· The yield stress of the composite is normally determined by the onset of plastic deformation in the matrix which is more ductile than the brittle fibers.
· In the isostrain orientation 
(1-direction) yield occurs at the elastic limit of the matrix. Up to that point the matrix and composite obey Hooke's law.

σ_yC= E_C1ε_yC = [V_f E_f + V_m E_m ]{σ_ym/E_m}, so that: 

σ_yC = [1 + (V_f E_f / V_m E_m) ]V_mσ_ym