Torsional fatigue damage evolution mechanism and life prediction model of 3D braided carbon fiber composites

Abstract

3D braided carbon fiber composite shafts hold significant promise for replacing traditional metal or polymer shafts, attributed to their high specific strength and excellent damage tolerance. This study delved into the torsional fatigue damage mechanism of 3D braided carbon fiber composite shafts and developed a life prediction model. The composite shafts were fabricated using 3D four directional braiding and vacuum assisted resin molding techniques. By conducting torsional fatigue tests in conjunction with 3D DIC (3D Digital Image Correlation), CT (Computed Tomography), and in-situ damage monitoring methods, the strain and damage evolution data throughout the fatigue process were acquired. The research results show that the braided structure gives rise to a periodic grid like strain distribution, and the high strain regions are associated with the locations of the braiding nodes. The stiffness degradation occurs in three phases: stable, linear, and accelerating. The damage commences from the fiber bundles on the specimen surface and undergoes an evolution process involving matrix fiber debonding, fiber buckling, and fiber fracture. The debonding cracks within the fiber bundle are the form of damage initiation. Fiber buckling has been proven to be a sign of rapid stiffness degradation. Through the bearing and damage analysis of fiber bundles, an energy based fatigue life prediction model was successfully applied. This model incorporates the stress and strain characteristics on critical fracture surfaces involving fiber resin debonding and fiber buckling fracture as core parameters. Experimental validation confirmed the accuracy of the model in predicting fatigue life under different braiding angles and various loading conditions.