Deformation and failure mechanisms of braided composite tubes in compression and torsion

Abstract The deformation and fracture behaviour of glass fibre–epoxy braided circular tubes is examined experimentally and theoretically for the loading cases of compression, torsion, and combined tension–torsion and compression–torsion. Failure maps are produced for compression and for torsion to summarise the effect of braid microstructure upon failure mode and upon the mechanical properties of the braid, including yield strength, modulus, strain to failure and energy absorption. In compression, two competing mechanisms are observed: diamond shaped buckling of the tube and fibre microbuckling. In torsion and in combined compression–torsion, the tubes fail by fibre microbuckling. The initiation and propagation stresses for diamond shaped buckling, and the critical stress for fibre microbuckling are successfully predicted using simple micromechanical models. Drawing upon the available experimental data, yield surfaces are constructed for in-plane loading of the braid, and a comprehensive mechanism map is constructed to illustrate the dependence of failure mode upon braid geometry and loading direction.