PSI - Issue 28

A.M. Al-Mukhtar / Procedia Structural Integrity 28 (2020) 124–131

125

2

A.M. Al-Mukhtar/ Structural Integrity Procedia 00 (2020) 000–000

of stress (see Fig. 1) [1][2][11]–[16][3]–[10]. The stress concentration provides the unique conditions for the crack initiation and propagation, see Refs. [3][13][15][17][18]. The undesired design of the parts causes also the stress concentrations. Hence, the propagation rate will be increasing under the cyclic loading until final failure. Cyclic loading produces failure that called fatigue. Fatigue fracture increases with the presence of stress concentration. Therefore, after initiating the crack, the propagation into a critical crack size causes a sudden failure because the remaining materials would not support the load. The propagation rate increases dramatically after the critical crack length. Therefore, the scheduled intervals inspections are necessary. Therefore, without the scheduled inspection, the crack length may increase into a critical size. 1.1. Fuselage Failure Phenomena The fuselage sheets are overlapped by riveting or spot welding. For such joints, the fretting fatigue is predominated especially when the metal are tired. Fuselage failure occurs from the fastener holes in aircraft fuselage [19][20][21][22][23]. The fuselage has many notches, see Fig. 1a. Figure 1b shows the crack initiating around the rivet, bolts and screw, see Refs [4][14][16][24]. In addition, the surface defects such as scratches will increase the local stress concentration [24]. The crack tends to propagate through the weld joint of the stiffening members [25]. It was shown that for a range of crack lengths, the shorter crack grows faster than the longer crack that may exist [23][26][27]. Therefore, the periodical interval inspections have to be shortened. However, for the mature machines, the inspections have to be increased.

a)

b) Fig. 1. a) Rivets around the fuselage; b) Spot-welds around the fuselage cabin [28]

1.2. Crack in Aluminum Skin Metals like aluminum have no endurance limit. Nevertheless, the fuselage still constructed from the aluminum alloys [29]. Therefore, there are unknown numbers of cyclic loading that can aluminum withstand before failure. In addition, during the thousands of times of pressurized and depressurized of the aircraft, the properties of the materials will be changed. The fuselage cabin will be depressurized and pressurized during the takeoff and flying into an altitude. These sequences called cycle. The metal will be subjected to what known fatigue. Physically, these cycles of pressurization and depressurizing of the cabin, and the numbers of take off and landings weaken the aluminum skin and causing fatigue crack [30]. The short crack is common in aluminum. It has a significant role [31][32]. The aluminum skin is a common place for fatigue cracking that causing a structural failure in aircraft [33][24][34]. Because, the skin has the most common sites for crack initiation, such as the bolts and the rivets [24][28]. Figure 2 shows the sudden failure of airplane fuselage that caused an emergency landing of Southwest's flight 812 on April 1, 2011. However, it occurred in a young plane that far away from such a kind of rupture [35].