- Guidance notes for the FEA part of the assignment and the Discussion.

1. Introduction
Although these components are superficially simple, there are a number of aspects to the CAE which means this part of the assignment is not as simple as it seems. The areas that are likely to cause difficulties are:

2. Approach

2.1 Developing the model:
Using the 'Sketch' tool in Solid Works sketch the cross section, exit the sketch tool, click on the Extrude Boss / Base and put in the required section length. Probably a few mm less than the actual length to allow for the actual end constraints. To avoid errors of excessive deflection and high stress on internal corners, it is important that appropriate fillets are put onto internal corners.
Because a torque load requires 'Reference Geometry' to define the axis of rotation and the most convenient way of providing this is do sketch of a circle (in the centre of the restrained end of the section) and extrude a small cylinder.

2.2 Application of the loads: The point to bear in mind here is that the load is a torque about the axis of the section. This can be applied as tangential loads at the corners of one end of the section or as a moment or torque on the surface at one end of the section. If this latter approach is being used then it must be remembered that torques need a reference to indicate their direction.

Remember that open sections have very much lower torsional stiffnesses than closed sections - so use suitably smaller moments for open sections.

2.3 Choosing Constraints: This is by far the most difficult aspect of this task. It is very easy to over constrain the model and make it far too stiff.
The section can then be meshed.
You will almost certainly have to run the model a few times with different constraints to check which gives the best representation of the experiments. You should carefully observe the experiments and see what happens to the sections under torsion. Also see what some textbooks say. Only when you have a clear idea of what is happening will you be able to set up reasonable constraints.
Even at this stage it will not be easy.
After each 'Run', check the deformed shape to see that it seems reasonable, before making any use of stresses and strains.

Note the differences in behaviour between the open and closed sections.

3 Discussion of Results

For the discussion of results you should obtain computed stresses for the locations where the strain gauges are fixed. For easiest comparison the use of the stresses is recommended (this will have to be calculated from the strain gauge readings). Maximum principal and minimal principal stresses will be needed.

To find values on the model surfaces with shaded displays, under the 'Tools Results' menu, use the 'Probe' to indicate values where the cursor is placed and the LH mouse button pressed.

Another parameter that should be compared is the torsional stiffness of the sections, expressed as Nm per radian per metre of length, obtained experimentally and theoretically.

When discussing the results a clear presentation of a summary in 1 table is vital. Many students lose marks because their discussion is confusing and the results are not clear. You must make it clear which results are computed, which are calculated from theoretical formulae and which are experimental.

It is suggested that as a part of the conclusions the results be summarised in one table containing the key stress values obtained from the three approaches used.

The warping of the sections should also be commented upon.

Return to module introduction.

David J Grieve, updated 4th January 2008, previously: 13th December 2006, 14th September 2006, 17th June 2003).