Understanding the Fundamental Mechanisms Governing Tensile Strength of High-Performance Small-Scale Carbon/Glass Fibers

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This project is jointly funded by the Mechanics of Materials and Structures program and the Established Program to Stimulate Competitive Research (EPSCoR). High performance carbon and glass fibers are widely used as reinforcements in composite material systems for automotive, aerospace and defense applications. The tensile strength of commercial fibers is significantly less than its theoretical limits. The composite systems are often overdesigned, thus any increase in the fiber tensile strength can yield significant cost and weight savings. Modifications of fiber surface treatment (sizing) during manufacturing is a potential route to enhance the fiber strength. Single fiber tensile testing at millimeter-scale is typically used to characterize the effect of sizing on the fiber strength. However, the longitudinal tensile failure of a composite is governed by the fiber strength distribution and defects at microscale lengths. This award supports the fundamental research that overcomes current challenges in characterizing the tensile strength of the fibers at the microscale using experimental and data-driven computational methods. Besides the scientific understanding, this project will also provide a guiding template for the fiber manufacturing process through controlled surface treatment. A direct consequence of improving the tensile strength would be lightweight structures for applications in aerospace, automotive, and sports equipment sectors. As part of this project, a specific effort will also be aimed at recruiting graduate and undergraduate students from under-represented groups through the Society for Women Engineers at the University of South Carolina. Furthermore, the experimental setup developed in this research will be incorporated into a lab course for undergraduate students.A comprehensive understanding of the discrepancy between experimental tensile strength of commercial fibers and its theoretical limits has been elusive, and whether intrinsic fiber strength follows a Weibull statistical distribution remains an open question. This research aims to elucidate the fundamental mechanisms that govern the tensile strength of fibers at microscale gage lengths. Experimental and data-driven techniques will be employed to study the strength distribution of the fibers at microscales. Microscale gage lengths will be accessed by developing a novel in situ transverse loading experiment on single fibers under scanning electron microscope combined with micro-digital image correlation. Data-driven machine learning techniques will be applied to establish the scaling laws of strength. This research will provide new insights into the functional form of the survival probability and strength-controlling mechanisms in fibers influenced by fiber sizing. This new fundamental knowledge of processing (sizing)-structure (defect distribution)-property (tensile strength distribution) relationship at microscale lengths will enable the establishment of scaling laws for strength and will serve as a guide for fiber manufacturing process to enhance the fiber tensile strength.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.