Comparing Poisson Ratio Measurement of 3D Printed Continuous Fiber Reinforced Composites: Digital Image Correlation (DIC) vs. Video-Extensometer
Authors: Islam Md Zahirul , Rahman Md Atikur , Gibbon Luke , Hall Eric , Ulven Chad A. 
Published: August 2024

Abstract
Continuous fiber reinforced epoxy-based thermosets have been abundantly used in various applications, ranging from aerospace to ground transportation, over the past decades due to their superior strength, stiffness, lightweight, thermal stability, and chemical resistance. Conventional manufacturing of composites uses expensive molding tools for resin and fibers shaping, necessitating mass production to manage costs. In contrast, 3D printing, ideal for rapid prototyping and product development, allows for mold-less fabrication of fiber-reinforced composites, offering high design flexibility and cost-efficiency. Extrusion based 3D printing, with in-nozzle impregnation of fibers, is the most promising fabrication technique for continuous fiber reinforced composites. However, most thermoset resins are liquid at room temperature, thus requiring the print ink to possess shape-retention capability, or the use of additional instant curing mechanisms, such as thermal or light-assisted methods, to enable successful 3D printing. Light-assisted 3D printing of continuous carbon fiber-reinforced (CCFR) thermoset composites has shown significant potential for producing high-performance composites using readily available, low-viscosity thermoset resins. Accurate mechanical characterization of these 3D printed fiber-reinforced composites is vital for their utilization as cutting-edge materials. In addition to measuring the ultimate tensile strength of 3D printed composites, this study aimed to determine the Poisson’s ratio of the composites. The Poisson’s ratio is defined as the ratio of transverse strain to longitudinal strain within the elastic region of the tensile testing curve. This ratio helps describe how a material behaves under axial loading in terms of its lateral deformation. The determination of the Poisson’s ratio necessitates measuring both longitudinal and transverse strains during the tensile test of the specimen. Longitudinal strain can be measured using an extensometer, while measuring transverse strain can be challenging and cost prohibitive. This study illustrates the measurement of longitudinal and transverse strain through two distinct approaches: employing digital image correlation (DIC) and utilizing a video extensometer. To measure strains using DIC, four dot marks, two horizontally and two vertically, were placed on the specimens. Images of the specimens were captured at specific time intervals during testing. The dots’ displacement was measured using a virtual extensometer created within the DIC software to calculate longitudinal and transverse strain. Furthermore, the measured longitudinal strain was compared with the extensometer-derived longitudinal strain to validate the analysis. In the second approach, both longitudinal and transverse strains were measured using a video-extensometer. The Poisson’s ratios measured using both approaches were compared, and they exhibited similar values. Thus, the cost-effective DIC-based approach holds promise for accurately measuring the Poisson’s ratio of fiber-reinforced composites.