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 le ading to the initiation of stress-corrosion cracks on the as-supplied surfa ces of three unidirectional E-glass/polymer composites with modified polyes ter, epoxy and vinyl ester resins subjected to a nitric acid solution witho ut mechanical loads. The composite materials considered in this study are c ommonly used in composite high-voltage insulators on overhead transmission lines with the line voltages ranging from 69 to 735 kV. The initiation of s tress-corrosion cracks in exposed glass fibers on the composite surfaces wa s 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-corr osion damage is observed in the composites. The E-glass/vinyl eater system appears to be more resistant to the initiation of stress-corrosion cracking in comparison with the other two composite systems investigated. This syst em exhibits the lowest number of stress-corrosion cracks and the lowest tot al surface area of exposed fibers on the composite surfaces. The E-glass/ep oxy composite shows the lowest resistance to stress corrosion with the larg est areas of exposed fibers. The effect of exposed fibers on the stress-cor rosion process in unidirectional E-glass/polymer composites used in high vo ltage insulators has not been previously reported. It is clear that in orde r to reduce the rates of failure of composite high-voltage insulators by st ress-corrosion cracking (brittle fracture), the presence of exposed fibers on their rod surfaces should be minimized. (C) 2001 Elsevier Science Ltd. A ll rights reserved.