Meaning
Temporal change in the natural strain provides a way to quantify how fast a polymer is being elongated. Researchers use hencky strain rate to describe the kinematics of extensional flow in melt rheology. This value remains constant if the velocity increases exponentially with the length of the specimen.
Flow Velocity
Constant rate testing is necessary to isolate the time dependent properties of the molten resin. In a controlled hencky strain rate experiment the sample is pulled such that the percentage increase in length is uniform over time. This approach allows for the direct measurement of extensional viscosity.
Melt Stability
Sensitivity of the polymer to the speed of stretching determines the stability of the extrusion process. High hencky strain rate conditions occur at the exit of a die or during the drawdown of a film. If the material cannot relax quickly enough it may exhibit brittle failure or elastic turbulence.
Processing Optimization
Matching the resin properties to the mechanical limits of the line requires a precise understanding of the deformation speeds. Accurate knowledge of the hencky strain rate helps in preventing defects like sharkskin or orange peel. It guides the selection of temperature profiles and screw speeds to maximize throughput.
Engineers use this data to determine the maximum line speed that can be achieved before the melt begins to fracture. The relationship between this rate and the extensional viscosity is a primary factor in the design of high speed fiber spinning equipment.
