Meso-scale strain measurements in fiber reinforced composites

Effective bulk properties of fiber-reinforced composites can be determined from individual constituent properties through spatial homogenization. Homogenization, in this regard, is conducted over a specifically selected volume of the material that is sufficiently small to capture complex local deformation response, while large enough to encompass all individual components, i.e. fibers and matrix. The physical dimension of such homogenization volumes is a key parameter in multiscale studies. Experimental measurement of the length scale at which the transition from micro to macroscale response occurs is challenging. In the present study, we propose a systematic approach to estimate the physical dimensions of a micro-to-macro transition length scale in terms of the number of fibers in the transverse plane of a cross-ply laminate subjected to remote tensile load. In-house fabricated cross-ply composite samples are loaded in tension in a miniature tensile frame inside a scanning electron microscope, while images are acquired from a small area of interest located on the transverse ply. Digital Image Correlation (DIC) is utilized to obtain full-field strain distribution within the area of interest at various global stress/strain intervals. Spatial averaging of strains at meso-scale is used to determine the micro-to-macro transition scale.