Modeling of mechanical behaviors for natural fiber reinforced composites under hygrothermal ageing

Abstract

In recent years, natural fiber reinforced composites have been widely applied to various industrial products for their excellent environmental-friendly performance. It is essential to understand the mechanical properties of natural fiber reinforced composites under their in-service environment. Compared with synthetic fibers, the hydrophilicity of natural fibers could result in a much larger quantity of water absorption from the moisture atmosphere, which would have adverse consequences for the durability of natural fiber reinforced composites [1]. The environmental temperature would affect the water absorption process, and furthermore, high temperature may break down the fibrous tissues. Therefore, the effect of hydrothermal ageing on the mechanical behaviors of natural fiber reinforced composites attracts great attention from researchers.The changes in mechanical behaviors caused by water absorption of various natural fiber reinforced composites, such as composites with fibers of bamboo, hemp, jute or flax, are investigated by means of mechanical measurements and scanning electron microscopy [2]. Experimental studies show that the volumetric swelling of natural fibers accompanies by water absorption process, which would weaken the interface between the natural fibers and the polymer matrix and then leads to the initiation and propagation of cracks. Moreover, besides the physical deterioration caused by the volumetric swelling of natural fibers, the hydrolysis reaction between the natural *Corresponding author (email: zhongzheng@hit.edu.cn) fibers and the water absorbed plays an important role on the mechanical properties of composites as well. The hydrolysis reaction, occurring for long hygrothermal ageing periods, irreversibly affects the natural fibrous structures and causes debonding between the natural fibers and the polymer matrix.For experimental observations, some researchers aim at quantitively modeling the evolution of mechanical behaviors of natural fiber reinforced composites during the hygrothermal ageing process to predict their durability in service. On the basis of the non-equilibrium thermodynamic framework, Pan and Zhong [3][4][5][6][7] have conducted a series of research on modeling the effect of moisture absorption on the mechanical properties of natural fiber reinforced composites. In their analyses, the internal variables are introduced into the Helmholtz free energy to describe the mechanical degradation during the water absorption stage in the hygrothermal ageing process, and the relation between the amount of the water absorbed by the composites and the volumetric swelling of the composites is formulated on the hypothesis of the elastic incompressibility. For example, for the unidirectional natural fiber reinforced composites considering moisture absorption, the specific Helmholtz free energy function is given by ( )