Comparative analysis of bending test methods for evaluating the stiffness of high-performance yarns

This study evaluates yarn stiffness measurement techniques, focusing on 3-point and 4-point bending tests, the cantilever method, and the hanging pear loop method. It examines the effects of force and bending moment distribution on the stiffness of yarns such as recycled polyester, carbon, Kevlar, Vectran, and glass. The 4-point bending test consistently provides higher stiffness values than the 3-point method., reducing shear deformation and stress concentration. This difference is especially pronounced for carbon, Kevlar, and Vectran yarns, while glass and recycled polyester yarns exhibit more stable stiffness behavior. Alternative methods, like the hanging pear loop and cantilever techniques, also show differences based on stress application. A proposed buckling force evaluation for thin composite yarns with high fiber volume fractions demonstrates strong correlations with other methods. The findings emphasize the importance of selecting appropriate testing techniques based on yarn characteristics and accuracy requirements.