Local Stress Measurement in Notched Sapphire by Raman Microspectroscopy

Abstract

A new system of Raman microspectroscopy was developed to measure the residual stress in a local area with about 2μm diameter in ceramics. The system was applied to a single crystal sapphire in order to evaluate the capability of the system. The shift of two Raman spectra, 645 and 418cm^-1, of A_1g mode was proportional to the lattice strain, ε₃₃, along the c-axis of trigonal crystals. The proportional constant was larger for 418cm^-1 than for 645cm^-1 spectrum. The distributions of loading strains due to four-point bending were measured across the minimum ligament of single-edge-notched specimens by scanning the focused Ar⁺ laser beam. The strain concentration near the notch root was successfully detected by Raman microspectroscopy. The measured distributions of strains agreed very well with the results of a finite element analysis. The developed system of Raman microspectroscopy was applicable to the strain measurement in the area having a steep gradient as observed near the crack tip in ceramics.