Anurag Bansal


January 1996

Document Type


Degree Name



Dept. of Materials Science and Engineering


Oregon Graduate Institute of Science & Technology


Composite Insulators are required to fulfill long-term structural roles in power transmission and substation applications. These insulators consist of a glass reinforced polymer (GRP) composite rod, with two metal end-fittings either mechanically crimped or adhesively bonded to the ends of the rod during assembly. In comparison with their porcelain counterparts, composite insulators offer significant advantages such as a high mechanical strength-to-weight ratio, improved damage tolerance, flexibility, and ease of installation. However, since they are a relatively new product, their design is still in an evolutionary stage, and their structural integrity and expected life to failure is a subject of great interest to both utilities and manufacturers. The objective of this study was to evaluate the short-term structural integrity of composite insulators subjected to externally applied multi-axial loads, in conjunction with the residual radial compression applied to the GRP rod during crimping. In order to achieve this goal, comprehensive axisymmetric- and three-dimensional finite element models have been developed in this study. The models assumed either a perfectly bonded interface, or an imperfect interface between the GRP rod and metal end-fittings. The internal stresses in the GRP rod of 115kV substation insulators, caused by radial compression applied during crimping, and seven different cases of expected multi-axial loads were analyzed. In addition, destructive and non-destructive tests were performed on five substation insulators in order to determine their mechanical strengths under three different modes of external loading, and to measure the extent of radial compression applied to the insulators during crimping. Furthermore, the finite element models were used to perform a parametric study of the influence of design variables such as the radius of the GRP rod, the magnitude and shape of radial compression applied during crimping, and the coefficient of friction at the GRP-metal interface, on the axial loading capacity and the internal stresses in the GRP rod of both substation and suspension insulators. In addition, a methodology employing the biaxial Iosipescu fixture was suggested for measuring the biaxial failure strength, in particular, the resistance to axial splitting (or debonding), of unidirectional composites used in insulators. Results obtained from this study indicate that the magnitude and shape of the radial compression profile of the GRP rod, and the mechanical performance under external loads, can be significantly different among insulators intended for the same application. The perfect interface models were inappropriate for computing the maximum internal stresses, since they assume a linear mechanical behavior and predict singular stresses at the GRP-metal interface corner. On the other hand, the imperfect interface models could accurately predict the structural non-linearity of insulators, and were in good agreement with experimental results obtained under three different modes of external loading. From the parametric design analysis, the effects of several variables were evaluated, and semi-empirical relationships were derived to extrapolate the numerical data within a well defined range of the design variables. It was shown that the biaxial Iosipescu test is a reliable technique for measuring the resistance of unidirectional composites to intralaminar splitting under biaxial loading conditions.





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