Stress determination in silicon wafers and a micromechanical study of compression failure in graphite-reinforced epoxy using micro-Raman spectroscopy

The determination of stress states in silicon wafers and the micromechanics of compression failure in graphite/epoxy composites are two diverse problems which can be addressed using micro-Raman spectroscopy (MRS). Previous use of MRS to determine stresses in silicon was limited to stress magnitudes by assuming the stress state. A new technique has been developed to predict the unknown in-plane stress state and the magnitude of the stress components in (111) silicon wafers using MRS. The approach is based on analyzing the combined signal from the initially degenerate peaks of the F{dollar}sb{lcub}rm 2g{rcub}{dollar} mode in silicon as a function of the angle between the incident laser polarization and the polarization selected from the scattered beam using an analyzer. The peak position of the combined signal when plotted as a function of the angle was found to contain the information required to estimate the magnitude of the individual stress components in the plane-stress condition. The technique can also be extended to study graphite fibers with certain limitations.; MRS was also used to study the compression failure of graphite/epoxy composites by kink band formation. Graphite fibers in an epoxy matrix with different interfaces were studied at a micro and macro level using MRS, scanning electron microscopy, mechanical testing and optical microscopy. The results showed that kink bands are initiated by a damage zone comprised of a few crushed and broken fibers which caused a local matrix shear instability. The damage zone angle was found to be different from the kink band angle and changed with the nature of the interface. The damage angle was used instead of the fiber misorientation angle in the simple composite mechanics approach to predict composite compression strength. Different failure mechanisms in these composites were predicted, for different damage angles. A simple geometric model was developed to explain the damage angle in terms of the fiber volume fraction and the interface properties and the model agreed well with previous work on the role of the interface in kink band formation.