Editing Dynamic mechanical analysis (section)

===Types of analyzers===
There are two main types of DMA analyzers used currently: forced resonance analyzers and free resonance analyzers.  Free resonance analyzers measure the free oscillations of damping of the sample being tested by suspending and swinging the sample.  A restriction to free resonance analyzers is that it is limited to rod or rectangular shaped samples, but samples that can be woven/braided are also applicable.  Forced resonance analyzers are the more common type of analyzers available in instrumentation today.  These types of analyzers force the sample to oscillate at a certain frequency and are reliable for performing a temperature sweep.

[[Image:Two types of DMA analyzers.png|thumb|left|Figure 4. Torsional versus Axial Motions.]]

Analyzers are made for both stress (force) and strain (displacement) control.  In strain control, the probe is displaced and the resulting stress of the sample is measured by implementing a force balance transducer, which utilizes different shafts.  The advantages of strain control include a better short time response for materials of low viscosity and experiments of stress relaxation are done with relative ease.  In stress control, a set force is applied to the sample and several other experimental conditions (temperature, frequency, or time) can be varied.  Stress control is typically less expensive than strain control because only one shaft is needed, but this also makes it harder to use.  Some advantages of stress control include the fact that the structure of the sample is less likely to be destroyed and longer relaxation times/ longer creep studies can be done with much more ease.  Characterizing low viscous materials come at a disadvantage of short time responses that are limited by [[inertia]].  Stress and strain control analyzers give about the same results as long as characterization is within the linear region of the polymer in question.  However, stress control lends a more realistic response because polymers have a tendency to resist a load.<ref name="book">{{cite book|last=Menard|first=Kevin P.|title=Dynamic Mechanical Analysis: A Practical Introduction|publisher=CRC Press|year=1999|chapter= 4 |isbn=0-8493-8688-8}}</ref>

Stress and strain can be applied via torsional or axial analyzers.  Torsional analyzers are mainly used for liquids or melts but can also be implemented for some solid samples since the force is applied in a twisting motion. The instrument can do creep-recovery, stress–relaxation, and stress–strain experiments.  Axial analyzers are used for solid or semisolid materials.  It can do flexure, tensile, and compression testing (even shear and liquid specimens if desired).  These analyzers can test higher modulus materials than torsional analyzers.  The instrument can do [[thermomechanical analysis]] (TMA) studies in addition to the experiments that torsional analyzers can do. Figure 4 shows the general difference between the two applications of stress and strain.<ref name="book" />

Changing sample geometry and fixtures can make stress and strain analyzers virtually indifferent of one another except at the extreme ends of sample phases, i.e. really fluid or rigid materials.  Common geometries and fixtures for axial analyzers include three-point and four-point bending, dual and single cantilever, parallel plate and variants, bulk, extension/tensile, and shear plates and sandwiches.  Geometries and fixtures for torsional analyzers consist of parallel plates, cone-and-plate, couette, and torsional beam and braid.  In order to utilize DMA to characterize materials, the fact that small dimensional changes can also lead to large inaccuracies in certain tests needs to be addressed.  Inertia and shear heating can affect the results of either forced or free resonance analyzers, especially in fluid samples.<ref name="book" />