THE RUDIMENTS OF RHEOMETRICS AND
DYNAMIC MECHANICAL ANALYSIS (DMA)
The TA Instruments RSA-G2 Solids Analyzer

 
 

 
Rheology is the science concerned with the study of deformation and flow of materials. Dynamic mechanical analyzers such as the present-day TA Instruments RSA-G2 or the earlier Rheometrics RSA II  impose mechanical deformation on a specimen and measure the resulting stress-strain response.
Deformation is the relative change in shape of a body, or strain, under the influence of an external force, or stress.
Flow is a continuous relative change in shape per unit time, or strain rate, under influence of external stress. The RSA-G2 is in fact a linear rheometer, or a precision instrument which holds a specimen of the material of interest in a geometric configuration, controls the environment around it and applies and measures wide ranges of stress, strain, and strain rate. An alternative definition of rheology, relating more directly to the function of the rheometer, is the study of stress-strain or stress-strain rate relationships.
A material's response to external forces can be purely viscous or Newtonian behavior, purely elastic or Hookean behavior or a combination of the two. Nearly all commercial materials of interest respond with a combination of viscous and elastic behavior and are referred to as viscoelastic materials.
Amoung others, polymer scientists use the RSA-G2 and rheology theory to study these rigid-solid, soft-solid, and highly viscous liquid materials in terms of a variety of material parameters such as modulus, compliance, and elasticity.
Modulus is a measure of a material's overall resistance to mechanical deformation, whilst compliance is a measure of the material's ability to respond to deformational stress and elasticity is a measure of a material's ability to store and release deformational energy. 		 
Fig. 1. The stress-strain relationship exhibited by a classic Hookean solid is seen in the illustration above.
Copy of, "A Treatise on the Mathematical Theory of Elasticiy, 2nd Edn" provides the theory as illuminated in 1906 while the excerp- ted, "Historical Introduction" provides an overview.

The TA Instruments RSA-G2 DMA seen in Fig. 2 is capable of applying a variety of deformation types over a wide range of temperatures and time scales, or frequencies, and calculates these material parameters providing a wealth of information about a material s structural property relationships and material performance characteristics.
     
  1. An informative 10 minute movie on the TA Instruments RSA-G2.
  2. Complete information on the operation of the, "RSA-G2".
  3. A brochure on one of the RSA-G2's predecessors, the, "Rheometrics RSA II Solids Analyzer" which includes info on the capabilities of the Orchestrator software used to program and run the machine. Included in Ch. 5 is calibration information using the obsolete Rhios software.
  4. A brief piece on, "DMA Theory, Measurement Techniques and Applications" assembled from TA Instruments literature.
  5. "Characterization of Polymers Using DMA" from EAG Laboratories
  6. Copy of, "Dynamic Mechanical Analysis in Materials Science: The Novice's Tale" where the abstract reads in part, "There are a few useful textbooks and online materials available on dynamic mechanical analysis (DMA) but no short and succinct article that will be useful for a beginner."
  7. Copy of', "Dynamic Mechanical Analysis - A Practical Introduction".
  8. A presentation on, "DMA Calibration".
  9. An overview of, "High Strain Rate Testing" from Veryst along with a paper on an open-source 2D Digital Image Correlation (DIC) tool used to monitor in-situ, full-field deformation and strain responses of structures during loading.
  10. Preliminary info on, "Polyphene" a polymer/graphene nano-composite that, amoung many other things, will be PEARL, Inc's. new loudspeaker diaphragm material. ("Polyphenene" is a portmanteau of polymer and graphene.)
   


Fig. 2. The new RSA-G2 is the most advanced platform for mechanical analysis of solids. The separate motor and transducer technology of the RSA-G2 ensures the purest mechanical data through independent control of deformation and measurement of stress. It is capable of performing the most accurate DMA measurements as well as many additional experiments including creep and recovery, stress relaxation, stress ramps, strain rate ramps, iso-strain, iso-force, fatigue, multi-wave, arbitrary waveform, and dielectric thermal analysis. Being capable of such a broad range of solids analysis techniques, the RSA-G2 is uniquely positioned to address a wide range of applications from the R&D bench to the quality control lab.
This high-performance instrument is the fourth generation of dual-head (separate motor/transducer) mechanical analyzers and features a new, forced-convection oven for precise and accurate temperature control, an extensive array of sample holding geometries accommodating a wide range of sample shapes and stiffness, along with immersion testing capability. Additionally, the RSA-G2 doubles as a DETA, or Dielectric Thermal Analyzer, for stand-alone or simultaneous measurements.


Fig. 3. The Wikipedia article seen here provides good overview of the basic theory of dynamic mechanical analysis.

Fig. 4. Viscoelastic material trends as a function of driving frequency.
A DMA frequency scan showing the changes in a viscoelastic material's apparent morphology as driving frequency is varied. Forces at low frequencies allow polymeric chains time to relax and respond whereas forces at higher frequencies do not, with the result that materials exhibit higher Young's modulus.
Base graphic excerpted from: Dynamic Mechanical Analysis by Menard, 2nd edition.

 Fig. 5. There is no such thing as a negative value loss modulus, the DMA used to run that data was out of cal' and wasn't properly measuring phase angles. Even so, the illustration is somewhat instructive but will be replaced as soon as possible.
Base graphic retrieved from Wikipedia: Dynamic Mechanical Analysis Fig. 5
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