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In the technique of Dynamic Mechanical Spectroscopy a material (usually a slab of polymer) is exposed to a periodical deformation. The deformation can be in tensile, compression or bending mode but torsional deformations are the most practical ones because they tend to produce a linear response more readily. In other words the deformation (strain) can be described a linear function of the applied force (stress). The coefficient that links the two is called the modulus:
Strain = modulus * stress
In DMS the modulus is measured as a function of the frequency of the deformation and/or the temperature of the experiment. Because the temperature is typically varied in a systematic way the technique is also known as Dynamic Mechanical Thermal Analysis. (DMTA)
The modulus is generally a complex number, because when the applied stress is sinusoidal (i.e. a single frequency is applied) the strain can lag behind in time. The phase shift is due to viscous as opposed to elastic effects.
When the material undergoes a glass transition these losses reach a maximum. The temperature at which this happens, however, is frequency dependent.
The mechanical excitation does not have to be a single sine wave, in fact more than one frequency response can be measured simultaneously in a process called multiplexing. Often a block wave is used rather than a sine wave.
This is an application of the Fourier transform principle. A requirement for its application is that the response is linear for all frequencies.
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