RATING OF COMPOSITE SUSPENSION INSULATORS RELATED TO THE LONG-TERM MECHANICAL STRENGTH OF RODS

The long duration mechanical strength of rods used in composite insula tors is a major problem for the line designer. The previous approach t o the problem and the requirements of Standards give an extrapolation for the long term performance of the rod which is not confirmed by acc urate analysis of test results. So, a new model for the load-time curv e is determined. Based on the analysis of the behaviour of the rod und er tensile stresses with acoustic emission and the determination of th e physical ''damage limit'', a mathematical model of the breakage proc ess is proposed. This model takes into account two damage mechanisms; at high stresses, the slope of the load-time failure curve is governed by the relaxation of the resin matrix, then the curve tends to an hor izontal asymptote corresponding to the intrinsic mechanical strength o f the glass fibers. The model can solve any problem related to the ult imate behaviour of the fibreglass rod after various load histories. Th e conditions required to have no reduction of the ultimate strength of the rod in relation with the SML of composite suspension insulators a re established.