3 General questions on FEA:
3 General questions on FEA:
(OP)
I've finished reading a few texts I had on FEA theory and I had a couple of general questions for those more knowledgeable than me:
1. For most of the solution techniques presented (i.e. Galerkin's, potential energy, etc.) the expression for the stiffness matrix tends to wind up being the same (i.e. Integral [B]T[D][B] dv). Are there any applications [i.e. the solution techniques used] where this expression would be affected? Or does this pretty much stay the same.
2. As far as solid elements go(like the 8-noded brick element) they tend to have only three degrees of freedom at each node (i.e. translational). Are there any higher order elements/shape functions that allow rotational degrees of freedom for solids?
3. The texts I have all form the stiffness matrix for the 8-noded brick element by using the isoparametric system (and they indicate this is for convenience of computations). Is it possible to formulate an 8-noded brick element using the same approach as (say) a [non-isoparametric] tetrahedral element? Would doing so produce significantly different results (in comparison to the isoparametric formulation; assuming we are talking about straight sides in both cases (i.e. identical elements))?
Thanks in advance.
1. For most of the solution techniques presented (i.e. Galerkin's, potential energy, etc.) the expression for the stiffness matrix tends to wind up being the same (i.e. Integral [B]T[D][B] dv). Are there any applications [i.e. the solution techniques used] where this expression would be affected? Or does this pretty much stay the same.
2. As far as solid elements go(like the 8-noded brick element) they tend to have only three degrees of freedom at each node (i.e. translational). Are there any higher order elements/shape functions that allow rotational degrees of freedom for solids?
3. The texts I have all form the stiffness matrix for the 8-noded brick element by using the isoparametric system (and they indicate this is for convenience of computations). Is it possible to formulate an 8-noded brick element using the same approach as (say) a [non-isoparametric] tetrahedral element? Would doing so produce significantly different results (in comparison to the isoparametric formulation; assuming we are talking about straight sides in both cases (i.e. identical elements))?
Thanks in advance.





RE: 3 General questions on FEA:
The answer to 2 is: What would rotation at the node of a solid element mean? Since the shape function is based on displacements of the nodes how would you set this up for nodal rotations? The reason you have rotational dof on shell and beam elements is that you are assuming some kind of strain variation through the thickness.
The answer to 3 is: You better not get different results (within some small percentage)for different formulations. If so there is something wrong with the element formulation.
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RE: 3 General questions on FEA:
Thanks kellnerp.
RE: 3 General questions on FEA:
htt
I don't think any other solver has attempted to do this, so this is very much an oddity and generally solid elements only have translational degrees of freedom at their nodes.
RE: 3 General questions on FEA:
corus
RE: 3 General questions on FEA:
Ansys has this element [link]htt
TOP
CSWP
BSSE
www.engtran.com
www.niswug.org
"Node news is good news."
RE: 3 General questions on FEA:
RE: 3 General questions on FEA:
RE: 3 General questions on FEA:
These elements don't like distortion. So the aspect ratio needs to be watched. Again this can drive up the number of dof.
Sorry about the tgml. Should have been: SOLID95
ANSYS also has a reinforced concrete solid, SOLID65.
TOP
CSWP
BSSE
www.engtran.com
www.niswug.org
"Node news is good news."
RE: 3 General questions on FEA:
I've been meaning to try a 20-noded brick element for reinforced concrete. I've heard that it will converge faster than the [alternative] 8-noded brick element.
For cracked reinforced concrete action I've always just adjusted the modulus of elasticity (for plate elements) to simulate the cracked behavior. never been sure how to do it for solids.
I use to work somewhere where there was a big disagreement about when to switch over to solid elements for reinforced [around the perimeter] concrete pedestals. [I worked with this one guy who modeled a 12' x 12' pedestal with plate elements. He had about 4 elements per direction (and he made one set of them massless so the total weight would be right for the dynamic analysis he was doing). he didn't seemed concerned that the out of plane stiffness for the elements in the other direction would cause the model to be overly stiff (he didn't designate the elements as plane stress only).]