Effects of the fillers on the interface strength of the hat-stiffened composite panel

Hat-stiffened composite skins have been widely used in primary structures of aircraft components. The bonding strength at the stiffener-skin interface is critical to ensure the advanced load-bearing capacity of the stiffened skin, whereas, the deltoid regions are usually the weakest locations because of the geometrical singularity. Inserting fillers into the deltoid regions could alter the bonding strength. In this study, a series of four-point bending tests are conducted on the co-cured hat-stiffened composite skin specimens with and without fillers and with different pre-fabricated debonding defects, furthermore, finite element simulations are implemented to predict the failure process. The stiffness, initial failure load, failure mode and failure process are analyzed to obtain the influences of the fillers on the damage tolerance of the hat-stiffened composite panels, and the influential mechanisms were revealed by the combined analyses of the test and simulation results. It is found that the usage of fillers can increase the initial stiffness of the panel without prefabricated defects by about 10%, but provide limited influence on the initial failure load. The pre-fabricated debonding defect along longitudinal direction has little influence on the stiffness and failure load, but the influence of the defect along transverse direction are higher. The initial failure of all hat-stiffened composite panels with/without filler and with/without prefabricated defects under four-point bending onsets around the deltoid region. The usage of the fillers changed the load transfer and the stress status in the deltoid region of the hat-stiffened composite panels and as a consequence, the initial failure modes were changed and the failure load were increased by the fillers. This work provides a technical support for the damage tolerance design and strengthening method of hat-stiffened composite structures.