The role of loops in 3D fabric composites

The role of interlacing loops on the performance of 3D composites has long been overlooked. One possibility is that loops occupy only a small fraction of the volume, yet experiments have shown that their influence can be disproportionate in determining damage behavior. The key lies in the fact that loops not only build the 3D network of reinforcements by connecting through-thickness yarns, they also cover and protect the composites from external attack. In the present work, the role is examined experimentally. Two types of three-axis woven composites were made, one combining solid carbon/epoxy rods along the axial direction, and the other employing carbon yarns in all axes. These composites are identical in loop patterns but distinct in yarn alignment. Composite geometry was first examined by introducing two unit cells that describe internal yarns and surface loops. Two terms describing the coverage of surface loops were defined and calculated for the fabrics under investigation. In order to elucidate the influence of loops, some specimens were ground to remove all loops on surfaces. Material characterizations based upon flexure and Izod impact tests were then carried out. The loop-retained and loop-removed specimens provide a sharp contrast in the configuration of damage. On the basis of the comparison, the role of loops has been examined. The results show that loops can provide at least two functions that enhance composite durability and damage tolerance.