Failure analysis and strengthening mechanism of Z-pinned composite T-joints under tensile loading

Z-pinning technology is applied to composite T-joints in this paper to explore the strengthening mechanisms. The structural properties of T-joints with skin thickness values at 2 and 4 mm were determined under stiffener tensile load. The results revealed that the initial failure strength and failure modes of T-joints were affected by the thickness of skin. Under stiffener tensile load, the actual stress situation of Z-pin tends to be complicated with the deflection of skin. As a result, a mixed mode I/II stress condition exists at the delamination crack tip during tensile loading, and an enhanced frictional zone is located near the delamination surface. When the deflection of skin is indispensable, the snubbing effect acts as a significant factor to strengthening. As the pullout process of pins from the flange is limited by snubbing, pins could fail by split. This paper discusses the strengthening mechanism of Z-pinning and elaborates a potential enhancing effect of Z-pins resulting from the complicated loading condition due to different structure dimensions.