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
-dependent model
and the
-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.
Keywords
natural fiber, moisture absorption, mechanical degradation,
swelling, energy dissipation
