The tough-brittle transition in thermoplastics

Abstract

The tough and brittle failure of thermoplastìcs has been studied by means of tensile and flexural tests over a wide range of rates and temperatures and by falling weight tensile impact tests. The sudden drop in fracture energy as the velocity increases from 10−3 to 10−2 in./sec is due to the change from isothermal to adiabatic conditions, and has no relation to plastic flow or chain-chain slipping. The transition from tough to brittle failure occurs when the yield strength is equal to the brittle strength, not necessarily when chain uncoiling is prevented; there is no general correlation between tough-brittle transitions and modulus transitions. The yield strength is much more affected by changes in temperature and straining rate than the brittle strength. The chances of brittle failure can be reduced by: 1.(1) increasing the molecular weight, which increases the brittle strength;2.(2) reducing the crystallinity, which reduces the yield strength;3.(3) adding plasticizer, which reduces the yield strength more than the brittle strength;4.(4) adding rubbery polymers, which reduces the yield strength;5.(5) reducing crosslinking, which reduces the yield strength more than the brittle strength;6.(6) removing residual strain, because the specimen has a low brittle strength perpendicular to the strain.Bulky side groups reduce the brittle strength and increase the chances of brittle failure. The effect of notches on the tough-brittle transition can be explained by stress analysis. It is necessary to take account of flaws, such as chain ends, in explaining the magnitude of the brittle strength.