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

The deformation and fracture behaviour of glass fibre-epoxy braided circula r tubes is examined experimentally and theoretically for the loading cases of compression, torsion, and combined tension-torsion and compression-torsi on. Failure maps are produced for compression and for torsion to summarise the effect of braid microstructure upon failure mode and upon the mechanica l properties of the braid, including yield strength, modulus, strain to fai lure and energy absorption. In compression, two competing mechanisms are ob served: diamond shaped buckling of the tube and fibre microbuckling. In tor sion and in combined compression-torsion, the lubes fail by fibre microbuck ling. 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 experimenta l data, yield surfaces are constructed for in-plane loading of the braid, a nd a comprehensive mechanism map is constructed to illustrate the dependenc e of failure mode upon braid geometry and loading direction. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.