Viscoelastic Response
Characterization methods apply an oscillating force to a material to determine its viscoelastic properties as a function of temperature or frequency. In structural testing, dynamic mechanical thermal analysis yields the storage modulus and the loss modulus. These values describe the ability of the material to store energy versus the energy lost as heat.
The technique is sensitive to the molecular motions that occur during the glass transition.
Phase Lag
The instrument subjects a specimen to a sinusoidal strain and measures the resulting stress. Because materials have both elastic and viscous components, dynamic mechanical thermal analysis detects a phase lag between the input and the output. A purely elastic material would show no lag, while a viscous one would show a delay.
The loss tangent, calculated as the ratio of the loss modulus to the storage modulus, identifies the point where the material is most efficient at dissipating energy.
Operational Mode
Testing occurs in various geometries including bending, tension, or shear depending on the stiffness of the sample. For a thin film, a tension clamp is appropriate, whereas a rigid thermoset might require a three point bending setup. During the sweep, dynamic mechanical thermal analysis tracks how the stiffness drops by several orders of magnitude as the polymer passes through its glass transition.
This information allows engineers to set the safe operating limits for components that must maintain their shape under load. It also reveals secondary transitions that are too small for other thermal methods to detect.
Calibration Requirement
Regular checks using standard samples with known properties confirm the accuracy of the force and displacement sensors. This verification ensures that the reported modulus values are consistent across different machines and laboratories.
