In https://doi.org/10.1179/1743284712Y.0000000022, we propose an extension to the rules-of-mixtures for better prediction of NFRP moduli and strengths. You can now measure the fibre diameter and correct the cross-sectional are based on the fibre area correction factor (FACF: ratio between mean apparent and mean true cross-sectional areas). Confirmed for jute in the above paper, but needs fibre area correction factors to be determined for other fibres.

John Summerscales.