Biaxial mechanical response of plain-woven CFRP tubes under combined tension–torsion based on acoustic-optical measurements

Multiaxial loading experiments and damage characterization of carbon fiber reinforced polymer (CFRP) composites are of significant importance for their operation under complex service conditions. In this paper, the mechanical properties, damage evolution, and failure mechanisms of plain-woven CFRP tubes under combined tension–torsion loads are quantitatively investigated by integrating three-dimensional digital image correlation and acoustic emission (AE) techniques. The results indicate that the distributions of strain fields within the woven structure of the composite tube exhibit distinct differences under different load conditions. Shear loading influences the average axial tensile strain, while tension loading significantly enhances the shear strength and shear modulus of the tube. Improved schemes for failure criteria are proposed that are based on the shear strengthening effect of the tension–shear stress plane, and the AE signal characteristics were analyzed to categorize the damage types based on peak frequencies. As the normalized cumulative acoustic energy can serve as a damage factor for the quantification of both damage evolution and damage envelope flow, it assisted in further establishing a stress state–dependent damage assessment model. A combined analysis of the multiscale fracture characteristics of the CFRP tubes demonstrates that various types of damage exhibit distinct initiation and propagation mechanisms under different dominant stress types, and the damage patterns for the same damage type also vary across different locations within the woven structure. These variations ultimately influence the direction of crack propagation.