Compressive vs. tensile stiffness of IM/epoxy.

[RPstress]

I have some conflicting information on this. One source gives 170 GPa tension and 146 GPa compression, another almost reverses this: 152 GPa tensile vs. 164.5 GPa compessive! This is standard sort of 12k tows 100% 0 degree tape 60% Vf in ordinary prepreg epoxy.

Does anyone know which way around it "should" be? (I suppose it's just possible both are right and it can vary.)

 

[SWComposites]

For carbon fiber/epoxy, tensile modulus is always higher than compressive modulus, though at low strain levels they are very close to the same value. Tensile modulus increases with increasing strain; compressive modulus decreases with increasing strain. The first set of values is about right, though maybe slightly on the high side (depends on the exact material).

 

[RPstress]

Thanks. The first set of values was fairly firm average design data for Toho-Tenax IMS 5131 in 977-2. The Tenax data for the fibre itself lists modulus as 290 GPa (42 Msi), which I guess has to be tensile. 0.6 * 290 = 174, so maybe 170 isn't too ambitious for the composite. The second set of values has been used for preliminary design, but it does look a bit bonkers.