Mechanical degradation of natural fiber reinforced composite materials under constrained swelling

Natural fiber reinforced composite materials (NFRCMs) have found more and more applications because of their excellent performances over traditional fiber reinforced composites.However, mechanical properties of these materials may be dramatically degraded in a humid environment, whether subject to mechanical loading or not.A nonlinear constitutive model is established for unidirectional natural fiber reinforced composites under large swelling deformation based on nonequilibrium thermodynamics, in which an internal variable is incorporated in the Helmholtz free energy to consider the irreversible energy dissipation induced by moisture absorption.The Helmholtz free energy is further decomposed into the base free energy of the isotropic matrix, the reinforcing energy of fiber stretching and the free energy of volume expansion.Two kinds of reinforcing energy (the I 4 -dependent model and the I 5 -dependent model) are employed to predict the degradation of the elastic modulus for the cases of free swelling and constrained swelling.It is found that the predictions from these two models are identical for the case of free swelling and agree well with available experimental data.As for the case of constrained swelling, these two models yield obviously different results.