Influence of Normal Stress on Creep in Tension and Compression of Polyethylene and Rigid Polyvinyl Chloride Copolymer

Abstract

New apparatus suitable for compression creep tests of slender specimens is described. The apparatus is designed to prevent buckling and to introduce a minimum of friction. Results are reported for tension and compression creep of polyethylene and annealed, unplasticized polyvinyl chloride copolymer at 75 deg F and 50 per cent relative humidity. The stress σ, strain ε, time t data from these tests were found to be describable with reasonable accuracy by the equation ε = ε0′ sinh σ/σε + m′tn sinh σ/σm where ε0′, m′ , n, σε , and σm are material constants. The results for polyethylene show that the creep in tension and compression were virtually the same. However, the creep in tension was similar but larger than in compression for polyvinyl chloride. These observations are interpreted in terms of the material structure and the influence of the normal stress on active shear planes.