Braid-trusion of hollow thermoplastic composites using an expanding mandrel approach

Braid-trusion involves the combination of a braiding machine with a pultrusion line to continuously produce composite structures with a constant cross-section and angle-oriented fibres. During the braid-trusion process, braids are subjected to tension and compression, which leads to braid deformation and fibre orientation variations. Successful braid-trusion of tubular structures requires a braid design methodology that accounts for braid deformations during its passage in the pultrusion die. Here, we present a braid design methodology for square hollow pultruded beams reinforced by triaxial braids. It is proposed that the internal perimeter of the braid, constrained by the braiding and pultrusion mandrels, must expand during pultrusion to account for braid consolidation. Two braid designs are proposed. The first has a constant braiding and pultrusion mandrel perimeter. The second has a braiding mandrel perimeter that is 0.9 times that of the pultrusion mandrel, referred to as the expanding mandrel approach. The design demonstrated the effect of an expanding mandrel in reducing fibre waviness by achieving similar braiding yarn lengths. Glass fibre/polyethylene terephthalate composites were pultruded with the constant and expanding mandrels. The constant mandrel braid-trusion failed while the expanding mandrel succeeded.