Constitutive modelling of viscoelastic unloading of short glass fibre-reinforced polyethylene

The simulation, by traditional uni-dimensional rheological models, of viscoelastic unloading to zero stress after tensile testing of short glass fiber-reinforced polyethylene and its pure matrix is poor. The models significantly underestimate recovery rates, even with small amounts of strain. The use of a finite number of relaxation times does not sufficiently increase recovery rates during unloading when models are generated from the responses of materials under load. Similar results and observations are obtained using rate jumps in loading and unloading. 3D models developed using local state methods require that an additional recovery potential should be used. This observation seems to prove that it is necessary to take into account the evolution of the polyethylene microstructure while the stress is applying, thus justifying the existence of the additional potential. The similar situation that exists for pure polyethylene means that the phenomenon must not be confused with material damage.