Multi-scale study on stress concentration and strength characteristics of 3D braided composites containing two symmetrical holes

3D braided composites are widely used in stress-concentrated structures. This study employs a multi-scale approach to investigate stress concentration in a 3D 4-directional braided composite plate containing two symmetrical holes. Two theoretical methods are developed to predict inter-hole transverse stress based on different correction assumptions. The stress distribution between the holes is investigated through theoretical analysis and finite element simulations. Results show that increasing the hole radius intensifies stress concentration, while sufficient hole spacing minimizes inter-hole interaction. The tensile strength of the composite plate is further evaluated based on the point stress criterion (PSC) and numerical simulations, revealing that the strength characteristics are significantly influenced by hole radius and braided angle, while a smaller hole spacing leads to a noticeable reduction in tensile strength. These findings provide insights into the mechanical behavior of 3D braided composites and contribute to the structural design of components with defects.