True intrinsic mechanical behaviour of semi-crystalline and amorphous polymers: Influences of volume deformation and cavities shape

A model enabling the determination of the intrinsic mechanical constitutive equations of uniaxially stretched polymers is presented. This model takes into account the cavitation-induced volume strain which can occur during the deformation of such materials. In particular, the true intrinsic axial stress and strain depends on the overall volume strain and a form factor depicting the evolution of the voids shape. Based on our model, the true intrinsic behaviour of high-density polyethylene (HDPE), polypropylene/ethylene-propylene rubber (PP/EPR), and polyethylene terephtalate (PET) was assessed in tension. Compared to the overall true behaviour, the intrinsic true behaviour of the materials did not exhibit anomalies at large strain levels with changing experimental parameters (strain rate and temperature), and can be accurately predicted by means of phenomenological constitutive equations as the one proposed by G’sell and Jonas (1979).

► Our mechanical system measures volume deformation in situ during tensile tests.
► Cavities shapes are revealed post mortem by an environmental SEM.
► Our mechanical laws integrate amount and shape of voids depending on polymers.
► True intrinsic behaviour can be simulated regarding strain rate and temperature.
► Von Mises equivalent true intrinsic laws are compared for three deformation paths.