Abstract

Some carbon fibre flat pieces were made and tested to see how far they bend for a given load. A load cell was used to measure the force.

Introduction

So, before I go mad and put several hundred pounds of material into my CF tub, I thought it best that I do some clever investigation into just how does a composite structure bend, yield and break under various loads. So, I made the pieces, tested the pieces, and did some graphs, and came to some conclusions.

Method

8 trial pieces were made of various thicknesses – different cores and layups. Each piece had a few things in common though:

• single layer of 300 gsm aramid for intrusion protection (if it were on the car)
• this was balanced with a 300 gsm piece on carbon on the opposite layer to the core
• one layer of 600 gsm either side of the core, and one layer of 200 gsm e-glass either side of the core for a little tolerance and flexibility.

I could then vary the thicknesses of the cores and extra layers of 600 gsm carbon I wanted to use. I opted for the following core thicknesses:

• 10mm closed cell foam (CCF)
• 3mm CCF
• 6mm (2x 3 layers of 3mm CCF)
• none

In order to test the bending load, I went to see my good friend Simon at Cornering Force. There are only so many people who like it when you call them at 8pm and say “Hey – do you want to stress test and break some carbon fibre?” After marking up each sample, we then put each on a support, resting on the bed of a mill, and pressed down on it with a load-cell. The load-cell had a 25x50mm contact face. The mill allows us to make vertical adjustments in 0.1mm increments easily, and the load-cell is accurate to the gramme.

Manufacture

The pieces were laid out on one sheet, and infused as one piece, and then cut to size afterwards. The vacuum feed was left on post infusion and for the curing cycle. The resin was degassed. This was to ensure the maximum amount of spare resin was removed until the gel stage. After gelling and the initial cure, they were post-cured at 60C for 12 hours, as per the manufactures instructions. This gave me the maximum strength possible for the sample for the least weight. I decided not to vary curing methods (e.g. length of cure) because a 12 hour, 60C cure is what I can achieve for a part as large as this.

The plan was to work out how much a piece would deflect, how it would yield and fail, and just what were the most significant factors in this – is it core thickness, is it amount of cloth? Did I turn around three times widdershins?

This was also a good opportunity to try a couple of techniques given to me by Vic at Scorpion CDL. I am always astounded at the wealth of technique he has and the quality of work done there.

Lessons Learned

I also had a slight infusion issue, which was the temperature of the resin I infused with. I had it at about 10C which was great for it not going off, but made it too viscous, and meant de-gassing it was a pain. It also meant the infusion was quite slow for such a small part. Further checking says 25C to 30C is a better temperature for this. I should have also wrapped more mesh around the input spiral which would have also alleviated this problem, but that was fixing the wrong problem – get the temperature right – let the resin flow.

Analysis and Conclusions

Coming in the next post.