Abstract
The stress corrosion cracking of model epoxy-glass 0°/90°/0° crossply and 0 unidirectional composites in aqueous sulphuric acid has been studied. Specimens, in the form of coupons cut from laminates were tested under conditions of uniaxial tension at constant load, whilst partially immersed in an acidic environment. Two different modes of failure, which resulted in fracture were observed. In the "so-called" Mode I failure, fracture occurred within that part of the composite immersed in the acid,whereas in Mode II, it occurred in the unimmersed part. Both failure modes were observed for crossply and unidirectional composites. In unstressed 0°/90°/0° specimens a third failure mode (Mode III) was observed, in which the damage took the form of transverse and longitudinal cracking of the unimmersed part. The failure mode depended upon the magnitude of the initial applied strain, the nature of the environment, and the type of environment cell. At initial applied strains of greater than about 0.15% only Mode I failure was observed. The similarity between the times-to-failure of laminates with those recorded for single E-glass fibres showed that the resin was not providing significant protection from the acidic environment. To account for this result it is postulated that the acid rapidly permeates these composites through environmental microcracks, which form parallel to the axis of the glass reinforcement. in the 0° plies. The formation of these microcracks is due to a reduction in the resin/glass interfacial strength in the presence of acids. Confirmation of this phenomenon was obtained from experiments on the transverse cracking behaviour of crossply laminates, immersed in aqueous acid and tensile tested at constant strain rate. At initial applied strains of less than about 0.15% and depending upon the experimental conditions, failure was by either Mode I or II. Mode II, which occurs in shorter times than Mode I was observed in acids (e. g. sulphuric acid) giving rise to relatively insoluble glass degradation products, providing evaporation of moisture from the unimmersed part of the specimen was possible. The Mode II failure mechanism has been identified with the transport of the aqueous acid along the glass resin interface from the immersed to the unimmersed part of the composite. Here precipitation of the less soluble glass degradation products causes a localised stress sufficient to initiate and propagate a stress corrosion crack. At these strains both Mode I and II failures occur at times significantly greater than those observed for glass fibres. Mode III failure is similarly attributed to the precipitation of glass degradation products within the composite. Microscopical examination of the stress corrosion fracture surfaces did not reveal any morphological differences between Mode I and II failure, apart from the appearance of more crystalline products in the Mode II fractures. Although glass fibre fracture is the ultimate failure mechanism in these composites, stress corrosion of the fibre matrix interface was found to be a necessary precursor.