PSI - Issue 3

G. Zucca et al. / Procedia Structural Integrity 3 (2017) 553–561

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Author name / Structural Integrity Procedia 00 (2017) 000–000

The load conditions were obtained from calculation report issued by OEM and applied to the aerodynamic center of the door (Fig. 21), whereas two cylindrical support that allow the rotation around X axis, were used to simulate the hinge and the leverage. The contribution to stress absorption due to the honeycomb of the MLG door was simulated combining a partial geometry of the panel with a constrain condition that not allows X axis translation, as shown in (Fig. 23).

Fig. 22: Cylindrical Constraint

Fig. 23: Door Constraint.

Von Mises Equivalent Stress, Vergani (2006) and Gugliotta (2002), failure criteria were used in stress field evaluation. Static stress map obtained from a mean stress condition shows how the fatigue limit for AA7075, issued at 150 MPa, website (2016), is exceeded of an irrelevant quantity (9 MPa) in a position far from fracture initiation (Fig. 24). Near the crack the value of Maximun Equvalent Stress is of 110 MPa (Fig. 25).

Fig. 24: Stress Map of Rear Hinge.

Fig. 25: Stress Map of Crack initiation.

4. Discussion The fracture surface examination showed a fatigue mechanism. It started from two corrosion pits and initially propagated normal to them until the crack pass through the initial section (zone B Fig. 11). Afterwards the propagation changed its path longitudinally respect the cross section (zone C Fig. 11). In turns first crack propagation acted as initiation for the second one. The corrosive phenomenon is the result of the combination of protective layer separation and saline atmospheric exposure. The protective layer separation in the initiation region was probably caused by an incorrect painting cycle, as suggested by the hinge status observed on other similar aircrafts (Fig. 26).