(670h) Necking and Drawing of LLDPE/Seps Rubber Bilayer Laminates

Semicrystalline plastics often show necking and drawing behavior in tension. In contrast, rubbery materials do not show necking, but instead stretch homogeneously. We examine the behavior of bilayer laminate composites of linear low density polyethylene (LLDPE) and styrene-ethylene/propylene-styrene (SEPS) rubber to test the extent to which the SEPS can modify the necking behavior of the LLDPE. Video recordings of tensile tests on dog-bone shaped samples were analyzed by a Digital Image Correlation (DIC) technique to quantify the degree of non-homogeneity in deformation. The LLDPE showed severe necking with a natural draw ratio exceeding 5. Upon bonding it to a rubber layer, the natural draw ratio reduced significantly. With a sufficiently large SEPS thickness, the neck was almost completely eliminated and the sample reverted to nearly-homogeneous deformation. We present a simple 1D model of the mechanics of the bilayer laminate in which the force within the bilayer is treated as a sum of the force of a Mooney-Rivlin rubber layer and an elastoplastic layer. By applying the Considère criterion and a mechanical energy balance to this model, the changes in bilayer necking behavior with rubber thickness could be predicted. The model predicts the decrease in natural draw ratio and the elimination of necking as rubber thickness increases, consistent with experiments.