Torsional damage behaviour of double-layer braided composite tubes with varying braid angles studied by in situ synchrotron X-ray computed tomography (CT)

Braid angle is a key design variable for braided composite tubes, yet its role in governing damage evolution in multilayer tubular architectures under torsion remains insufficiently understood. Here, time-resolved in-situ synchrotron X-ray computed tomography (CT) has been used to investigate the torsional behaviour of double-layer 1/1 braided CFRP tubes with braid angles of 30° (DL30), 45° (DL45) and 60° (DL60). Increasing braid angle progressively increased wall thickness and tow crimp, while the effective fibre volume fraction varied only modestly and non-monotonically. These architectural changes produced distinct mechanical responses: DL45 and DL60 exhibit higher tube-level torsional stiffness whereas DL30 displays the highest shear modulus and torsional strength. Damage accumulation is strongly braid-angle dependent. At higher strain levels, intra-tow cracking, inter-tow debonding and inter-laminar debonding all markedly reduce as braid angle increases. The predominant damage mechanism also changes: DL30 develops a rapidly connected damage network (intra-tow cracking and inter-tow/laminar debonding) promoting microscale fibre kinking/buckling and further fibre breakage, leading to an abrupt post-peak loss of strength; DL45 showed more distributed damage accumulation and the highest post-peak strength; while DL60 exhibited delayed, weakly connected damage evolution with the lowest overall level of damage accumulation. These results suggest that angle-graded multilayer designs could combine lower-angle regions for reduced tow crimp, higher shear modulus and strength, with intermediate or higher-angle regions for damage suppression and stable post-peak performance.