Issue 16

F. Carta et alii, Frattura ed Integrità Strutturale, 16 (2011) 34-42; DOI : 10.3221/IGF-ESIS.16.04 36 Another kind of reinforcement, called doubler , is usually positioned in the separation zone between stringers, oriented parallel to them (see Fig. 3). Figure 3 : Representations of the possible placements of the doublers: in the middle of the bay (between the stringers) or under the stringers. Because of their shape, the doublers have not a stiffening function, but they slow down the crack grow rate in the zone between the stringers (the so-called bays). The doublers are typically bonded to the skin and experimental studies are showed they are more efficient with a thick section rather than a thin section [2]. E XPERIMENTS total of 35 stiffened panels, representative of a typical fuselage skin of a long-range family aircraft, were manufactured and tested in the laboratories of EADS-IW Ottobrunn. The fatigue crack propagation (FCP) rate was investigated for twenty-four of them. The remaining eleven panels were tested for the residual strength. The panel shown in Fig. 4 has been tested and it is characterized by a skin (1224 mm wide and 1455 mm long) with seven equally spaced bonded stringers. In addition to the stringers, bonded doublers are positioned below and between the stringers in order to provide additional reinforcement. All the doublers are placed orthogonal to the direction of crack propagation alike the stringers. Figure 4 : Representation of a “seven stringers” panel with doublers bonded between and under the stringers (a) and with an additional glass fiber reinforcement (b) . The tests were performed by means of a servo-hydraulic INSTRON 8805 machine with a 1MN load cell. The clamping was specifically designed for the 7-stringers panels (see Fig. 5). An anti-bending device was installed to prevent the out of plane deflection of the panel during the test (see Fig. 5). The skin, like the doublers, were made of 2024-T3 aluminum alloys with 1,4 mm thickness the former and 0,8 mm the latter. The stringers were “J”-shape extruded profiles made of high strength 7349-T76511 aluminum alloy. An anti- bending device was installed to prevent the out-of-plane deflection of the panel during the test (see Fig 5). In the experiments, the fatigue crack propagation was investigated starting from a through-the-thickness, 50 mm-long, machined notch. The crack was placed across the middle stringer; this stringer and the underlying doubler (when present) were also cut. The loading parameters were the same for all the tested configurations (constant amplitude loading, 280 kN maximum force and 0.1 load ratio). The tests ended when the crack was “four bays” long or in case of panel failure. A