Micromechanical phenomena during hygrothermal ageing of model composites investigated by Raman spectroscopy. Part I: Twaron fibres with different surface treatments

Raman spectroscopy combined with gravimetry has been used to investigate hygrothermal ageing in model composites based on an epoxy resin containing a single filament of a Twaron PPTA fibre. Three Twaron fibres have been studied: untreated fibre (HM), fibre with a surface finish (HMF) and fibre with an activated surface (HMA) using two specimen configurations: the diffusion slab (DS) and the double pullout geometry. Time evolutions of the Raman strain profiles ε(x,t) and the water uptake M(t) have been determined for specimens immerse in liquid water and exposed to water vapour. Experimental data are modelled using relationships that describe Fickian diffusion into a solid parallelepiped. Simple micromechanical models have been used to rationalise the Raman strain profiles in terms of the mid-fibre strain ε(x=0,t), the dimensionless parameter, n (controlling the stress transfer along interface) and the fibre strain energy G(t). For some fibres and at longer exposure times debonding was observed and the time dependence of the debond length Ld was quantified. Eliminating the exposure time, the swelling behaviour of the matrix is assessed and a concept for determination of the interface fracture energy is proposed. Time dependencies of n, Ld and G(t) are used to rank surface treatments of Twaron fibres as to durability of their interfaces during hygrothermal ageing.