Meshing
The four main types of elements are tetrahedral, bricks, shells and beams. As a general rule tetrahedral elements are used with automatic meshers for complex geometry such as bearing housings. Brick elements are also used for complex geometry but require more user input. Shells are generally used for constant thickness structures such as pressure vessels. Beams are widely used for steel structures. Other elements include springs, wedges etc
There are pro's and con's of each type of element and to demonstrate this the following example shows a simple cantilever beam which is fixed at one end with a load on the other. The analysis is solved with each type of element and the results compared.
Consider a flat bar fixed at one end with a load on the other and calculate the maximum stress.
There are pro's and con's of each type of element and to demonstrate this the following example shows a simple cantilever beam which is fixed at one end with a load on the other. The analysis is solved with each type of element and the results compared.
Consider a flat bar fixed at one end with a load on the other and calculate the maximum stress.
Our calculated stress is 1875 MPa. Now let’s compare this with FEA using different types of elements.
Tip
Section Modulus data tables are widely available in text books and suppliers catalogues for a variety of sections such as Angles, RHS, SHS and RSJ’s etc.
Compare the flat bar in FEA using different types of elements.
In summary, the beam meshed model FEA result was exactly the same as the hand calculation. The other types of elements were not. This is because we introduced a stress singularity due to the fixed constraint. Stress singularities are caused by fixed constraints and sharp corners. The stress will not converge on a singularity and will keeping rising to infinity as the mesh is refined.
There is no such thing as a fixed constraint or a sharp corner in the real world. Even components that are deliberately designed to have a sharp corner actually have a very small fillet radius. This is something to bear in mind when de-featuring geometry by removing a fillet radius. You may be introducing a stress singularity.
If you have a stress singularity such as the cantilever beam fixed at one end and a load on the other, the stress should be taken some distance away from the fixed constraint. The main thing is that you recognise it as a stress singularity and not real stress.
For meshing complex blocky geometry, it would be very time consuming to apply anything but tetrahedral elements unless solver time must be kept to an absolute minimum. Parts that are very thin are ideal for shell elements and parts that are long and slender are ideal for Beam elements.