [SMTP:scott@c...]
> Sent: Monday, December 14, 1998 5:39 PM
> To: xansys@o...
> Subject: [xansys] Re: FW: Hex & Tet elements
> From: "W. S. Zemitis"
> Aside from the other factors mentioned, another consideration which
> confuses
> the issue of tet vs. brick superiority is the completeness of the
> polynomial
> describing the displacement functions of each type of element.
> For example, the 10 terms in the displacement function for the 10-noded
> tet
> element represents a complete 2nd order polynomial. The 8-noded brick
> displacement function, while lacking several of the second order terms of
> the 10-noded tet, has a third order term (xyz) absent in the tet.
> Likewise,
> though the 20-noded brick is a "quadratic" element like the 10-noded tet,
> it
> has a more complex 20 term displacement function that includes all 10
> second
> order terms, 7 third order terms, and 3 fourth order terms.
> When this information is considered together with the other factors
> influencing accuracy and efficiency, the "best" element choice becomes
> highly problem dependent.
> WSZ
> CAEA
> -----Original Message-----
> From: John Swanson
> To: 'xansys@o...'
> Date: Monday, December 14, 1998 11:58 AM
> Subject: [xansys] Re: FW: Hex & Tet elements
> >From: John Swanson
> > With mid-side node elements (10 node tet, 20 node bricks), the
> >results should be the same (very close) for the same nodal density.
> >The tet mesh will have more nodes because of the nodes in the
> >center of the faces and in the center of the brick, and therefore will
> >be somewhat more accurate.
> > The advantage of fully automatic mesh generation offered by the
> >tet elements normally outweighs the additional time the solution
> >might take (computers are cheaper than people, and time is always
> >critical).
> >John.Swanson