Failure analysis in secondary bonded T-stiffened composite panels subject to cyclic and quasi-static compression loading

Adhesive bonding technologies are widely used for the assembly of stiffened panels manufactured in advanced composites for structural applications in aeronautics. However, stiffened panels are prone to the occurrence of defects or damage in the skin/stiffener junction, which will reduce the damage tolerance properties and affect structural integrity. The presence of unstable irregularities in the bonding region contributes to the decrease in the level of adhesion, limiting the resistance of the adhesive/laminate interface when subjected to mechanical loads. This article presents an experimental fracture analysis of flat panels with a longitudinal T-stiffener integrated into the skin by secondary bonding. The panels were produced in quasi-isotropic carbon/epoxy laminates with an artificial insert film replacing the adhesive film in the center of the bonding, to induce the initial damage. The tests were performed under cyclic loading followed by static axial compression loading at room temperature up to collapse. The panel selected for visual and fractographic analysis reached buckling instability with 14% of the final load, in the time interval when the failure propagation induced slight reductions in stiffness. The results obtained from this work showed the influence of the failure mechanisms combined with the formation of the failure modes and fractographic aspects that characterized the complexity of the fracture morphology provided by debonding of the skin/stiffener junction. The information revealed was relevant to the understanding of the failure process resulting from a critical defect on secondary bonding joints, applied in the integration of composite stiffened panels for aeronautics applications.