Study of bolted hole edge stresses in composite variable stiffness laminates - based on variable angle lay-up and parametric analysis

Composite variable stiffness laminates have become an area of significant research focus in recent years, with bolted connections being an important method of joining composite structures. In this study, the single-bolt connection structure of variable stiffness laminates was selected as the subject of investigation. A reconfiguration mesh cell was applied for finite element modeling of the hole-edge structure. The stress distribution at the hole edge in fixed stiffness and variable stiffness laminates with single-bolt connections was examined, and radial, circumferential, and shear stresses were calculated. Stress distributions for different lamination schemes were obtained under various layups. A combination of experimental and numerical simulations was used to analyze the effects of bolt aperture and end spacing on the mechanical properties of variable stiffness laminates. Numerical simulations revealed three stress concentration zones in fixed stiffness laminates and only one in variable stiffness laminates, indicating that a variable-angle trajectory design in composite bolted structures can significantly reduce stress concentration and fiber damage. Experimental results showed that the ultimate tensile strength of the bolted connection structure was approximately 57.5 MPa when the bolt aperture was 4 mm and 39.3 MPa when it was 6 mm. The findings indicate that larger bolt aperture reduce tensile stress but degrade the mechanical properties of the laminates. This provides a basis for the design and analysis of bolted structures in variable stiffness laminates.