An Evaluation of the Critical Conditions for the Initiation of Stress Corrosion Cracking in Unidirectional E-glass/Polymer Composites

The purpose of this work is to establish the critical surface conditions leading to the initiation of stress corrosion cracks on the as supplied surfaces of three unidirectional Eglass/polymer composites with modified polyester, epoxy and vinyl ester resins subjected to a nitric acid solution without mechanical loads. The composite materials considered in this study are commonly used in composite high voltage insulators on overhead transmission lines with the line voltages ranging from 69 to 735 kV. The initiation of stress corrosion cracks in exposed glass fibers on the composite surfaces was observed in the absence of externally applied mechanical loads. However, the crack initiation rates are strongly dependent on the amount of exposed fibers. After the initial stage of crack initiation, no further stress corrosion damage is observed in the composites. The Eglass/vinyl ester system appears to be more resistant to the initiation of stress corrosion cracking in comparison with the other two composite systems investigated. This system exhibits the lowest number of stress corrosion cracks and the lowest total surface area of exposed fibers on the composite surfaces. The E-glass/epoxy composite shows the lowest resistance to stress corrosion with the largest areas of exposed fibers. The effect of exposed fibers on the stress corrosion process in unidirectional Eglass/polymer composites used in high voltage insulators has not been previously reported. It is clear that in order to reduce the rates of failure of composite high voltage insulators by stress corrosion cracking (brittle fracture), the presence of exposed fibers on their rod surfaces should be minimized.