Multi-fiber strains measured by micro-Raman spectroscopy: Principles and experiments

• An original theoretical model between the nominal Raman shift of fiber bundle and the multi-fiber strains was established.
• A measurement principle of application of micro-Raman spectroscopy to multi-fiber strains in a fiber bundle was given and verified by dual-fiber tensile experiment.
• The Raman shift as a function of strain is obtained using digital image correlation and micro-Raman spectroscopy. The FWHM for the Kevlar 49 fiber showed a quadratic increase with the fiber strain.

Based on widely used axial strain measurement method of Kevlar single fiber, an original theoretical model and measurement principle of application of micro-Raman spectroscopy to multi-fiber strains in a fiber bundle were established. The relationship between the nominal Raman shift of fiber bundle and the multi-fiber strains was deduced. The proposed principle for multi-fiber strains measurement is consistent with two special cases: single fiber deformation and multi-fiber deformation under equal strain. It is found experimentally that the distribution of Raman scattering intensity of a Kevlar 49 fiber as a function of distance between a fiber and the laser spot center follows a Gaussian function. Combining the Raman-shift/strain relationship of the Kevlar 49 single fiber and the uniaxial tension measured by micro-Raman spectroscopy, the Raman shift as a function of strain was obtained. Then the Raman peak at 1610 cm−1 for the Kevlar 49 fiber was fitted to a Lorentzian function and the FWHM showed a quadratic increase with the fiber strain. Finally, a dual-fiber tensile experiment was performed to verify the adequacy of the Raman technique for the measurement of multi-fiber strains.