A Comparison of the Mechanical Behaviour of Weft-Knitted Glass and Carbon Fiber Fabric-Reinforced Polyamide-12 Composites Produced with Commingled Staple Yarns

Abstract

The mechanical response of knitted glass- (GF) and carbon fiber (CF) fabric-reinforced polyamide-12 (PA-12) composites was compared both under static (tensile, flexural) and dynamic (perforation impact) conditions. The GF- and CF-content of the composites produced by hot pressing of 8 layers of planar weft knits containing staple reinforcing GF or CF and PA-12 fibers was practically the same, viz ca. 50 vol%. Dynamic-mechanical thermoanalysis (DMTA) showed a strong stiffness anisotropy: the stiffness of the composites in the wale-direction (W) was markedly higher than in the course-direction (C). The same strong anisotropy was also found for the static tensile and flexural characteristics (stiffness and strength). The mechanical anistropy and the large difference between the stiffness and strength due to tensile and flexural loading were traced to the structure of the knits (“unbalanced” reinforcement) and their non-planar structure in the composite sheets. The mechanical performance of the knitted CF-reinforced composite was superior to the GF-reinforced version under dynamic perforation impact conditions as well.