Radiation degradation of insulating materials under stress

Effects of stress on the radiation degradation of insulating and jacketing materials of electric wires were studied. Dumbbell-shaped samples of four kinds of insulating and jacketing materials such as chlorosulfonated polyethylene (CSM), ethylene-propylene rubber (EPR), chloroprene (CR) and silicone rubber (SiR) were exposed to various conditions of irradiation under stress. The irradiation was carried out at room temperature and at dose rates ranging from 1 to 10kGy/h in air, and at 4.5 kGy/h under oxygen pressure of 0.5 MPa up to dose of 2.0 MGy. Stress were added by straining the samples from 0 to 100% of the initial length. Tensile properties were measured by using Shimazu Autograph (type DCS-500) with a cross head speed of 200 mm/min, and the breaking energy was calculated from the stress-strain curves. The tension set (permanent strain) was not seen without irradiation, but observed to increase with the dose and initial strain for all materials and not to depend on irradiation conditions. Those of EPR and SiR become equal to the initial strain above 1.0 MGy. These tension sets are not annealed by heating at 100-120 deg. C for 24 hours. The elongation and strength at break decrease with the dose and initial strain. And the influence of the initial strain on the degradation is seen more clearly in the elongation for CSM and CR, and in the tensile strength for EPR and SiR, respectively. The influence of the initial strain was also examined by the value D50% which is defined as a dose at which the breaking energy calculated from the stress-strain curve becomes to 50% of the unirradiated sample's value. It is shown that D50% decreases with the initial strain linearly and that the irradiation at a low dose rate or in oxygen over pressure gives lower values for CSM, and that D50% for EPR does not depend on the initial strain and irradiation conditions. (author). 3 refs, 8 figs