The aim of this work is to evaluate the moisture absorption effect on the mechanical properties of a hybrid sandwich composite formed by orthophthalic polyester resin, reinforced by bi-directional woven fabrics of glass and jute fibers, with a central layer of polyester fabric (coremat). For the composite characterization, were performed tensile and three-point-bend tests, damage mechanism analysis, and moisture absorption tests. A larger absorption was observed for the hybrid composite compared to commonly observed fiberglass composites. This behavior is related to jute fiber as well as the coremat. In the mechanical tests a strong influence of the moisture content on the mechanical properties was verified. The main characteristic of the three-point-bend test was the premature shearing fracture of the coremat under both dry and wet conditions.

RESUMOO estudo visou a determinação das causas da falha de um isolador polimérico composto, empregado em uma linha de transmissão (LT) de 138kV. O trabalho foi desenvolvido seguindo a metodologia recomendada para análise de falhas, envolvendo caracterização metalúrgica e ensaios químicos. Foi possível, através dos ensaios realizados, identificar o mecanismo de corrosão sob tensão (CST) como o responsável pelo processo de fratura frágil sofrido pelo isolador. Este mecanismo é frequentemente associado às falhas em isoladores poliméricos compostos (resinas epóxi reforçadas com fibras de vidro ricas em boro), em presença de umidade. O isolador em questão apresentava uma série de características construtivas inadequadas que possibilitaram a penetração de umidade na terminação inferior e consequente contato com o núcleo do bastão PRV (polímero reforçado com vidro) levando à sua ruptura em serviço. Os resultados deste trabalho motivaram a substituição de todos os isoladores poliméricos com características semelhantes por parte da concessionária de energia elétrica.Palavras chaves: análise de falha, isolador polimérico, CST. Failure analysis in polymeric insulator ABSTRACTThe objective of this study was to determine the cause of the failure in service of a polymeric insulator used in a 138 kV transmission line (TL). The study was conducted following the most usual recommended methodology for failure analysis, metallurgical characterization and chemicals tests. The tests identified that the mechanism of stress corrosion cracking (SCC) was responsible for the insulator´s brittle fracture. This mechanism is often associated with the existence of flaws in polymeric insulators compounds (epoxy resins reinforced with boron rich glass fibers) in the presence of moisture. The tests revealed that the design characteristics of the insulator were inappropriate because they allowed the penetration of moisture into its lower termination and subsequent contact with the GRP (glass reinforced polymer) core, leading to its disruption. These results led to the replacement of all insulators with similar characteristics by the electric power transmission utility.

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