Measurement of strain distributions near the steel/epoxy interface by micro-Raman spectroscopy under tensile load condition

Micro-Raman spectroscopy was applied for evaluating the stress distributions in the vicinity of the interface of the steel/epoxy bonded joint under tensile loading condition. Herein, single-walled carbon nanotubes (SWNTs) embedded in a polymer can be used as a mechanical sensor, in which the position of the D* Raman band varies with the strain or stress transferred to SWNTs from the surrounding matrix. In order to evaluate the strain distributions, however, it is required to elucidate the effect of the multiaxial stress on the D* band shift, because a multiaxial stress field appears in the vicinity of the interface and, the validity of this method has been confirmed only under uniaxial loading condition. Hence, at first, the D* band shift of a bulk epoxy/SWNT composite was measured under biaxial loading condition using a cruciform-type specimen. It was found that the D* band shift could be standardized in terms of the strain in the polarized direction even though under the biaxial condition. Then, on the basis of the result, this method was applied for evaluating the strain distributions of the steel/SWNT composite bonded joints under uniaxial tensile loading condition. The observation indicated that the strain singularity appeared in the vicinity of the interface, similar to the results of the finite-element analysis, and the observed strain almost agreed with calculated one in the range of 0.03–10 mm distance from the interface.