Correlating microstructure to thermomechanical response for large-format printed carbon-fiber reinforced composites

Abstract

This study correlated the thermally induced distortion of large-format additive manufacturing (LFAM) printed composite structures to the alignment of reinforcing fibers. During LFAM material extrusion, shear forces near the nozzle wall align reinforcing fibers in the print direction (x-axis). This produces a complex microstructure comprised of a “shell” of highly aligned fibers near the outer edge of the bead and a “core” of randomly oriented fibers in the center. Given the inherent anisotropy of high aspect ratio reinforcing fiber materials, the orientation of these fibers dictates thermomechanical response of the LFAM part as it is heated to elevated temperatures. Understanding the extent and cause of this distortion is crucial for applications such as autoclave composite tooling so that resulting parts can maintain proper dimensional tolerance. This study characterized the microstructure of LFAM parts using microscopy, method of ellipses, extreme small-angle x-ray scattering (ESAXS), and micro-computed tomography scanning. These results were compared for type, quantity, and cost of data. Each technique captured similar trends in microstructural data, which established ESAXS as a viable option to characterize LFAM microstructure at much lower costs without sacrificing through-thickness measurement. Digital image correlation (DIC) was used to measure thermomechanical response of the LFAM part and correlate spatial variation in thermomechanical data to microstructural trends. Results showed higher y-direction expansion occurred near bead edges where fibers were highly oriented in the x-direction. Findings from this work address the knowledge gap in compensation modeling so that LFAM tooling can maintain correct dimensions during thermal cycling.