Investigation of hygrothermal aging phenomena in model composites by Raman spectroscopy

Raman spectroscopy combined with gravimetry has been used to investigate hygrothermal aging in model composites based oil an epoxy resin containing a single filament of a reinforcing fibre. Three fibre types, namely PBO, M5 and Twaron, were investigated using two specimen configurations - the diffusion slab (DS) and the double fibre pullout (DFPO) geometry. Variables such as the fibre type, surface treatment in the case of Twaron fibres, and the penetrant type (water vapour and liquid phase), were screened by establishing development of the Raman strain profiles epsilon(x, t) and the water uptake M(t) with time. Experimental data were successfully modelled using relationships that describe Fickian diffusion into a solid parallelepiped. Relative water uptakes determined gravimetrically were reduced to time dependencies controlled by the specimen geometry, diffusion coefficient of the matrix D and the equilibrium uptake of water, M((infinity)). Using simple micromechanical models, the Raman strain profiles were rationalised in terms of the midfibre strain = epsilon(x = 0, t) and the dimensionless parameter n, which controls the stress transfer along the interface. The fibre strain energy G(t) was also obtained from the area under the Raman profile. At longer exposure times and for some fibres, debonding was observed and the time dependence of the propagation of the debond length L-d was quantified. Generic conclusions on composite behaviour were drawn by eliminating the exposure time. The swelling coefficient beta was derived from the linear dependence of midfibre strain on the water uptake and a change in the fibre strain energy was related to the debond propagation. (C) 2003 IoM Communications Ltd. Published by Maney for the Institute of Materials, Minerals and Mining.