This is how the stress on an
individual fibre varies with strain

This is how the stress on a matrix
on its own varies with strain

The composite stress is given by the rule of mixtures up to ε_fu \[ \sigma _{1} =f\sigma _{f}+(1-f)\sigma _{m}\]

Fibre fracture

On exceeding ε_fu the fibres
fracture into shorter lengths,
leading to an increase in strain at constant composite stress.

If the matrix fails before the entire load is transferred to the matrix, then this is the composite failure stress. \[ \sigma _{1u} =f\sigma _{fu}+(1-f)\sigma _{mfu}\]

Composite failure