Damage evolution in multilayer braided composite tubes under torsion studied by in-situ X-ray Computed Tomography (CT)

In this study, the failure mechanisms of tubular braided carbon fibre reinforced polymer (CFRP) composites under torsion are investigated by time-lapse synchrotron X-ray CT imaging combined with an in situ torsion testing. Fibre tracing is used to identify and distinguish the fibre tows in different orientations. To our knowledge, this is the first time the sequence of damage has been tracked in 3D for torsional failure of multilayer braided CFRP tubes. Damage is first observed at 1% shear strain. Soon after this point the macroscale stiffness falls, and the peak stress is reached (~1.25%). The macroscale structural stiffness decreased with the progressive accumulation of the gap of inter-tow debonding causing kinking bands when the stress reached the peak value. With increasing torsional strain (a) damage initiates as radial intra-tow cracking (predominantly) and inter-tow debonding (to a lesser extent), then (b) propagates by inter-tow debonding and peripheral intra-tow cracking, (c) kink bands form due to the loss of lateral constraint afforded by the developing inter-tow debonding beyond 1.5% shear strain. Such 3D image sequences can be used to design better architectures and to set up image based finite element models of the damage sequence and other braid architectures.