Elastic moduli and damage mechanisms in 3D braided composites incorporating pultruded rods

This paper is aimed at elucidating the processing-property relationships of composites through experimental characterization. The fabrication processes involve three primary steps: the pultrusion of 1 mm diameter unidirectional rods, the formation of three-dimensional preforms incorporating the rods, and the impregnation of resin into the preforms. The use of the rods is intended to address crimp problems often seen in textile composites. A modified two-step set-up to incorporate the rods has been developed. A series of fabrics with varying braiding fibers, pitch lengths and braider sizes has been made to investigate their respective influence on processing and damage behavior. In comparison with conventional textile composites, the axial yams are straight and are packed in a rather dense and orderly manner. The composite moduli have been analysed by using the fabric geometry model and compared with the experimental results. The material characterization has been carried out on the basis of flexure, short-beam and compression tests. Damage configurations and accumulation for each loading case have been examined. Unique features include pull-in and push-out of the rods, which are the dominant modes in the short-beam tests. Buckling of the rods is the major damage in the flexure and compression tests. The buckled rods form kink bands analogous to fiber microbuckling in compressed unidirectional composites. The kink bands were found to propagate along the interlacing loops on the surface. These unique damage characteristics associated with the use of the rods are discussed in detail. (C) 1998 Published by Elsevier Science Limited.