Measurement and modeling of internal stresses at microscopic and mesoscopic levels using micro-Raman spectroscopy and X-ray diffraction

In this study, we use X-ray diffraction (XRD) and micro-Raman spectroscopy (MRS) to measure internal strains in sensors embedded in polymer matrix composites. Two types of strain sensors embedded in either chopped graphite fiber∕epoxy matrix composite (MRS) or unidirectional graphite fiber∕polyimide matrix composite (XRD) were investigated. For XRD measurements, the sensors were in the form of spherical aluminum inclusions with diameters ranging from 1 to 20 μ m. Due to large cross section area of an incident X-ray beam, only average stresses are reported using the XRD approach. Complementary to XRD experiments, MRS was pursued to measure internal strains in Kevlar-49 fibers embedded in chopped graphite fiber∕epoxy matrix composite. In recent years, MRS as an experimental tool for microstrain measurements has drawn considerable attention mostly due to its excellent spatial resolution. The resolution of MRS typically ranges between 1 and 10 μ m, which means that strains can be measured in individual sensors. The principle of this method relies on a change of certain molecular vibration frequencies as a result of an applied stress. Several examples are presented and discussed to demonstrate the potential of combining micro and macrostrain measurements and modeling to capture the stress distribution in heterogeneous materials.