Impact of ductility on the performance of bonded composite repairs in cracked aluminum panels under overload

This paper investigates the impact of fatigue overload on effectiveness of adhesively bonded composite repair for aeronautical aluminum alloys (2024 T3 and 7075 T6). The focus is on understanding how overload-induced plasticity influences the efficiency of these repairs. The study is divided into two parts: an experimental phase and a numerical phase. The experimental analysis involves fatigue tests on single edge notched tension (SENT) specimens made from both aluminum alloys, with the cracks repaired by a carbon/epoxy composite bonded on the cracked plate. Fatigue life curves were derived from these tests. Two levels of overload were tested (9 and 14 kN). The finite element method (FEM) is employed in the numerical part to assess the size of the plastic zone behind the crack front in the repaired specimens. The experimental results showed that the composite repair significantly enhances fatigue life, especially under overload conditions. For an unrepaired plate with an overload of 9 kN, the effect of this overload is negligible for Al 7075 T6 and for Al 2024 T3 the fatigue life increases by 75%. Furthermore, the combination of patching and overload cycle may lead to an almost infinite fatigue service life as the overload amplitude increases. The simultaneous application of the 9 kN overload and the composite patch resulted in a twofold increase in fatigue life for the 7075 T6 and a fivefold increase for the 2024 T3. This fatigue life becomes practically infinite for both aluminum alloys when the applied overload is 14 kN. This behavior is attributed to the growing plasticity around the crack tip, which intensifies with the overload amplitude as shown by the finite element analysis.