The hypersonic frequency range Brillouin spectroscopy

Brillouin spectroscopy enables the elastic constants of pol3oners to be determined at frequencies of several gigahertz, i.e. three orders of magnitude higher than those pertaining to ultrasonic measurements, which are known as h3qiersonic frequencies. 

The principle of the method is to use Fabry-Perot spectroscopy to measure the frequency shift in laser light scattered through 90° after passage through a parallel polymer sheet. 

For an isotropic pol3mer the h3qDersonic sound velocity is determined as a function of direction, and the elastic constants are obtained by fitting the data to a set of equations known as the Christoffel equations, which essentially relate the values of Vs to the stiffness constants Cs through Cs = pV, where p is the density [24]. For a detailed discussion of this technique the reader is referred to papers by Kruger, Pietralla and co-workers [25,26]. 

Leaderman, H., Elastic and Creep Properties of Filamentous Materials and Other High Polymers, Textile Foundation, Washington, DC, 1943. 

Turner, S., Mechanical Testing of Plastic (2nd edn), G. Godwin, Harlow, 1983. 

Hadley, D.W., Pinnock, PR. and Ward, I.M. (1969) Anisotropy in oriented fibres from synthetic polymers. J. Mater. Sci., 4, 152. 

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