The role of loops in 3D fabric composites

The role of interfacing loops on the performance of 3D composites has long been overlooked. One possibility is that loops occupy only a small fraction of the volume, pet experiments have shown that their influence can be disp roportionate 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 al l axes. These composites are identical in loop patterns but distinct in yam alignment. Composite geometry was first examined by introducing two unit c ells that describe internal yarns and surface loops. Two terms describing t he coverage of surface loops were defined and calculated for the fabrics un der investigation. In order to elucidate the influence of loops. some speci mens were ground to remove all loops on surfaces. Material characterization s based upon flexure and Izod impact tests were then carried out. The loop- retained and loop-removed specimens provide a sharp contrast in the configu ration of damage. On the basis of the comparison, the role of loops has bee n examined. The results show that loops can provide at least two functions that enhance composite durability and damage tolerance. (C) 2000 Elsevier S cience Ltd. All rights reserved.