Stress intensity determination in lap joints with mechanical fasteners

Accurate determination of stress intensity factors for cracks emanating from fastener holes in metallic joints is often difficult due to rapidly varying structural geometry as well as fastener load transfer in the joint. Detailed finite element models that include specific aspects of fastener geometry may be developed for lap joint analysis, but such representations are often impractical for large lap joints involving many fasteners. A methodology is proposed for efficiently depicting mechanical fasteners in lap joints using finite elements. Fastener material properties are determined, based on an empirical force-displacement relationship, for highly refined fastener models as well as for idealized spring element representations of fasteners. Parametric studies involving single shear lap joints with three rows of fasteners indicate that both fastener representations provide comparable load transfer and relative displacement between mating sheets For a variety of fasteners. The two fastener characterizations are used in combination to develop computationally efficient models for predicting stress intensity factors for cracks in lap joints. Residual strength predictions for large bolted lap joints with multiple site damage are obtained that are consistent with experimental results from the literature.