Stress Distributions In Micro-region Of Carbon Fiber Composites-by Carbon Nanotube Raman Strain Media

Raman Spectroscopy has many special advantages of nondestructive, non-contact, high spatial resolution, broaden spectrum range, and Raman shift unaffected by the frequency of laser source. Furthermore, Raman spectrum is sensitivity to strain/stress in the sample. Advanced composites and micro-devices which possess micro/nano-structure have been used widely, thus, how to measure the mechanical quantities such as stresses and strains of these micro-elements attracts great interest. As Raman active materials, multiwalled carbon nanotubes(MWCNTs) are used as Raman strain media, when MWCNTs are under strain, this may cause a Raman shift, by which strain may be possibly direct measure. The underlying basis of Raman spectroscopy for strain measurement is that Raman shift reflects the strain information. Therefore, stress distributions of micro-region in carbon fiber composites can be detected by Raman strain media. The main contents and conclusions show that:(1)785nm laser wavelength is used to detect MWCNTs to avoid fluorescence background; and no more than5%laser power is used to detect MWCNTs to avoid heat effect.(2) Within micro-region of carbon fiber composites, stress near carbon fiber is not a specific value. Stress distributions are found by point-to-point Raman detection both in the directions of perpendicular to and parallel to carbon fiber axis. Stress remains unchanged within the area far away from eight times of diametric distance in both directions. According to G’band shifts of MWCNTs within different positions of matrix, stress distributions of different distances are known by Raman mapping. Residual stress is negative under zero strain, namely composite is in compression. With the increment of tensile strains, composite itself overcomes residual stress and stress is positive when tensile strain is1.0%. (3) Micro-areas nearby a circular hole are detected by point-to-point Raman spectroscopy, and the experimental results are found in good agreement with elastic mechanics theory. CNTs strain media coefficient ((?)media) can be obtained by measuring the Raman-shifts in two polarization directions(0°and90°). In order to investigate the stress distributions in the micro-regions around the circular hole, these areas are further detected by Raman area mapping, and the distributions of G’ band shifts are obtained under zero and0.1%strains respectively.