Why Carbon Fiber Reinforced Plastic (CFRP) composite material is weak when subjected to normal force?
Dear Shrestha,
Your question is rather generic because because there are so many different kinds of CFRP! If you assume that it is made by unidirectional continuous fibres and the force is normal to an axes parallel to the fibres, a simple answer is that fibres in this case are just acting as a defect rather than a reinforcement.
I think whether defect or reinfoecement dependent on the interaction between the poumers and CFRP. Adjust the interaction will change the dispersion state of CFRP in the polymer matrix.
Shrestha,
I also have to mention if you are refering to sustaining loads in the direction prependicular to the fiber direction ( in unidirectional CFRP for example), then as Alberto and Ulrich mentioned, the fibers do not provide any reinforcement to your matrix. If you mean normal loads in compression, again your fibers wont provide in the compression either and they will fail in buckling mode.
For CFRP with unidrectional fiber reinforcement, your tensile strength highly depends on the strength of your fiber, and strength and fiber volume fraction will dectate your final strength in tension.
If you load your composite perpendicularly to the fiber direction Young's modulus is determined by the series model that is mainly determined by the matrix properties. Check out the lower bound modulus of https://en.wikipedia.org/wiki/Rule_of_mixtures
This is how it works for the stiffness but the effect also applies for the strength but the relations isn't quite as simple in that case. Consult a composite mechanics textbook for more details on the transverse strength.
Sorry, but you are not correct with the judgement in your question, if you employ 3D- or 4D-reinforcement in the materials.
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