Anisotropic failure analysis of 2.5-D braided composites under compression

2.5-D braided high-alumina fabric-reinforced silica matrix composite materials are being developed as functional structural composites with thermal load-bearing capacity for the top thin face sheet of an integrated thermal protection system (TPS). This study seeks to elucidate the mechanical behavior of a 2.5-D braided top face sheet composite material subjected to off-axis compression, while the sandwich TPS is under deflection caused by the bending moment resulting from the temperature gradient in the thickness direction and the aerodynamic pressure on the top face sheet panel. The effective compressive modulus and ultimate strength along the warp direction are significantly lower than those along the weft direction. Both decrease as the off-axis loading angle increases, indicating anisotropy. A nonlinear compressive stress-strain response with extensive damage along the warp yarn direction, while the weft yarn direction exhibits a quasi-linear response with minor damage before fracture. Within the framework of continuum damage mechanics, fiber kinking and transverse inter-fiber cracking of fiber bundles with the linear damage evolution law are all considered simultaneously for the progressive damage model. To account for the interaction between multiple damage mechanisms in the fiber yarn, a joint degradation induced by multiple damage mechanisms is first proposed. The analysis reveals that the anisotropy of failure under compression is primarily due to the warp fiber bundle's much larger undulation than the weft fiber bundle, which causes severe bending-dominated damage. Using the calibrated model, the qualitative effects of material thickness and yarn geometry size on the compressive mechanical properties of materials are analyzed.