It has been stated previously that an increase in the volume fraction of fibres in a composite results in an increase in stiffness. In the graph on the left it can be seen how the volume fraction of fibres affects the modulus of the composite in both the isostrain and isostress conditions. The modulus in the isostrain condition is linearly related to the volume fraction of fibres. In the isostress condition the relationship is not linear. Note how the isostress condition never produces a value of modulus higher than that obtained in the isostrain condition.