Talk Composites - The Forum for Advanced Composites

Seemingly impossible bend in CF sheet during bonding

http://www.talkcomposites.com/Topic32890.aspx

By dazedproductions - 5/20/2019 12:23:31 PM

So, I have a 95 winnebago T4 camper.  They have a notorious and well documented issue with cracking on the pop top ASA plastic roof.  I was prepping for a trip away and realised the crack mine has on one side opened up again

I took a 2 pronged attack.  I resealed the crack with ASA plastic rod (soldering iron to weld).
Secondly I had some 4-layer carbon fibre sheet left over that I decided to attach to give some support (mix of the profinish 210g and the twill 2/2 200g) with epoxy resin.  Wet lay on a mirror finish sheet of aluminium.  Over the length of the sheet, obviously some flex but its pretty rigid.  About 1.5mm thick measured.
I know from previous peoples attempts that bonding to ASA can be tricky but I had ET538 honeycomb bonding adhesive around so I figured I try this with the backup plan being putting a few holes/rivets through the top.
I roughed up the ASA, cleaned, put the adhesive on the CF and stuck on with gaffer tape holding in place (not easy to get a clamp on)
Left for 48 hrs and was shocked to find this:



I can't work out how this is physically possible!  the forces to do this must be huge.  Neither the CF or the ASA seem to expand much with temperature.  It clearly bonded well!  I welcome thoughts as to how this is possible.  May yet put matching on the other side but with a different adhesive.  Has it reacted to the ASA?
By dazedproductions - 5/31/2019 6:11:19 PM

 
oekmont - 5/21/2019 2:41:46 PM
How do you come to believe, that asa has a small thermal expansion value? Plastics usually have some of the highest thermal expansion factors. Asa has 80-100 × 10^-6/C°, aluminium 23, steel 10-12 and cfrp is close to zero.
To make things worse, cfrp is multiple times stiffer, so it accumulates the forces that build up over the joint. Gfrp would be a better choice, as it is less stiff and has a greater thermal expansion.


Thanks for the response, bad and overly quick research I think is the answer to your question!