Publication
The constant improvement of polymer composite materials has allowed their integration in many industrial fields where their mechanical and chemical properties are apparently attractive. Mixed metal-composite insulators are now increasingly used for high voltage networks instead of conventional porcelain structures because of their high strength and reduced maintenance requirements due to their strong resistance against chemical degradation and dynamic loading. The composite insulators investigated in this study are made of an epoxy rod reinforced with glass fibres, two steel fittings being strongly crimped to both ends of the rod to transfer loads between the suspended electrical components and the fixation points. Due to the plastic deformation of the steel induced by the crimping process, a purely mechanical link results from the residual radial stresses at the interface between the composite rod and the metallic fitting. The strength of the joints plays an important role during the tensile and bending loading of the assemblies (clamping of the high-voltage line on the electrical tower and guiding of the electrical transformation substation). Experimental and numerical studies on the crimped joints are performed simultaneously in order to determine the strength of the structures and to characterise the damage and fracture mechanisms. The traction and bending tests are achieved to highlight the failure modes appearing for very high loads. In parallel, the numerical analysis provides complementary information on the stress state and the sensitivity of some influent parameters. The proposed dual approach allows the identification of the critical loads corresponding to the failure initiation and the possible mechanisms associated. The results for the tensile tests performed on structures subjected to a crimping load between 290 and 460 kN exhibit a nonlinear increase from 110 to 160 kN of the maximum pull-out load, and a transition of the failure mode of the link from a sliding of the rod out of the end-fitting for weakly crimped insulators and a delamination of the composite for strongly crimped ones. For still higher crimping conditions, the maximum tensile load decreases due to the damage introduced before tensile loading. In parallel to the experimental study, the numerical model developed for the crimping and traction steps provides the stress state in the joint during the solicitations. The comparison between the numerical predictions and the experimental observations highlights the strength limit of the bond by delamination of the outer layers of the composite. The use in the numerical model of a tensor damage factor depending on the stress state in the composite provides additional information on the failure initiation resulting from the stress combination and allows the characterisation of the critical crimping and loading conditions. The analysis of the crimped joints under tensile loading is extended to the study of insulators subjected to bending loading. Experimental tests are performed for two different sizes of insulators showing different final failure modes. For 51mm-thick insulators, a collapse of the compressive part of the rod occurs while for the 63mm-thick assemblies a shear crack propagation on the mid-plane of the rod can be observed. In addition to the fracture behaviour, the tests show a progressive embrittlement of the joint due to the damage onset appearing at 60 and 50% of the maximum mechanical loading of 15.7 and 21 kN found for the 51mm- and 63mm-thick rods respectively. Moreover, the numerical simulations contribute to the understanding of the failure zones and allow the identification of the critical loads and the stress state associated with the damage observed. Owing to the numerical and experimental analysis, structural modifications are proposed for improving the current bearing capacities of the crimped structures in order to shift the damage initiation and to increase the maximal strength of these assemblies under industrial use. In the tensile loading case, an optimal contact area between the rod and the end-fitting ensures an increase of 50% of the maximum mechanical load for the 18.7mm-thick insulator. For the bending case, the damage initiation at loading rates which are much lower than the bearing capacity can be postponed by changing the conical form of the metal end-fitting. However, the maximum bending load cannot be improved since it is directly limited by the diameter of the rod.
Thomas Keller, Landolf-Giosef-Anastasios Rhode-Barbarigos, Tara Habibi