Chopped fiber composite analysis

[CHagen]

I was curious if there was a simple way to analyse a chopped fiber random orientation composite. Can it be analysed nearly correct as an isotropic material? Idea being to make a with a chopped carbon epoxy paste and squeeze it into a mold.

 

[SWComposites]

The flow of the material in the mold will partially align the fibers in the direction of flow and along the mold surfaces; so the material will likely not be purely isotropic. You could possibly use properties from specimens loaded transverse to the flow direction in a plate to give approximate conservative values.

 

[patprimmer]

I agree. After moulding it will no longer be random. Degree of orientation depends on a lot of factors, but can be quite high.

Regards
Pat
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[CHagen]

Maybe instead of squeezing into a mold it could be packed into two halves of a mold and then joined together to preserve the randomness..? If it was very random could it be considered as isotropic and give a close estimate? Also is there any known materials properties for chopped carbon epoxy like this or would I have to test it? Thank you for the insight.

 

[patprimmer]

It would be isotropic in 2 planes only, but that is probably what you require.

I may have some data in an old buried archive.

Regards
Pat
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[CHagen]

Im having trouble understanding why only 2 planes. If you find the data I would really be grateful.

 

[patprimmer]

When the mould closes it will flatten the fibresout so they tend toward being parallel to the surface, so with originally random orientation, as you close the mould you will not disrupt the random orientation perpendicular to the mould closing, but you will disrupt it in the direction the mould closes.

Come to think of it, say you where moulding a bucket shape, you would not disrupt the orientation across the bottom in any way but the thickness direction, but the sides would be oriented a bit in line of draw as the core pushed into the cavity.

Regards
Pat
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[SWComposites]

For material properties, the material supplier should have some sort of representative data (likely on the optimistic side). Data sheets should be available on the web, or you could contact the vendors.

SW

 

[CHagen]

The material I was going to use was chopped fiber held together in thin flakes if that changes anything...

 

[CHagen]

From the sound of it it seems like the fibers/flakes align in the direction of flow as the molds are assembled. Is that true?

 

[SWComposites]

More or less. The amount/extent of alignment is highly dependent on the particular material, part shape, mold details and injection parameters. If you have not worked with this type of material before, I highly suggest talking to a technical rep from one of the material suppliers and also to a mold designer. There are some analysis codes (Moldflow, etc) which can predict alignment, but of course you need to supply the material properites, part and mold geometries, etc.