= Critical fibre length
{title2=$l_c$}
The <critical fibre length> is the minimum embedded fibre <length> needed to develop a specified tensile <stress> in a simple interfacial-load-transfer model. For diameter $d$, fibre strength $\sigma_f^*$ and constant limiting interfacial <shear stress> $\tau_i$,
$$
l_c=\frac{\sigma_f^*d}{2\tau_i}.
$$
Each half of the fibre builds axial <force> from its end through interfacial shear; balancing that transferred <force> against the strength load gives the formula. It is a model-dependent criterion, not a universal <length> of the material. Nonuniform shear, imperfect bonding and fibre orientation modify actual composite behavior.
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