Dynamic Mechanical Analysis (DMA)

Dynamic Mechanical Analysis (DMA) is a testing technique and related analytical instrument that measures the physical properties of solids and polymer melts, reports modulus and damping, and is programmable to measure force, stress, strain, frequency and temperature. DMA is also described as rheology of solids and also Dynamic Mechanical Thermal Analysis (DMTA) when combining the information with temperature response.

Theory

DMA instruments apply an oscillating force (stress) and record an oscillating sample response. Modulus is calculated from the elastic response; e.g. sample response “in phase” with applied oscillatory stress. Damping is calculated from the viscous response; e.g. sample response “out of phase” with applied oscillatory stress.

Equipment

A sample is held in place between two grips or confining elements. Next, an oscillatory (dynamic) force is applied to the sample. This is applied using an electric motor moving rotationally (typically back and forth) and linearly (typically up and down), and contains a frequency (speed of oscillation), and contains a force (energy input into sample).

Resulting strain (displacement) is measured typically using LVDT measurement but can also be a force transducer. Typically plots Storage Modulus and Tan Delta (damping) vs. Temperature.

Typically calculates Tg (the “glass transition”, typically melting of amorphous phase) and Beta transitions (low temperature declines in Modulus).

Methods

  1. Temperature Scan: Modulus and damping is recorded as the sample is heated
  2. Frequency Scan: Modulus and damping is recorded as the sample is vibrated at increasing speeds
  3. Stress Scan: Modulus and damping is recorded as the sample stress is increased
  4. Strain Scan: Modulus and damping is recorded as the sample strain is increased
  5. Transient Testing (not oscillatory): Creep-Relaxation and Stress-Recovery
  6. Multiplexed Scan: Combinations of above methods

Samples: Solid ASTM test bars / Films / Fibers, Melted polymers, Fluids.