degrees of freedom in solid 3

[enthu1980]

Hi friends,

I have just started learning fea.
I have a basic fea question.
My question is why there are 3 degrees of freedom in solid. Why there are no rotational degrees of freedom in 3D elements unlike in 2D elements.

 

[corus]

There are three translational degrees of freedom in a 3D universe. For two dimensional bodies there are two degrees of freedom with some assumption made about the third, ie. plane strain or axisymmetric bodies. For shells there are 6 degrees of freedom - 3 translational, 3 rotational. The rotational degrees of freedom define the displacement in the thickness direction. 3D solids don't need this as the behaviour in that direction is defined by the translational displacement.

corus

 

[Denial]

Taking Corus's answer a bit further, it is by introducing the concept of rotation that we are able to model (some) three dimensional objects as two or one dimensional objects.

 

[Cmfg]

Solid Elements with 6 dof per node hasn't been very succesful numerically. Although they are very useful in some applications where you require to combine with shells (delamination problem), or when are combined and beam elements (modeling of bridges).
Formulation of elements with rotational dofs are more complicated than traditional elements, because you have to maintain continuity of the displacements and their derivatives. Ansys had a solid element with 6 dofs some time ago, the formulation was based on a hybrid of stresses.

 

[prost]

kudos to Denial for his/her succinct and accurate description of the reason why rotational degrees of freedom are sometimes used in the basis or parent elements

 

[xerf]

In general one can distinguish between solid elements which typically have only translational DOFs and structural elements which may include also rotational DOFs (e.g.,beams, structural shells).

The type of the DOFs (traslational, rotational , temperature etc.) contained in element formulation has little to do with the element dimensionality (2D, 3D) but rather depends on the underlying theory the element formulation is derived from.

I suspect you used a structural element type for your 2D model and a solid continuum element type for the 3D model.