Panel geometry

The geometry of the cracked structure in the present work is shown in Fig. 1. It was considered a 6061-T6 aluminum alloy plate; a through thickness crack is used to simulate the defect in the structural components.

The fracture is made with the technique of Electrical Discharge Machining (EDM), based on the erosion of metals by spark discharges. This technique allows an accurate control of the crack size and tips shape. The size of the panel is: Hp = 180 [mm], Wp = 180 [mm] and thickness tp = 1.5 [mm]. The central crack has a length 2a = 40 [mm] repaired by carbon- epoxy composite patch bonded with adhesive having an estimated thickness ta = 0.1 [mm]. Tables 1 and 2 show the mechanical properties of the patch and adhesives used. The height and width of the patch are Hr = 40 [mm] and Wr = 60 [mm], respectively. It is well known (Mahadesh & Hakeem, 2000) that the rectangular shape of the patch is not the best choice; nevertheless the rectangular shape is a good compromise between mechanical performances and manufacturing aspects. The patch size is the result of various numerical tests not reported here for the sake of brevity. It represents the minimum patch size, covering integrally the crack length, that leads to a significant improvement of the mechanical behavior under the considered loading conditions as shown in the next Sections.

Table 1. Patch properties

Two types of carbon-epoxy patches were used. Type 1 is a two unidirectional layers +45°/- 45° patch, and Type 2 is a single fabric layer patch with the fibers oriented along the ± 45° directions.

The panel is subjected to various tests with a sinusoidal uniaxial load at 1 Hz Tables 3 and 4 report the mechanical properties of the aluminium alloy panel and the loading conditions, respectively