Torsional cracks development in carbon-fiber 3-D braided composite tubes

Torsional cracks development is key to design high torsional strength tubes, especially for heterogeneous materials such as fiber reinforced composite tubes. Here we report torsional cracks initiation and development in carbon-fiber 3-D braided composite tubes. 3D digital image correlation (3D-DIC) and micro computed -tomography (micro-CT) techniques were used to observe inner crack distributions. The correlation between cracks and braided structure is revealed. We found that the torsional damage propagates along the helix of braided bundles in torsion direction and inhibits in vertical direction within repeated braid units. The bundles are subjected to axial tension in the torsion direction and axial compression in vertical direction. This leads to slippage and cross-section deformation in braided yarns at intersection points. Strain distribution has the same periodic form with the braided structure, i.e., low-strain locates on fiber bundles and high-strain concentrates on the edges and interlacing gap of bundles. Interface cracks are main damage modes and induces final torsional failure both after resin and yarn damages.