An analysis of internal strains in unidirectional and chopped graphite fibre composites based on x-ray diffraction and micro Raman spectroscopy measurements

In this paper, the method for the determination of internal strains in polymer matrix composites from the strain measurements in the embedded sensors has been examined. Two types of strain sensors embedded in either chopped graphite fibre/epoxy matrix composite or unidirectional graphite fibre/polyimide matrix L composite were investigated. For the chopped fibre composite, we used Kevlar-49 fibres (similar to 10 mu m in diameter) as strain sensors, while aluminium inclusions with diameters ranging from 1 to 20 mu m were embedded in the unidirectional composite. Both composite plates with embedded sensors were subjected to external loads generated by a four-point bending fixture. Strains inside the sensors were measured using either x-ray diffraction (XRD) or micro Raman spectroscopy (MRS). A model based on the equivalent inclusion method (EIM) was used to extract the internal strains in composites from the measured strains inside the embedded sensors. It has been demonstrated that the geometrical features and the material properties of the embedded strain sensors may affect the accuracy of the extraction of the composite internal strains. The average interactions between the sensors were found to have only a minor effect on the strain determination in a composite.