National Physical Laboratory

Dynamic Mechanical Analysis

Dynamic Mechanical Analysis is a technique that measures the modulus (stiffness) and damping (energy dissipation) of materials as they are deformed under periodic stress. Polymeric materials, which are viscoelastic in nature, are subject to time, frequency and temperature effects on mechanical properties which can be analysed by this method.

Material properties which can be measured by the technique are:

  • Modulus
  • Damping
  • Glass transition
  • Softening temperature
  • Degree of cure
  • Viscosity
  • Gelation
  • Sound absorption
  • Impact resistance
  • Creep
  • Stress relaxation
DMA curves indicating analysis points
DMA curves indicating analysis points. The inflection point in the storage modulus curve is the preferred analysis point for Tg


DMA can be used to analyse a wide variety of materials in different forms. It finds application in Research and Development for the investigation of material structure, their development, selection for specific end-uses, comparative evaluations and material lifetime predictions. It is also used in Quality Control for process simulation and optimisation, incoming and in process material certification and troubleshooting.

Design engineers working with new materials need to understand their properties and to be able to predict their performance over the lifetime of the specific application. Often short term test information is used to project long term high temperature performance. These may have severe limitations. DMA can continuously monitor material modulus at different applied frequencies and hence provides a more realistic indication of properties.

Dynamic Mechanical Analysis (DMA) Instrumentation
Dynamic Mechanical Analysis
(DMA) Instrumentation

DMA allows a quick comparison of material properties between two dissimilar or two similar materials processed differently. Since samples can be tested in the form of bars, films, fibres and viscous liquid samples, the technique comes closest to the actual application envisaged and results therefore reflect the actual application.

The technique can be used to determine the linear viscoelastic region of viscoelastic materials such as silicone gums. A run at constant temperature should yield constant modulus with increasing stress or strain. A departure from this value gives the limit of linear viscoelasticity.

Curing behaviour such as onset of cure, gel point and vitrification can be studied by supporting the thermosetting liquid on a structure such as a fibreglass braid and determining modulus as temperature is increased.

Films as thin as 5 microns can be studied. Weaker transitions due to molecular motions of side groups or smaller sections of polymer chains are revealed. Effects of film orientation can be studied. Stress/strain curves can be generated for films and fibres to a better accuracy than is possible with standard physical testing devices, since the mass and inertia of the clamps is much smaller.

Monofilament fibres can be analysed in stepwise isothermal multifrequency sweeps to yield curves which can be used for prediction of a variety of mechanical properties.

Damping transitions can be used to measure desirable end use properties such as noise abatement, vibration dissipation and impact strength.

The most important application of DMA is in projecting material behaviour using superpositioning. Viscoelastic materials exhibit behaviour during deformation and flow which is both time and temperature dependant. Fatigue and creep properties are of great importance to design engineers wishing to use these materials. To analyse long term ageing of these materials they must often be subjected to the actual time periods under the effect under study. DMA allows treatment of data designated as the method of reduced variables or time-temperature superposition which overcomes the difficulty of extrapolating limited laboratory tests at shorter times to longer time, actual application conditions. For this application, the samples are run simultaneously at several frequencies at elevated temperatures. The data is then shifted to give a master curve where the material property of interest can be predicted over a broad time scale. Results show good fit to William-Landel-Ferry (WLF) equation in most cases.

International Interlaboratory Trial

As part of VAMAS Technical Work Area 5 (Polymer Composites), anĀ international round-robin exercise is currently in progress for TgĀ determination by DMA


Contact

Sam Gnaniah
Tel: +44 (0)20 8943 6174

Last Updated: 26 Jun 2015
Created: 24 Jul 2007

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