Is there a minimum length below which there is no appreciable increase in composite strength? E.g. if i use carbon fibre strands 0.05mm long in a resin matrix, will it actually increase the strength and stiffness? I suspect no! What are the tradeoffs if any?
No it wont
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No it wont
References :
There is a relationship between length and strength benefit, but it is not a simple correlation. The aspect ratio is also important as is the relative stiffness of the fibre and the matrix.
I am more familiar with fibres in concrete, where the fibres do their work after the matrix (concrete) begins to crack. The strengthening mechanism is related to the cros sectional area of fibre bridging the crack and its embedment in the matrix to either side of the crack. A very short fibre will have little embedment and will pull out, while a longer fibre will be fully embedded and will rupture. In a concrete matrix fibres shorter than 10-15mm would be fairly innefective, as would fibres >60-75mm as they couldn’t be placed effectively.
A fibre 0.05mm long would have very little effect unless you are working at the microscopic scale, i.e. very small components with very small crack widths.
References :