Hello,

for my roadster (1100 kg), which is frequently used on the racetrack, i want to replace the separating sheet metal between the boot and the convertible roof room.
Pictures: http://www.pic-upload.de/gal-703069/7v69gx/1.html

Beside the weight reduction, my target is to enhance the torsional stiffness of the hole car at the rear end.
My solution would be a laminate which combines the shear field theory (panel connected to frame structure) and a frame construction with diagonal braces to use the benefits of the unidirektional fiber placement at reduced material costs.
The following laminate setup is planned:
layer 1 - 200 g/m^2 Carbon Fiber Twill Weave - 0/90
layer 2 - 200 g/m^2 Carbon Fiber Twill Weave - +/- 45
layer 3 - 140 g/m^2 Carbon Fiber UD Tape width ~ 50 mm for every frame brace
layer 4 - 140 g/m^2 Carbon Fiber UD Tape ...
layer 5 - 140 g/m^2 Carbon Fiber UD Tape ...
layer 6 - 140 g/m^2 Carbon Fiber UD Tape ...
layer 7 - 200 g/m^2 Carbon Fiber Twill Weave - +/- 45
layer 8 - 200 g/m^2 Carbon Fiber Twill Weave - 0/90

Would the laminate setup be enough shear stiff and have you suggestions for:
- the number of layers,
- ply weights,
- the laminate setup an strength theory layout in general,
- and especially choosing the right frame structure for optimal force paths ?

The clearance holes will be stung in the dry fabric and the distance to edges will be increased.
The frame nodes will be crossing UD tapes, so the laminate would gain twice the thickness at the clearence holes for enhanced application of force.
Another benefit of the planned laminate setup could be that it has less denting danger.
Production via resin infusion on a mould with the original geometry.
With the use of expensive carbon fiber, i would like to achieve a stiffer sheet/panel right in the first try.

Greetings from Germany,
Alexander

It looks like you are still thinking in metal terms.

Use a core, preferably nomex.

Hi,

after discovering the shear field theory, i was a bit curious about trying it out.
But you are right, it's a metal layout concept.
So it's going to be the balanced laminate i planned before.
Consisting of 8 layers of UD carbon fiber with varying fiber angles in 45° (0,90,+/-45).
At the clearence holes/mounting points i would fed the same 8 layers as patches with a radius of about 70 mm in the middle of the laminate. 
Mmmh, maybe it's better to step the patch radius a bit...

Now to the core, why should i use one or what are the load cases (where it could be usefull)?
In the case of torsion of the car around the roll axis, shear of the laminate (transmitted via compressive and tensile stress in plane) and so bending at the side wings of the laminate occurs, which are connected to the wheel wells.
For the case of shearing the left side against the right side of the car in the same plain, it would apply maybe a little bit bending stress to the laminate, but the most part would be shearing in the direction of the laminate thickness.
If we torque the left side against the right side of the car around the pitch axis it would apply a out of plane torsion load on the laminate. (i think this was the case years ago before adding underfloor braces, when i accelerated heavily in lower gears)
(Against denting under compression, the original geometry of the panel with beadings would be sufficient)

Not sure if the core (Soric) should stop before the edge all around and mounting points to equalize the height difference with additional fiber material or if it should reach till the edge of the laminate, because with the 8 additional patches at the mounting points, the compressive stress on the core might be spreaded enough to take all loads from the mounting points ?

Greetings,
Alexander

By adding a core you increase the stiffness, bending and buckling capacity of the shape i.e. Ix, Zx and rx/ry with very little weight penalty so a lot of the time its a case of if you can then why not!