Multiscale investigation of torsional failure mechanisms in 3D braided carbon fiber composite shafts via integrated CT, 3D-DIC, and AE analysis

This study systematically investigated the torsional damage evolution and failure mechanisms of 3D braided carbon fiber/epoxy resin composites through an integrated multiscale methodology combining static torsion mechanical testing, computed tomography (CT) damage analysis, three-dimensional digital image correlation (3D-DIC), and acoustic emission (AE) monitoring. Experimental results revealed that the 3D braided carbon fiber-reinforced composite specimens exhibited approximately linear elastic behavior during torsion. An increase in braiding angle (15°–45°) enhanced shear modulus and strength by 67 %, but reduced failure strain to 0.61 % while shifting the failure mode dominance from ductile matrix deformation to brittle fiber fracture. CT analysis demonstrated that compressive fiber bundle failure governed mechanical performance, with damage progression initiating as interfacial debonding (at 60 % load), progressing through crack bifurcation (at 80 % load), and culminating in fiber buckling failure. 3D-DIC quantitatively characterized the strain heterogeneity regulated by braiding topology, showing that maximum shear strain decreased by 67 % with increasing braiding angles. Notably, 45° specimens developed mesh-like strain distribution pattern, revealing the directional regulation of load transfer paths through spatial fiber entanglement. The proposed AE signal processing framework integrating Hilbert-Huang transform with frequency-domain calibration techniques successfully identified three characteristic damage modes: matrix cracking (100–200 kHz), interface debonding (200–320 kHz), and fiber fracture (320–420 kHz). Statistical analysis indicated matrix damage dominated the failure process (75.5–80 % contribution), occurring during early loading stages, whereas fiber failure emerged near final rupture. Higher braiding angles were found to suppress matrix damage through enhanced fiber interlocking effects.