Effect of profile and reinforcing the surface layers on bending behavior of 3D multi-cell spacer weft-knitted composites

3D multi-cell spacer-knitted reinforced composites are composed of knitted layers which are joined together by multiple fabric connecting layers. Despite numerous capabilities, they demonstrate inferior bending performance due to their highly looped construction as well as low fiber volume fraction. Bending characteristics could be enhanced by inserting the warp and weft yarns through the fabric’s surface layers in wale and course directions. This research aims to investigate the role of reinforcing warp and weft yarns and structure profile on bending behavior of the composites produced from 3D multi-cell spacer-knitted fabrics. 3D multi-cell spacer-knitted fabrics were produced with rectangular cross-sectional shape in single- and double-decker profiles on a computerized flat-knitting machine. The surface layers of both 3D multi-cell spacer-knitted fabrics were reinforced by glass straight yarns in course and wale directions. The produced fabrics were used to fabricate thermoset 3D composites using epoxy resin via vacuum-assisted resin-transfer method. Composite samples were subjected to three-point bending load. Statistical analysis revealed that the composite profile and presence of the reinforcing warp and weft yarns affect significantly the flexural properties of 3D-knitted composites. Accordingly, introduction of the reinforcing straight yarns causes an increase in maximum bending force by 75 and 62.5% for single and double-decker structures, respectively. In order to understand how the tensile and compressive stresses are distributed on the 3D-knitted composite structure as well as to predict their mechanical behavior, a macro-mechanical model was created in the ABAQUS software.