Resin types

[carbongr]

After a test we did to a part we manufacture, we discovered that when applying constant force to a certain point the deflection gets an initial value, which slowly increases in respect of time and gets its max value after about5 minutes.
We did the same test to another identical part (but made from another manufacturer), which had its maximum deflection immediately after applying the same force at the same point.
Worth mentioning that the initial deflection of our part was 1mm less than the other part, but the final was 1mm more.
Another clue is that these two parts do sound differently when you hit them with a small metal piece (i.e. a small hammer). The one from the other manufacturer sounds brittle (like hitting porcelain or glass) when our sounds more like plastic. Our part vibrates much more when it hits the ground from a certain height too.

Is these caused due the probably different epoxy resin we use, or is it due to different Carbon/Epoxy ratio (we use 68% carbon fibre in this particular part)?
P.S. the pars are solid rods.

Thank you very much in advance.

 

[CoryPad]

Different resins and different fiber volume fractions will affect the testing results you describe.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Demon3]

CoryPad said it well. That ringing noise means high modulus and low damping. This can be due to many factors, main ones are carbon fiber loading (amount), the degree of cure of the resin and the glass transition temperature of the resin. It may be that your resin is less cured than the competitor's.

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[CompoMan]

Different epoxies+hardenerss+catalysts combination will result in your matrix with different degree of crosslinked density - therefore different physical and mechanical properties. You can roughly compare their xlinked densities from storage modulus (DMA) traces at temperature above (>30C)their TgE.

 

[RPstress]

Was there permanent deflection in your part? Or did it eventually relax back to its original shape on removal of the load?

Having asked that, it seems odd behaviour. A part with a lot of carbon fiber (and 68% by volume is a fair bit - or is that wt%? 68% carbon by mass is respectable, but not that high) in shouldn't slowly deform, unless somehow resin-dominated loadpaths are taking the load. Even then five minutes seems odd for the length of time.

If these are something like unidirectional fibre rods loaded in bending, then the resin may be creeping in shear. If your resin is particularly soft it might do this, though any decent resin (you mention epoxy) wouldn't usually do so. Are you sure your resin is cured? If it's not cured properly then quite possibly it might do this. A little beam DMA test would be the usual simple way to check this, though measuring residual exotherm with DSC should also work.

The increased vibration of your part when 'bonked' is at odds with the possibility of soft resin. A part which has a material which will creep usually has higher damping, and will vibrate less for less time.

Properly cured carbon/thermoset parts which aren't too thick have a fairly distinctive sound when struck, sort of like a cross between aluminium and ceramic.

Can you tell us a bit about your resin? A really high damping material intended as an adhesive, such as 3M's AF 32 (which is sometimes used as damping layer), might just give this sort of behaviour. Also, are your fibres continuous?

 

[carbongr]

Thank you everybody for the help.

We investigate the issue and have discovered that the properties of the cured pure epoxy are different to the ones when the epoxy was new. Now it is only 6 months old, stored in 20 C and in the original container all this time.
The rod we made with just epoxy has the properties i explained in my first post (elastic, plastic deformation, not brittle). We found a similar rod made again from the same epoxy (3-4 months old) which is very hard, solid, and definitely very brittle.

So i guess our epoxy has expired in just 6 months (self life 12 months according to the manufacturer). Or is it something else going on and somehow we can restore the initial properties of the epoxy? Is it a shame as we are talking about more than 100kg of epoxy-hardener.

 

[Compositepro]

Epoxy and hardeners are chemically reactive materials. They will react with each other but also often with air and moisture. If the containers are not tightly sealed or have been opened frequently with air space left in the containers they will go bad. Shelf life is determined on unopened containers.

A good practice is to use nitrogen to purge the air out of containers that have been opened, before resaealing.