CONTROL OF STRAIN RATE IN A SHEET-INFLATION RHEOMETER

In the experimental study of nonlinear viscoelasticity, it is found that a material's response depends on the size, rate, and kinematics of the deformation. While uniaxial extension is now fairly widely used in the study of polymeric liquids, there has been less success with the generation of controlled, uniform, biaxial extension. Several methods have been used to generate this type of deformation. The earliest was the sheet inflation technique, which has been used to study raw elastomers, rubbery thermoplastics and bread dough. The problem of controlling the strain rate has limited its application to such materials. The sheet inflation techniques may still be the most attractive basis for a simply performed test that can yield meaningful data on a wide range of materials, including molten thermoplastics. The authors address the problem of controlling the strain in such a way as to maintain a constant strain rate.