Stress-strain and volume change characteristics of an assemblage of polymer pellets

A series of experiments were performed on an assembly of four commercially available polymer pellets at room temperature. An automated deformation con trolled triaxial testing system was used for the entire testing program. Lu bricated end platens were used to assure uniform strain conditions during t he application of compressive loading on the confined cylindrical specimen of a given polymer pellet type at a target bulk density. The test results i n terms of the deviator stress-strain and volume change characteristics of polymer pellets were obtained for use in the modeling and analysis of solid s conveying in plasticating screw extruders. Testing procedures are describ ed that can be used routinely to obtain the strength and deformation behavi or of an assemblage polymer pellets. The deviator stress-axial strain and v olumetric strain-axial strain curves are presented for four confining stres s levels (100, 200, 300 and 400 kPa) for these materials using both dry and saturated specimens. The water saturated specimens were used in parallel f or obtaining reliable measurements of the volumetric strain variation durin g the application of deviator stress. The effect of using water as the pore fluid on the mechanical behavior was also discussed. The angles of interna l friction of these four polymeric materials were deter mined and the use o f Mohr-Coulomb failure criteria was evaluated These confining pressure depe ndent friction angles are compared with the angle of repose measurements. T he stress transfer coefficients as a function of wall friction for the pote ntial states of confinement of these polymer pellets in the solids conveyin g portion of an extruder are also summarized.