yrighted material licensed to BR Demo by Thomson Reuters (Scientific), I - 6 - TR62662IEC:2010(E) GUIDANCEFORPRODUCTION,TESTING ANDDIAGNOSTICSOFPOLYMERINSULATORS WITHRESPECTTOBRITTLEFRACTUREOFCOREMATERIALS 1 Scope This technical report presents an analysis of the risk of influencing factors for brittle fracture of composite insulators that are mostly loaded in the tensile mode (suspension and tension insulators). Guidance is given to reduce the risk of in-service brittle fractures. This phenomenon is limited to tension and suspension insulators. However,the general information given concerning the importance of various parameters can be used as a Inc. guideline for the design and production of any kind of composite insulator. 2 Normativereferences The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments)applies: IEC 61109,Insulators for overhead lines-Composite suspension and tension insulators for a.c. systems with a nominal voltage greater than 1 ooo V-Definitions, test methods and acceptance criteria. IEC/TR62039,SelectionguideforpolymericmaterialsforoutdooruseunderHVstress 3 Terms and definitions For the purposes of this document, the following terms and definitions apply 3.1 fibre reinforced plastic material FRP composite material consisting of reinforcing components e.g. glass or synthetic fibres that are carryingpartof a composite insulator 3.2 stress corrosion cracking SCC failure of material subjected to a constant tensile stress in a corrosive environment 3.3 brittle fracture abnormal and sudden breakage of FRP core materials with well-defined characteristic fracture patterns NoTE,Before brittle fracture,no apparent plastic deformation takes place. In the case of FRP core materials, brittle fracture is caused by SCC 3.4 failure mechanism Uncontrolled when p principal and fundamental process that leads to a characteristic failure, e.g.brittle fracture print yrighted material l TR62662@IEC:2010(E) - 7- licensed to BR Demo by Thomson Reuters ( NOTE Afailuremechanismmayhaveseveral modes offinalfailure. 3.5 failure mode specificfailure scenario oroptional path of afailuremechanism 3.6 sealing system technical arrangement to prevent the ingress of moisture, gases, etc., at a material transition point exposed to the environment 3.7 failure mode effect analysis (Scientific),. I FMEA standardized risk assessment tool generallyused forfailure prevention 3.8 damage 4 Description of brittle fracture Brittlefracture is the commonly used term forstress corrosion-induced failure of insulator core rods manufacturedfromresinbonded glassfibrematerial(RBGF,commonlyknownas fibre reinforced plastic FRP).This failure mechanism results ina complete mechanical separation ofthecore(normallyneartheenergizedendfitting),andcanoccurattensileloadswellbelow theratedmechanicalstrengthoftheinsulators.Inadditiontoa(minimum)tensilestressof approximately 5oMPa,thebrittlefracture mechanismrequires the presence of acid from either external or internal sources. The chemical process of stress corrosion is an ion exchangemechanismwherebyions intheglassfibresarereplacedbyhydrogen ionsfromthe acid (see IEC/TR 62039). IEC2042/10 Figure 1-Typical brittle fracture Uncontrolled when material - 8 - TR62662@IEC:2010(E) Identificationof brittlefracture 5 The macroscopic features associated with the brittle fracture of an FRP core rod have been described by CiGRE [2] 1. A typical brittle fracture is shown in Figure 1. The fracture surfaces typically have the following characteristics: by asmooth,clean,planarsurface perpendiculartothe core axis,comprisingaportionof the normal tensile fracture (fibrous) in the remaining rod cross-section. In addition to these macroscopic features, confirmation of the brittle fracture mechanism is possible through the identification of several distinctive