MACROCRaCK foRMATION has been observed by the U.S. Navy in a number of porcelain base insulators used to support large radio communications towers. These cracks do not, per se, impose a severe structural limitation because the primary stresses in the insulator are compressive. In the presence of the high voltage rf fields, however, the heat generated around the cracks may lead to
structural degradation. Hence, towers containing cracked insulators are shut down. The prevention of macrocrack formation in por• celain base insulators is a primary requirement if continuous opera• tion of the radio towers is to be achieved. One possible approach to prevent in-service macrocrack formal ion in ceramic components is overload proof testing. This is an evaluation procedure which subjects the insulator to a load greater than the service load. If the insulator survives the proof test, it An investigation of proof testing has been conducted on large cone and cost porcelain insulators. The cone conto• uration was unsuitable for overlo ao proof testing to the loads nee aed fceffective lifetime predictions The on.,merit of subjecting this configuration a proot test is the assurance of no •rr,• mediate macmcracking during 'nstal:a• tion This is achieved by testing to a loac marginally larger than the service load Conversely, the post configuration exhibited the basic prereqo.stes tor ~dec:•tive proof testing but detailed stress analysis of this system is needed betoe specific proof testing procedures car: be recommended. Finally. acoust.c emission measurements were 2 Tia'G' asset for monitoring both macrocrec-c• ing events and stress devetopmsnt our• i"ng the proof tests.
xhould sa1isfy engineering requirements of service load and lifetime.' The proof test condition for components made of por• celain must take into account the subcritical crack growth that occurs in this material due to moisture in the environment. 2 Subcrit• ical crack growth leads to a time dependence of the strength, and the lime to failure is determined by the time necessary for a crack to grow from a subcritical to a critical size. 1 A mathematical framework for the proof testing of components made of materials that exhibit subcritical crack growth indicates that the important parameters in proof testing are: 1-•1 the proof test load. the test environment and the loading cycle. In addition, for compressive proof testing, a failure criterion must be specified, because crack size criticality does not lead to complete fracture. In this paper, the proof test approach is applied to two types of porcelain base insulators commonly used to support radio towers. The variables of the study are loading rate and proof test load.

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