Meaning
Harmonic analysis of stress response signals during large deformation testing quantifies non-linear viscoelastic behavior in polymer melts. Compared to linear dynamic mechanical testing, fourier transform rheology applies fast fourier transformation to oscillatory strain data to extract higher harmonic intensity ratios. The technique ceases to yield meaningful data when sample edge fracture disrupts shear flow inside the cone-and-plate geometry.
Signal Analysis
Third-harmonic intensity ratios measure structural departures from linear viscoelastic response under large shear amplitudes. Digitized torque signals pass through mathematical filters to isolate subtle non-linear frequencies. Accurate signal processing distinguishes physical structural changes from mechanical transducer noise.
Branching Sensitivity
Long-chain branching in polyolefins alters melt elasticity and melt strength during blown film extrusion. Characterization using fourier transform rheology detects low levels of long-chain branching that standard melt flow index measurements fail to reveal. Identifying long-chain structure guides resin selection to eliminate bubble instability on high-speed conversion lines.
Processing technicians correlate the third relative harmonic with strain hardening behavior observed in industrial extensional rheometers.
Strain Response
Increasing strain amplitude drives the polymer matrix beyond its linear viscoelastic limit into non-linear deformation regimes. Non-linear stress signals reflect macromolecular alignment and chain disentanglement dynamics under high shear rates. Uncontrolled shear histories alter the measured harmonic spectrum during quality control screening.
