Effects of axial load on torsional fatigue of 3D braided carbon fiber composites: Mechanisms and life prediction

Three dimensional braided carbon fiber reinforced epoxy composites are gradually replacing traditional materials in transmission components. This study experimentally investigates the influence of axial loads on torsional fatigue damage evolution and lifetime by integrating three dimensional digital image correlation, acoustic emission monitoring, and micro-computed tomography analysis. A fatigue life prediction model incorporating axial load effects is subsequently established. Results indicate that axial loads significantly modify the torsional fatigue behavior of 3D braided composites by redistributing local strain and altering damage progression. Under 0 MPa axial load, the material exhibited a fatigue life of 2787 cycles. Compressive axial loading (−40 MPa) reduced torsional fatigue life by 47 % compared to the no-axial-load condition, accelerating stiffness deterioration. In contrast, tensile axial loading (40 MPa) extended the fatigue life to 5007 cycles by suppressing interface debonding propagation. DIC analysis revealed that compressive loading induced a three-stage strain accumulation in resin-rich regions. Tensile loading maintained strain field stability through stress redistribution. Combined AE and Micro-CT analysis demonstrated that compressive loading increased the matrix cracking proportion to 84 %, expanded post-failure crack volume by 38 %, and formed spiral divergent damage zones. Tensile loading constrained the damage zone width to 7.6 mm and reduced the interface debonding energy proportion. Finally, a multiaxial fatigue life prediction model accounting for mean stress effects induced by axial loads was developed based on microscopic damage mechanisms. The model successfully addresses combined axial-torsional loading effects in 3D braided composites. Experimental validation confirmed that all predicted results fell within the two-fold scatter band.