SESs – making sure the basics are covered
Alternative tubing
When using alternative tubing, remember that for the purposes of the SEF all steel is
treated equally regardless of what grade of steel is used. This means that not only
must EI be higher whilst meeting the minimum wall thickness, but the cross sectional
area must also be equivalent.
Also remember if you replace the lower side impact tubing and you wish to attach the
lap belt to this then the tubing must still have a wall thickness of at least 1.6mm.
Attaching the main hoop to the monocoque
The attachment points would normally replace the welds to the side impact tubes. To
do this, the strength of the attachment point must therefore be equivalent to the tensile
strength of a side impact tube. In your analysis you need to show the following as a
minimum:
- The bolts are equivalent to the tensile strength of a side impact tube
- The 2mm plates must have a similar weld length to a side impact tube weld
- The shear strength of the monocoque must be equivalent to the tensile strength of
a side impact tube, this can be established by calculating the perimeter of the
backing plate, and multiplying this by the thickness of the composite that must be
sheared and then by the shear strength of the composite.
Also include a picture which shows a cross section through the mounting point.
Side impact structure - monocoques
To prove equivalency for a side impact structure, first calculate the EI and tensile
strength of a baseline side impact tube. Then for a fixed length tube find out how
much load it can take before any of the tube experiences yield. Simple beam theory
calculations can be used for this. Then work out the total for the 3 tubes.
With a composite or metal monocoque structure, approximate the side impact
structure to a simple vertical planar panel. This will normally have two faces and a
core. Again use simple beam theory to establish EI and the stress in the faces of the
panel, average material properties can be used. If there are more panels then repeat
the same process with these. The properties of the individual panels can be summed
together.
Although the EI and strength determined by this method should be equivalent, it can
be lower for the composite structure than the 3 side impact tubes, but it must be of a
similar order of magnitude. If it isn’t exactly equivalent, then the shape of the panel
or the fact that a box like structure has been used, can be taken account of in the final
judgement and can be confirmed by using FEA, other more detailed analysis, or real
test results.
Front Bulkhead support – monocoques
Follow the same approach as for the side impact structure. Approximate only the side
of the front bulkhead support to the tubes that are replaced. Prove that the properties
of the side are a similar order of magnitude to the three tubes that are replaced. If the
performance of the side is slightly lower than the tubes, then the performance of the
top and bottom can be factored in.
Attaching the harness to the monocoque
Although the front impact attenuator is specified to decelerate the car at 20g, it is
possible that the deceleration for higher speed crashes will exceed this. A driver
should survive a 60g deceleration if the car is designed well. Spreading this load for a
100kg driver through 6 mounting points means that the load that a harness mount
must take has to be at least 10kN as an absolute minimum (preferably 15kN).
The shear strength of the monocoque must therefore be at least 10kN, this can be
established by calculating the perimeter of the backing plate, and multiplying this by
the thickness of the composite that must be sheared and then by the shear strength of
the composite.
Pull out tests on real components can also be used for proof.
Andrew Deakin – 13/4/08
Formula Student Technical Rules