SES Monocoque Side Impact Structure

[Lux5018]

Hi,
It is my first post here therefore I will first introduce myself.
My name is Luka Jerman and I am a member of University of Maribor Formula Student team (University of Maribor Grand Prix Engineering) from Slovenia.
This year I am working on the suspension of our car. However due to my experience with composite calculation (FEM with conventional shells in Abaqus, material properties obtained with tests that formed a part of my bachelor thesis) I am helping a little bit with the monocoque (it is our first one).

During our design of the monocoque we have defined the proper layup of it based on the torisonal stiffness that was calculated with finite elements. Because we cannot calculate the shear strengths we started with the plunger tests. To obtain the right shear strength we had increase the plies in our layup to 5 plies (200g fabric) on each side with 20mm of rohacell in the middle.

Based on what other teams are doing we thought we wouldn't have a problem with the laminate bending stiffness and strength. We tested the laminate test sample with the same layup as the monocoque (5 plies on each side with 20 mm rohacell core) the width of the panel was 200 mm as stated in the rules, the distance between the outer cylinders was 505 mm. The test results showed us that we aren't even a bit close to the required stiffness and strength that would be equivalent to the steel tube/tubes. If we wanted to reach the appropriate stiffness the core should be at least 40 mm thick which is a bit too much and we haven't seen anyone having a core that thick.

We can reach teh apropriate EI with the I from the monocoque side impact structure (chamfer at the bottom increases the I) with the same layup.
Is it the case that we have to manufacture a test sample that has the same second moment of inertia as the side impact structure of the monocoque?

Please help.

[mech5496]

Luka,

The rules state that the test should be done with a FLAT panel, so I really doubt you can include the chamfer; at least we never had. It would be useful to ask the rules commitee though.

[Lux5018]

Thanks for the reply. Maybe I stated it in a weird way :D

I know that we need a flat panel for the test therefore I want to know if we can increase just the core so that the flat panel has the same moment of inertia as the monocoque structure.
The monocoque structure still has thinner core but is a little bit larger and has the chamfer on the bottom that increases the I.

[Lux5018]

Ok we have measured the cross-section of the monocoque side impact and it's I is equal to 1,865E-7 m^4.
Therefore we could make the representation of the side impact in the form of a flat panel that has a core thickness of 42,5 mm with approx. 1 mm thick skins (5 plies of 200g cfrp).
From our calculations 40 mm core would be just enough for the flat panel with the width of 200 mm to produce the required stiffness.

I am a bit confused with everything:
-if we make a plate with 42,5 mm thick core can we use a core with the thickness of 20 mm because our monocoque side impact has a geometry that is different to the plate (a chamfer on the bottom) that provides larger second moment of inertia?

Rule T3.30 states:
Monocoque Buckling Modulus – Equivalent Flat Panel Calculation
When specified in the rules, the EI of the monocoque must be calculated as the EI of a flat panel with the same composition as the monocoque about the neutral axis of the laminate. The curvature of the panel and geometric cross section of the monocoque must be ignored for these calculations.
Note: Calculations of EI that do not reference T3.30 may take into account the actual geometry of the monocoque.

From it I understand that my method woudn't be appropriate and that we need to have a sandwich panel with a core thickness of at least 40 mm (i have tested more plies, increasing the core works better).
I wolud really like to know how others do it and what is right. The 40 mm core seems a lot based on my observation of other teams side impact structures.