Effects of Mechanicl Stresses on the Dielectric Breakdown Strengths of PET and FRP

Organic materials to be used for the electrical insulation of superconducting magnet coilsin large fusion reactors must withstand electrical stresses under high mechanical loads, extreme temperatures, and intense nuclear radiation. In this paper, measurements at room temperature are reported on the effects of mechanical compressive and tensile stresses on the dielectric strengths of bi-stretched polyethylene terephthalate (PET) and epoxy glass cloth (FRP, G-10) which are widely used for the insulation of superconducting magnet coils. For PET, the breakdown strength increases with increase of compressive stress in the region of elastic deformation but decreases in the region of inelastic deformation. The value of compressive stress showing maximum dielectric strength is independent of film thickness and is approximately 100 MPa, smaller than the stress appearing in large fusion reactors. In the case of FRP, the behavior of breakdown characteristics under compressive stress is nearly the same as that of PET but the relationship between characteristics of electrical breakdown strength and mechanical properties of FRP isnotclear because it is a composite of two materials: epoxy resin and glass filler. The effect of tensile stress on the dielectric strength is small for PET, but significant for FRP.