Cyclic loading effect on the mechanical behaviour and morphology of the high density polyethylene

Abstract

High density polyethylene pipes (PEHD 80) are still widely used in the distribution and transportation of natural gas. While in use, these materials are exposed to internal pressures, to earth pressures, to soil movements and to the dynamic effect of the liquid displacements. These stresses induce inevitably, considerable deforma-tions of the tube’s material and affect its mechanical prop-erties. In the present work, the effect of cyclic loading on the behaviour of PEHD 80 tubes is experimentally investi-gated. Samples are tested to cyclic loadings followed by the pulling (tension) until failure of the material. The curves expressing the mechanical behaviour obtained indi-cate clearly that the PEHD 80 exhibits large hysteretic loops for the first loading cycles, and then the loops de-crease according to the increase of the cycle number. It was concluded that cyclic loading causes a considerable increase in the linear stiffness and the cyclic softening of the material. At microscopic levels, the observations have showed parallel orientation of the macromolecular chains to the tension axis.