Studies on Creep of Polyethylene

Especially, on the Effects of γ-Ray Irradiation

Abstract

The creep properties were studied for high-density polyethylene by the use of the test machine specially designed to work under the constant stress condition. The tests were carried out at constant temperatures from 20°C to 90°C on several stress levels. The instantaneous strains occurring just after the weighting were evaluated based on the applied stress and Young's moduli measured by the vibrating reed method.
The creep curves, exclusive of ultimate stage to failure, consist generally of two stages, both being expressed by Nutting's equation. The first stage can be approximately expressed by Andrade's equation, but the period of this stage becomes shorter with increasing temperature and is hardly measured at temperatures above about 80°C. For the second stage, the creep strain ε_c2 can be denoted by the equation, ε_c2=a₂₀ sinh (ασ)tⁿ², similar to Findley's equation, as a function of stress σ and time t. The a₂₀, α and n₂ are respectively the material constants dependent on the temperature. In the range of the test temperature, a₂₀ decreases, α increases and n₂ slightly decreases with increasing temperature.
Creep tests were also made with the samples irradiated from Co⁶⁰ (dose rate, 10⁵r/h). The creep character depended markedly on both the radiation dose and the atmosphere of irradiation. The samples irradiated in the vacuum showed higher creep resistance, and the creep strain decreased with increasing crosslinking as ε_r=ε_oe^-αx, where ε_r is the creep strain of the irradiated sample, ε_o the creep strain of the nonirradiated sample. x the degree of crosslinking and α a constant independent of stress and temperature. On the other hand, the irradiation in air did not change the creep properties in a monotone, and the sample irradiated by some constant dose showed the minimum creep resistance. These phenomena, however, can be explained from two inverse effects of radiation, the crosslinking and the degradation, in which the former increases with increasing dose and raises the creep resistance, but the latter occurs in the surface layer predominantly when the thinner sample is irradiated with low dose rate, and decreases the creep resistance.