Creep and recovery behaviour of ultra-high molecular weight polyethylene in the region of small uniaxial deformations

The creep and recovery behaviour of an ultra-high molecular weight polyethylene (UHMWPE) has been studied in the region of small uniaxial deformations. At deformations as small as 5 × 10^?4 the stress-strain behaviour is non-rectilinear and the recovery cannot be described by a theory of fading memory. A new one-dimensional constitutive relation is presented which describes quantitatively the multistep creep and recovery behaviour of this material in the case where the specimens are not mechanically preconditioned. The multistep in strain-stress relaxation behaviour of the UHMWPE has also been investigated for the case in which the second step in strain is approximately half the magnitude of the first step. Calculations of the strain necessary to give the observed stress in a two-step stress-relaxation experiment have been made assuming that the stress-relaxation experiment can be represented by a series of multistep creep experiments where in each step the stress is adjusted so as to maintain a constant deformation. The agreement between the experimental values and the calculated values are very good. The proposed equation, which describes plasto-viscoelastic behaviour, appears to be able to describe quantitatively the creep and recovery behaviour of a wide range of semicrystalline polymers.