The influence of mechanical conditioning on the viscoelastic behaviour of short-fibre glass reinforced epoxy resin (GRP)

Abstract

In this paper an attempt has been made to understand the influence of conditioning on the viscoelastic behaviour of glass reinforced epoxy resin (GRP) by comparing the creep characteristics of unconditioned and conditioned specimens. This has been done by experimentally measuring the creep strains and by calculating the stress/strain isochronals and the isochronous compliance/stress trajectories for both types of specimen. It is shown that the creep strains obtained from GRP specimens conditioned at 26 and 40% ultimate tensile strength fall within the creep strain scatter band obtained from the six unconditioned specimens. The isochronals clearly show, first, that the conditioning shifts the isochronals such that a greater strain is obtained for a given creep stress, and second, that the isochronals remain straight up to a creep stress equal to the conditioning stress. The isochronous compliance/stress trajectories of the conditioned specimens exhibit compliance saturation. This saturation is found only at elevated stresses in the unconditioned specimens. Compliance saturation is discussed in terms of the degradation of the in-situ matrix and subsequent fibre dominated composite deformation. An operational composite model that estimates a limiting initial compliance necessary for subsequent viscoelastic behaviour is tentatively proposed. By contrasting the isochronous compliance/stress trajectories of GRP with those of glass-fibre-reinforced thermoplastic, GM-PET, a qualitative analysis of the engineering applicability of three non-linear viscoelastic models is given.