Creep of Highly Anisotropic, Low-density Polyethylene

Abstract

A SYSTEMATIC study of the creep properties of polymers possessing known anisotropy does not seem to have been attempted, probably because of the experimental difficulties involved. The few results reported are usually measurements of the creep extension in the direction of the applied tensile stress, made by following the separation of the specimen grips. End effects make this an unsatisfactory arrangement, especially when small samples are used. We have therefore developed an apparatus for measurement of the creep strain of specimens having a gauge length of as little as 0.5 in. The extensometer uses a linear displacement differential capacitor transducer, with a sensitivity better than 10⁻⁵ in., in a counter-balanced arrangement to reduce specimen loading to a minimum. The transducer sensitivity is equivalent to a strain of 2 × 10⁻⁵ for a gauge length of 0.5 in. The creep load is applied to the specimens using a lever loading system based on a well-proved design¹. The results reported are for creep in the time range 5–100 s after application of the load. Other tests have shown that the transducer and control unit are stable, within the limit of sensitivity, for a period of at least 5 months.