On the use of selective stitching in stiffened composite panels to prevent skin-stringer debonding

Composite laminates are generally characterized by poor mechanical properties along the thickness direction, therefore they are highly susceptible to delaminations. A potential increase in structural efficiency may be obtained by using 3D reinforcements technologies. In this paper, an experimental/numerical study is presented focusing on the effects of a single reinforcement seam of stitches along the edge of a stringer foot, in a stiffened composite panel. Three-point bending tests have been performed on a skin-stringer configuration by considering variations of the skin thickness, the stitching technique, the pitch and the yarn diameter. Then, a Finite Element Model has been developed capable to simulate the mechanical behaviour of stiffened composite panels with selective stitching and to assist the design of more complex geometrical configurations by estimating the damage behaviour, as well as the onset and the propagation of delamination taking into account the effect of the selective stitching. The numerical results, in terms of load vs applied displacements, have been found in good agreement with the experimental data proving the effectiveness of the introduced numerical model. The numerical results have confirmed the potential beneficial effects of stitching in terms of delay of the crack initiation and growth, of smoother delamination profile, of increase in the stiffness of the structure with a reduction of the delamination area.