Improved model and stiffness prediction of 3D four-directional braided ceramic-matrix composites

Based on the CT scan results of 3D four-directional braided ceramic-matrix composites and theoretical analysis, referred to the existing interlaced model, an improved 3D cell element model was established. This model truly reflects the mesoscopic structure of the internal material. The yarns' cross-section along their axes model varied cyclically in shape and area, yarn's cross-sections alternate transformed form parallelograms to pentagons, each yarn's axis presented interlaced relationships, close to the extrusion deformation law of tight yarns in materials. By measuring the average yarn packing factor and using the finite element method, the elastic properties of yarns and materials were obtained. The predicted value agrees well with the test data. The finite element simulation reveals that the yarns undertake the main load in the materials' cell model. Some of the yarns and matrixes interfaces tend to appear stress concentration phenomenon. These areas could mostly produce crack propagations and local damages. The determination of the mesoscopic stress field also provides a foundation for the analysis of failure mechanism and the strength of the materials.