Issue 35

R. Sepe et alii, Frattura ed Integrità Strutturale, 35 (2015) 534-550; DOI: 10.3221/IGF-ESIS.35.59 537 Coordinates Strain gauges S1 S2 S3 S4 S5 S6 S7 X [mm] 680 710 995 1235 1525 1770 1800 Y [mm] 1144 1144 1144 1144 1144 1144 1144 Z [mm] 1.2 1.2 38 1.2 38 1.2 1.2 Table 1 : Strain gages positions on panel (see reference system in Fig. 2). The test machine used to test the full scale aeronautical test panel is shown in Fig. 3. Such machine consists of five hydraulic cylinders, one in of the diagonal direction and the other four aligned along two orthogonal directions, hinged onto supports rigidly connected to an octagonal box frame. In the diagonal direction the maximum load is 1000 kN, while in the normal directions it is respectively 250 kN in X direction and 500 kN in Y direction (for the reference system see Fig. 2). Its overall sizes are about 5000x5000x2000 mm 3 , without considering the hydraulic and electric control units. The machine is able to test flat specimens up to an extension of 2500x2500 mm 2 , including the gripping zone. Figure 3 : Tested panel loaded by the multi-axial full scale fatigue machine. The gripping system consists of 32 items, 8 for each specimen side; this localized loading practically determines the required stress distribution on the specimen side. Each grip transfers the load to the specimen by friction and by four special shaped pins (Fig. 4). The four traction cylinders are connected to the 8 grips by a rocker arm balancing system. The levers have equal arms, so the cylinder load is uniformly distributed; it would be possible to apply a different load distribution on specimen side with a non equal arm lever system. Moreover the localized grips and the balancing system allow free panel side elongations. The diagonal cylinder applies the shear load by means of a rope winded up to 128 pulleys. Some pulleys are assembled on the grips and some others are assembled on a plate linked to the cylinder as shown in Fig. 5. Two pulleys for one grip, in symmetric position with regard to the panel, avoid non expected moments. The pulleys are different in diameter and vertical position to avoid interference with the rope. They can rotate to balance different rope length elongations. Just two locked rope pieces are used, one for two consecutive panel sides. A rolling bearing system ensures the “simply supported edges” boundary condition. It allows the in plane specimen translations and prevents the out of plane one. This system is supported by a secondary frame connected to the principal one. The secondary frame reacts to out of plane forces that can arise by geometric and material asymmetry or by buckling phenomena. Moreover the machine is not constrained by an external bed because it is entirely balanced. The cylinder