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Effect of varying carbon fibre and matrix ratio

In a composite material, the reinforcing fibres provide strength and stiffness while the matrix keeps the fibre in position and transfers the stress to the fibre.

Designers and engineers specify where the strength/stiffness needs to be and the layer of fibers are added and aligned in the direction and angle required to provide best effect. The matrix keeps everything in it's proper place so that a shape can be built up and so that the fibres will stay in place to do their job.

The matrix also serves to protect the fibre from outside damage such as abrasion or knocks and crashes. The matrix prevents water infiltration between the network of fibres, which cold damage the composite.

A third function of the matrix is to provide a smooth surface for finishing. Carbon fibre and other composites are rarely left unfinished because current matrix materials such as epoxy are not resistant to Ultra Violet degradation and will be damaged by exposure. For this reason care is taken to provide a pleasant smooth surface on which to apply either a transparent coating or paint and decals.

Since epoxy matrix adds weight without much strength nor stiffness, reducing the amount of resin results in a composit improves the strength to weight ratio.

Because Carbon fibers are quite strong and very stiff and matrix is neither very strong nor very stiff, builders strive to use only as much matrix as is required to keep the assemblage together.

To accomplish this the assemblage of mandrel, mold, or other temporary support on which the composite is being shaped, along with the fiber cloth, mats, roving and whatever form the fiber is being applied, is placed in a vacuum chamber, or a custom vacuum set up is constructed around the assemblage in order to inject the liquid matrix material and using vacuum, apply even pressure to insure that any extra matrix material is squeezed out while insuring complete impregnation of the fibres.

Another advantage of using pressure systems is the reduction of voids while still minimizing the amount of matrix often epoxy, required. Voids can significantly weaken a Composite. Link to the Effect on Carbon Fiber Epoxy Composite. pdf file.

Carbon fibre can be supplied in pre-impregnated rolls. This Pre-Preg, can be shaped and placed without having to worry about dripping and complete impregnation of the fibres. The assemblage then needs to be heated and sometimes subjected to vacuum to set the composite and ensure a smooth finish.

The greater the % amount of fiber to the total composite the stronger/stiffer it is and the greater the percentage of matrix the weaker the final composite becomes.

Graph comparing the strength of a composite at different % of matrix to fibre

graph of strength of composite

As a force is applied, the materials undergoes deformation. The lower the % of fibres in the composite the less force it takes to deform it and bring it to its failure point. The greater the % of fibre the better it resists deformation and the more force it takes to deform and cause it to fail.

Carbon Fibre and other common reinforcing fibres are rigid and not elastic. There is very little deformation before failure. Matrix is flexible and its deformation curve is curved before failure.

This explanation is a simplification and if you are very keen and want more details here is a link that explains the effect of varying fibre and matrix ratios. It's a PDF file.

Amateur boat builders are at a disadvantage

In order to reap the most benefit from using carbon fibre, amateur boat builders must manage to reduce the amount of matrix in the composite without introducing flaws, or starving the composite of matrix, which would weaken the structure.

They can either use vacuum techniques or pressure clamping, or careful wrapping in plastic.

I don't claim to be an expert. This article combines information that I have found in my research on Carbon Fiber, I am not a primary source. I try to be accurate but I MAKE MISTAKES, I know this might come as a surprise to some of you, but there it is. If you plan to build a mast or other boaty things, do your research. Consult an expert, and be careful. Have Fun.

email me if you find mistakes, I'll fix them and we'll all benefit: Christine