Tensile dilatometric studies of deformation in polymeric materials and their composites

Abstract

The theory for volume changes in deformation for polymeric materials is presented, together with a brief literature review of the general area of tensile dilatometry. The theory has been used to enable the prediction of the volumetric response of a material to a deformation, which allows for the detection of the onset of cavitation (volume increasing)-type mechanisms in materials displaying such responses. A series of experiments has been performed using an instrumented tensile dilatometry technique on PMMA and on talc-filled reinforced polypropylene at 23 and 60 °C. The engineering constants, tensile modulus and lateral contraction ratio were measured and found to be viscoelastic. The determination of strain in three mutually perpendicular directions during the instrumented tensile test resulted in the measurement and prediction of the volumetric strain response with applied load. A significant cavitation-type mechanism was recorded in the case of the talc-filled reinforced polypropylene, whereas PMMA showed a deviatoric type mechanism. The volume strain has been found to be directly related to the bulk modulus for these materials. Finally, a new method of presenting volumetric strain versus applied stress data is shown and its relevance explained.