Principles of DMA

The usefulness of dynamic mechanical analysis over other forms of mechanical testing is the weU-defined periodic loading regime that is used. A periodic force or displacement, usually sinusoidal, is apphed to the sample and the resultant displacement or force is measured. This measurement includes the amplitude of the signals and also the phase difference between them [6]. 

Note that only instruments producing linear displacement will be considered here. Rotational instruments are required to produce torsional shear and these are mainly sold as rheometers and normally used for the analysis of liquids. Solid samples can be tested on such instruments with suitable bar clamps, but it maybe difficult to obtain measurements through the glass transition, due to stifihess range considerations. 

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Figure 5.1. Basic principle of DMA operation a sinnsoidal strain is applied to a sample, and the resnlting sinusoidal stress is measured. The strain and stress are out of phase (courtesy of TA Instruments).

S.2.3.2. Dynamic Mechanical Analysis and Rheology The principles of DMA are equally applicable to liquids. In fact, dynamic mechanical methods are an important way to determine (to a first approximation) flow viscosity data for thermoplastics in the melt state, in lieu of using more expensive and... 

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