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Buchado - 2/18/2020 10:53:35 AM

Hi all,
We usually have datasheets for the carbon fiber and resins we use, but the fiber/resin ratio is normally variable, so I was wondering, is it safe to assume that the final composite piece has its mechanical properties proportional to the fiber/resin ratio and the fiber and resin properties?
As an example, let's say some carbon fiber has a 30Mpa modulus, epoxi 20Mpa modulus, a piece with 50% fiber/resin ratio would have a 25Mpa modulus?

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polaraligned - 2/21/2020 11:48:25 PM

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Buchado - 2/18/2020 10:53:35 AM

Hi all,
We usually have datasheets for the carbon fiber and resins we use, but the fiber/resin ratio is normally variable, so I was wondering, is it safe to assume that the final composite piece has its mechanical properties proportional to the fiber/resin ratio and the fiber and resin properties?
As an example, let's say some carbon fiber has a 30Mpa modulus, epoxi 20Mpa modulus, a piece with 50% fiber/resin ratio would have a 25Mpa modulus?

Well, for starters, I have no idea of which modulus you are speaking of (tensile, flexural, compressive).   The tensile modulus of something like Hexcel IM7 is 276 GPa, and the tensile modulus of a quality infusion epoxy runs < 4GPa.  So the fiber is something like 70 times stiffer in elongation than the matrix.   A tensile modulus listed on the fiber data sheet drops dramatically for even a high quality layup at 70/30, so you can't use the properties of the raw fiber for a design.  That 276 GPa above drops to 168 GPa in a low resin laminate.  The fiber properties dominate the properties of the laminate after the initial drop in properties. 

IIRC, the difference in stiffness between a quality hand layup without vacuum bagging and a quality infusion is about a 20% gain in stiffness for the same amount of CF and layup.  I have used Helius Composite simulation software in the past and have simulated the difference in resin ratios using micro-mechanics.  The difference is not dramatic, but noticeable.