· During plastic deformation of the composite the fibers continue to deform elastically and the matrix deforms plastically.
· The flow stress of the matrix governs its "elastic" behavior during plastic deformation with the tangent modulus (dσ_m / dε_m )_ε = E_m' , where: 
σ_m = K ε_mⁿ
· The effective Young's modulus, E'_C1 , during matrix flow is therefore: 
E'_C1 = [V_f E_f + V_m E'_m]
· The second term is normally small compared to the first so that: 
E'_C1 ~ V_f E_f .
· For the graphite fiber reinforced epoxy (gfrp) composite with V_f = 0.8 and 
E_f = 300 GPa the effective modulus with the matrix plastically deforming is: 
E'_C1 = 240 GPa ~ E_f .