Mechanical failure behavior on <scp>CFRP T</scp>‐joints of stitching reinforcement subjected to low‐velocity impact and post‐impact tension

Abstract T‐joints made from carbon fiber reinforced polymer (CFRP) composites have been applied in aircraft fuselage and wing boxes, but CFRP structures have been facing problems of low impact resistance and interlaminar delamination. To investigate the mechanical failure behavior of the T‐joints, low‐velocity impact (LVI) on the weak corner of T‐joints and post‐impact tensile testing are conducted carefully. With the help of a self‐designed mold, we fabricate 27 kinds of stitching CFRP T‐joints with three numbers of stitched sides, three needle distance, and three stitching positions relative to the corner. Especially, the number of stitched sides includes one‐side stitching, double‐side stitching, and three‐side stitching. Their impact resistance is compared comprehensively based on the impact response, damage morphology, and crack length. Further, to reveal tensile damage mechanisms, the digital image correlation (DIC) technique is employed to record the strain evolution. By comparison, the most optimal enhancement effect is found in the stitching strategy of three‐sided stitching closer to the deltoid and denser needle distance. Moreover, the tensile strength of the T‐joints surpasses twice that of the unstitched ones, owing to the effective suppression of the propagation of interlaminar delamination under LVI and the enhancement of stability in their crack propagation during tension through the thread. Highlights T‐joint enhancement focuses on stitching sides, distance, and needle distance. Three‐side stitching around deltoid area has excellent reinforcement effect. Stitching inhibits effectively interlaminar delamination propagation under LVI. Stitching enhances the stability of crack propagation during tension.