On the assessment of the load-bearing capacity of Z-pinned composite T-joint under out-of-plane tension

Z-pin technology, as a promising through-thickness reinforcement, offers an efficient solution for the weak composite parts joints, especially for large integrated composite structures. This paper presents a two-dimensional finite element model to analyze the enhancing efficiency of Z-pins for composite T-joints. The modeling used a new parameter definition method for delamination growth simulation in Z-pinned joining interface via a regional cohesive zone model. The finite element model was validated by experiments for unpinned and Z-pinned T-joints under web pullout tension. The parametric analysis on the structural configuration shows that shear stress ratio in Z-pins declines with the span ratio increasing and the skin thickness ratio decreasing, which leads to the failure mode change of a single pin from pull-out to shear fracture so that the improvement on the out-of-plane tension declines gradually. Within 1.7-2.1 span ratio (the constant 4mm skin thickness) or 1.8-2.5 thickness ratio (the 200mm constant span), the ultimate tension is improved by at least 15%, where Z-pins exhibit the best efficiency. When span ratio is above 2.4 or thickness ratio below 1.2 within the range studied herein, Z-pinned T-joints fail by skin fracture rather than the bonding interface in which cases further increasing of Z-pin reinforcements becomes unfavorable.