Laser Raman Microscopy (LRM) is the only experimental technique able to measure stress or strain on individual fibres at the microscopic level. The Raman wavenumbers of certain vibrational modes of crystalline materials – such as carbon, SiC or Kevlar fibres embedded in composites – are stress/strain dependent, most frequently through a simple linear relation that can be established experimentally and constitutes the fibre-specific Raman Calibration Curve (RCC). The RCC of a fibre can be used reversely to translate shifts captured from the same type of fibre in a composite material, to stress/strain.
As an optical technique, LRM can only capture strain or stress on fibres that are accessible by the incident monochromatic light radiation. In an epoxy matrix system with a certain degree of transparency LRM can be focused on- and collect spectra even from fibres below the composite’s surface. Stress distributions along individual embedded fibres width then be established through the collection of consecutive spectra along the fibre length while 2-D stress maps can be built by scanning areas of the material with the Raman probe.
We use the LRM technique also for measuring the residual stress of matrices and fibres by comparison of the stress-free spectrum of the material to reference values, as well as for measuring bridging and pull-out stresses during fracture of brittle matrix composites. Value of micro-stress measured on intact fibres can be further used to calculate the interfacial shear stress (ISS) by means of a straightforward balance of shear-to-axial forces argument.
- "Effect of Fatigue on the Interface Integrity of Unidirectional Cf-Reinforced Epoxy Resin Composites" by C. Koimtzoglou, K.Dassios and C. Galiotis , Acta Materialia (2009), doi:10.1016/j.actamat.2009.02.038 Download the paper (pdf)
- “Thermal stress development in fibrous composites” by G.Anagnostopoulos, J.Parthenios and C.Galiotis, Materials Letters, 62/3 (2008), 341-345 Download the paper (pdf)
- “Effect of Stress and Temperature on the Optical Phonons of Aramid Fibers” by D. Bollas, J. Parthenios and C. Galiotis, Physical Review B (2006), 73, 094103 Download the paper (pdf)