Broken part

General Composites Discussions Composites Talk
#1

Hi all,
Recently, I made ​​a part in carbon / kevlar by infusion, to replace an aluminum piece, but it is broken, I’m not sure why failed, I would like to know if it was my fault or part was too weak.
Some air came in during the infusion (I have no idea how it happened), but It didn’t seem too serious, this happened when the resin was gelled,vacuum was lost completly, (and then reapplied).

I ask because I plan to repeat the piece trying not to have any air in, but maybe it will break in any case… then would have to make a piece stronger.

Red, main fracture.
Blue delaminaccion.

Both pieces had the same dimensions except for the thickness, 8mm in the case of the aluminum part and 5mm carbon / Kevlar part.

Vf = 61%

this side is on compression

one layer of 100g/m2 plain charcoal
two layers of 823g/m2 triaxial coal
four layers of 600g/m2 twill carbon / kevlar
one layers of 823g/m2 triaxial coal
one layer of 100g/m2 plain charcoal

this side is on stress

What should I do?
Trying to do the same piece again, but this time with a perfect infusion, because a 5mm should be enough, or repeat adding more layers.

Regards and thank you very much

#2

Delamination in the Kevlar, could have been moisture in the Aramid. But also the air that came in during infusion can be the reason for the delamination.
Do you need the Kevlar? Why not only carbon.
Make sure your vacuum is below 20mbar for min 1 hour to get the moisture out of the Kevlar. Also you vacuum has to be 100% tight.
If you have a dynamic load on your part lower your VF to about 50%

#3

Hi, thanks for reply.

Yes, it’s for dynamic loads, in fact, I’m looking that my composite part increase rebound/energy returns, after apply a load, versus aluminum part (I don’t know how to say, would be something like the carbon fiber legs of Oscar Pistorius, “energetically efficient”)

http://www.fastcodesign.com/multisite_files/codesign/imagecache/inline-large/post-inline/1280-oscar-pistorius.jpg

So I tried get a high Vf, I thought that high Vf give more rebound, but certainly the fibers appear to be a bit dry.

About aramid, I used it for two reasons, it has a lower density that carbon, therefore, it would be working a bit here as “core material”, I could get a part with better strength / weight rate that using carbon only, and don’t penalize rebound, not ?
Also I would like that part be as safe as possible, and if broken, does not do so abruptly. Could kevlar works for this?
Also the piece could be impacted, but this really is not the reason for the choice.

Then, considering safety as the primary goal, Would it be a good idea not to use the kevlar? could also use 3mm foam 80kg/m3, but don’t think that was good for safety or rebound.

I’m not sure how long I kept vacuum, but it probably under an hour, the next time will keep all night, (less than 20 mbar), Bag appear completely seal (I degas resin ), any problem happened when I was not present, because the vacuum disappeared very fast.

So, Do you think that has been a problem of construction and really it should have been so resistant like aluminium part with this amount of fiber?

Regards.

#4

Don’t use Kevlar for this or if you use it don’t put it on the total surface.
Otherwise you may change the shape of you part and create some grooves.

You can’t make similar parts in two different material and have the same caracteristics

#5

A structural part made like this will not work , it’s the wrong design with wrong materials, plus cutting the slots

#6

Hi, probably kevlar has not been a good idea, it is delaminated in several places, I already knew that kevlar does not stick at all well with the resin, but carbon in hybrid fabric should help here, Not? Is this normal? Maybe resin other hadesion best properties would have given a better result?

Really, I cannot change design, slots are needed here. Note that I’m not designing a piece, I’m replacing designed piece, I just could change the thickness and materials really.

What materials would you recommend to use?

#7

What kind of resin? What did you do to try and get a higher Vf?

#8

I infused part for three hours in total approximately, just before the end (80% infusion completed), I stop rein input, the I apply heat with a hair dryer and try to compact the fibers applying more pressure with my hands, while try to move resin, towards hose output.

I thought that 1kg/cm2 (1bar), it don’t seemed enough for compact fibers (5100g/m2 in toral).
Then I apply vacuum (and heat )to the inlet hose, and left some more resin.

#9

Epoxy? Vinyl esters? Polyester?

#10

I’m sorry, I thought that had responded lOl!
Epoxy resin infuison E3000RI and hardener E300RI (3 hours)

#11

That’s a pretty slow hardener. Did you heat treat the resin before stressing it?

#12

Slow hardener, Is it bad for some reason?
I always prefer to have time to make the infusion, degassing etc…
About thermal treat it was cured at 80 ° C in the mold, before demolding, for 8 hours around …