A new experimental and numerical investigation on the fatigue response of composite and metal aircraft structures

The static strength and fatigue crack resistance of aircraft skin structures depend on the materials used, the joint type or joining method. Most of the skin panels of commercial aircraft are made from aluminum alloys and composite materials, mainly carbon fiber reinforced epoxy (CFRE). In this research, published in the journal Mater. Sci. Composites, it is investigated with experiments and finite element analysis (FEA) the fatigue resistance of four joint configurations that are typical in aircraft repair patches (metal substrate-metal patch, metal substrate-composite patch, composite substrate-composite patch, and composite substrate-metal patch), using riveted, adhesive bonded, and hybrid joining techniques. The results of this investigation are important to improve the understanding of the behavior of, for instance, aircraft repair patches, as well as raising awareness on how to improve current standards in the field.

Particularly, the fatigue tests conducted in this study were tension-tension because of the typical nature of the loads on aircraft skin panels susceptible of experiencing fatigue. The experimental results indicate that the fatigue life of hybrid joints is superior to adhesive bonded joints, and these in turn perform significantly better than the conventional riveted joints. The superior performance of hybrid joints is thanks to the fact that the adhesive bond provides better (smoother) load distribution than the rivets alone, while the rivets in the hybrid joint induce compressive residual stresses in the joint, and, compared with the adhesive bonding alone, the rivets in the hybrid joint ensure its load-carrying capacity, in the event of premature adhesive failure. The numerical results obtained from FEA for the adhesive bonded and hybrid joints agree well with the experimental results. From the FEA, the strain energy release rate (SERR) for adhesive bonded joints is higher than that for hybrid joints, in both the fatigue crack growth mode I and mode II (opening mode and shear direction mode, respectively). Most joints show higher SERR in mode II. This indicates that the joints experience fatigue crack in the shear direction, which is responsible for crack opening.

Reference:

Pitta, S., Rojas, J.I., Roure, F., Crespo, D., Wahab, M.A., On fatigue life of riveted, adhesive bonded, and hybrid aircraft joints – An experimental and numerical study, Steel and Composite Structures 43(1) (2022) 19–30. JCR-SCI (2020)