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Raman is based on the inelastic scattering of monochromatic light. A laser excites the material, which is usually in the visible region of the spectrum. The frequency of scattered light is analysed compared with incident values. The technique is similar to IRS in determining the nature of molecular structures and is a complementary technique to IRS when characteristic frequencies are weak or for highly absorbing materials. Samples require minimal preparation, but need to be stable to high intensity light and contain no species that fluoresce when excited by visible radiation.
Raman spectroscopy can be used to determine near surface strain distribution. The use of the technique for determining strain relies on detecting frequency shifts in Raman modes (phonons) under mechanical stress. The frequency and intensity (peak height) of the scattered peak will change with stress. Frequency is dependent on the strain state of the material (i.e. residual strains will cause the peak to shift). In general, compressive stresses result in an increase in Raman frequency and conversely tensile stresses cause a reduction. The relation between strain or stress and the Raman frequency tends to be linear under uniaxial and biaxial loads. By monitoring the Raman scattering frequency at different positions on the sample, a strain map can be produced with a spatial resolution of 0.01 mm. Raman spectroscopy systems are capable of measuring frequency changes of ~0.02 cm-1.
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