Characterizing Viscoelastic Relaxation during Thermoset Post Cure Cycles
Characterizing viscoelastic relaxation during post cure cycles prevents internal stress lockup, ensuring long-term dimensional stability for structural thermosets.
Shift Factor Calculations
Meaning
A mathematical method determines the relative shift of viscoelastic material response curves along the time or frequency axis as a function of temperature. Performing shift factor calculations allows engineers to apply the principle of time-temperature superposition to accelerate material characterization. This technique shifts short-term experimental data collected at elevated temperatures to predict long-term performance at lower temperatures.
High-precision industries use this method to model the aging of sealants and composite structures.
Thermal Equivalence
Polymer structures exhibit similar deformation behaviors under long times at low temperatures as they do under short times at high temperatures. The output of shift factor calculations defines the translation distance required to align multiple isothermal curves into a single master curve. Engineers utilize either the Williams-Landel-Ferry equation or the Arrhenius equation to calculate these values.
Selecting the appropriate model depends on whether the material is above or below its glass transition temperature. When these shift factors are plotted against temperature, they reveal the activation energy of the molecular relaxation processes.
Model Calibration
Accurate curve fitting requires a complete dataset across a wide range of test frequencies. Instrument software automates the process of aligning the data, but manual adjustments are often necessary to resolve anomalies near transition points. If the shifts are applied incorrectly, the resulting master curve will overestimate the material life.
This calibration process provides the data necessary for finite element simulations of rubber and plastic parts.
Yield Prediction
Using these mathematical projections avoids the prohibitive costs of testing materials for several years. Simulated performance curves show whether a sealing elastomer will maintain its sealing force over a decades-long service life. Product development cycles are shortened when long-term behavior can be simulated in a few days.