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Thread: Composite laminate tensile testing toughts and doubts.

  1. #1
    Hi Guys.

    I'm in the process of modelling and designing some structural parts of the car made out of carbon and glass epoxy laminates. This is our first step towards gaining knowledge on structural analysis on composites.

    Today, we tested some glass epoxy laminates in tension, in order to get things ready and understood when the carbon finally comes.

    We tested specimens of vacuum infused 18oz/sqyd cloth (plain weave) on epoxy resin, in 0 and 45 deg directions. And we were surprised by a couple of things:

    1- Altough the ultimate resistance was +/- 300MPa as expected, Young modulus resulted on a fairly low meaurement. Our initial guesstimation from common data pointed to somewhere 20~30 GPa, we've got 8GPa on average. The entire stress strain curve was linear, as expected.

    As an educated guess, i think our fibers aren't as tightly packed as 6K or 3K carbon commonly used, in fact, one can see small "windows" between the warp and fill strands on the dry cloth. That could contribute to a weak fiber to resin ratio or void formation. I'll recheck for resin and cloth density next week to see if this is the main culprit. FYI, avg densities are on 1.8g/cm^3.. quite on the ballpark side.


    2- On the 45 degree test, things went a lot different. Seems to be that the best approximation for the stress strain curve is a bilinear tendency. Basically we get two fairly different young moduli with a "transition" curved zone between the two linear regions, being the first a lot stiffer than the second part. Things like interlaminar failure at the chucks or simply "dark magic" have been our first guesses to explain such behavior.

    Any toughts?

    Best Regards

    EDIT: Reading again on ASTM D 3039/D 3039M Standard Test Method for
    Tensile Properties of Polymer Matrix Composite Materials, i found references about bilinear curves being typical for some composites, altough i've never seen such behavior on glass epoxy stress strain curves before.
    -----------------------------------
    Drivetrain Leader 07-08
    Technical Director 08-09
    Team FSAE USB - Caracas - Venezuela.



  2. #2
    A couple comments...

    How many ply's did you infuse together? 18 oz is a pretty heavy weave. Like you said, you can see windows through the fiber so you're are basically building voids into the layup. More ply's of lighter glass will compact and take up the voids better. Fiber volume would be an interesting measurement if you have a place to do it.

    Also, I doubt this is the case but are you sure you've got a balanced weave?
    Shaun Kapples
    Mechanical Engineer
    Alumni, FSAE at UCF

  3. #3
    Thanks for your response kapps, another thoughts:

    Yes, 18 oz is a bit overkill. We are using it on a paralell project from some active and ex team members (wind turbine vanes) and using the vacuum infusion knowledge we got there on the FSAE team. The cloth was a balanced weave.

    We were forced to use such heavy cloth due to the fact that epoxies produced here are a little bit too viscous to be vacuum infused. And the costs of importing infusion ready epoxi here gone pretty high.

    Already measured fibre volume fraction, 37% give or take. I've done that calculation last night just after posting this. Seems to be a bit low figure afaik. And effectively, that could be the culprit responsible of the +/-45 bilinear stress strain curve.

    We are thinking of using Vinyl Ester resins on a near future, theyre not that expensive, and are a lot less viscous than our epoxies, so, infusing well on tightly packed fabrics as 6k Carbon. Going V.E/Glass can be a good deal regarding stiffness and resistance compared with Glass/Epoxy. In the other hand, i don't know yet how much is carbon compatible with V.E resins.

    I remember a couple parts vacuum infused by Lehigh (some sexy flexure A arms) i saw on this board some time ago. They were carbon unidirectional/ VE resin.
    -----------------------------------
    Drivetrain Leader 07-08
    Technical Director 08-09
    Team FSAE USB - Caracas - Venezuela.



  4. #4
    just a side comment, make sure you dont use a high vacuum on the infusion on something tight like 6K carbon. A regular 12psi than a normal 20+ psi is probably good for infusing.

    People doing infusion tend to like using "open" weave since the resin will pass through the plies easily. I recommend using resin and cloth that are designed for infusion if you are planning on doing testing instead of using regular cloth and wet layup resin.
    RiNaZ

  5. #5
    RiNaZ:

    We're infusing exactly at 10~20 kPa absolute (2~3 psi), wich matches with the 12PSI vaccum you're talking about..

    How do you infuse at 20+psi without an autoclave or some sort of external pressurization?

    Regarding the infusion specific resin, is a little bit hard to get it here due to restrictions imposed on imported goods here. But i'm curious about that what you said on infusion tailored fabrics. I've never seen such remarks on cloth specs on our usual providers, can you be more specific?
    -----------------------------------
    Drivetrain Leader 07-08
    Technical Director 08-09
    Team FSAE USB - Caracas - Venezuela.



  6. #6
    Different manufacturer make their own resin and fiber cloth and some would make small batches and advertise it as an infusion kit for people starting out. So if you're buying in bulk, you probably wont see it mark as infusion cloth by your providers.

    what i would do is to get with your provider and see if they could recommend cloth that is suitable for infusion.

    I dont know how experience you are in composite, but usually, people starting out in infusion process tends to get more dry fabric in their infusion because of their technique, which messes up with their test result.

    But if you've got a lot of experience in infusion, dont pay attention to what i said above
    RiNaZ

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