Microscale residual strains in monolayer unidirectional fiber composites

Thermal residual stress is common in fiber reinforced metal matrix composites and significantly affects their mechanical properties. The calculation of these stresses typically assumes continuum mechanics holds. As the fiber diameter in most composites approaches the grain size of the matrix, the continuum assumption can become invalid. Since the mechanical properties depend on the residual strain state of the composite, it is therefore necessary to determine the residual strains using spatially resolved microscale measurements. In order to quantify these residual strains, X-ray diffraction of both the fiber and matrix was employed using a sampling volume less than the fiber diameter. Results were compared to macroscopic measurements including many fibers. The measurements were performed in transmission using high-energy synchrotron X-rays yielding strains representative of the entire thickness of the composite. Evolution of these residual strains after application of load was also investigated. Spatial variations in residual strains showed significant deviation from the macroscopically observed residual strains.