The AFX Master
09-24-2010, 08:00 PM
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.
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.