A Shear-Lag Model for the Pullout Behavior of Pre-twisted Straight Fibers and Its Application in the Toughening Analysis of Twisted Fiber-Reinforced Composites

Abstract

The pre-twisted straight fiber exhibits exceptional mechanical properties, including high tensile stiffness and remarkable flexibility. In applications such as artificial muscles and fiber-reinforced composites, these fibers are typically embedded in an elastic matrix, functioning as key reinforcing or deformation-driven structural components. In this study, a shear-lag-based model is developed to describe the pullout behavior of a pre-twisted straight fiber from an elastic matrix, incorporating geometric nonlinearity and tension–twist coupling induced by large pre-twist angles. Based on this model, the stress transfer mechanism between the twisted straight fiber and the surrounding matrix is systematically analyzed. Furthermore, the derived force–displacement relationship during fiber pullout is employed to perform crack-bridging analysis, revealing the toughening mechanisms in twisted fiber-reinforced composites. Results show that pre-twist of fiber introduces distinct tension–twist coupling, which generates hoop interfacial shear stresses and allows the fiber to undergo larger tensile deformation. It leads to greater crack-opening displacements in the bridging zone and a significantly enhanced toughening effect. The present work provides new insights into the stress transfer and toughening mechanisms of twisted fiber-reinforced composites, offering valuable guidance for the design and fabrication of high-performance composite materials.