Experimental characterization of damage mechanisms of seamless net-shaped circular pre-form and overlapped stitched composite pipes

Seamless net-shaped circular composite pipes and overlapped stitched tubes were investigated for three-point and four-point bending to discover deformation mechanisms, damage initiation and propagation. The change in fiber tow angles is the main mechanism of dissipations of energy in seamless tubes. The overlaps were stitched utilizing a zigzag pattern, with a density of 0.0016 stitch/mm². The tubes’ load-displacement responses were considered as growing from a linear into nonlinear behavior due to two primary factors: (i) ovalization and consequent reduction of moment of inertia of the cross section, and (ii) damage due to plastic deformation and local buckling. Nonlinear behavior in load-displacement response was observed as semi-brittle and brittle hinges (kinks) were formed and developed. In overlapped stitched tubes, damage initiated due to matrix transverse cracks, and propagated to delamination and local buckling as the overlapped stitched zone experienced failure. Ovalization initiates more rapidly in seamless tubes as a result of less circumferential stiffness, compared to the initiation of ovalization in overlapped stitched tubes, but grows slower in seamless tubes as fiber reorientation occurs. The performance of the seamless composite tubes was superior to that of overlapped stitched tubes due to the elimination of debonding, braid breakage and sudden catastrophic failure.