Issue 16

F. Carta et alii, Frattura ed Integrità Strutturale, 16 (2011) 34-42; DOI : 10.3221/IGF-ESIS.16.04 34 Damage tolerance analysis of aircraft reinforced panels F. Carta, A. Pirondi Industrial Engineering Department, University of Parma, v.le G.P. Usberti 181/A, 43124 Parma – Italy A BSTRACT . This work is aimed at reproducing numerically a campaign of experimental tests performed for the development of reinforced panels, typically found in aircraft fuselage. The bonded reinforcements can significantly reduce the rate of fatigue crack growth and increase the residual strength of the skin. The reinforcements are of two types: stringers and doublers. The former provides stiffening to the panel while the latter controls the crack growth between the stringers. The purpose of the study is to validate a numerical method of analysis that can predict the damage tolerance of these reinforced panels. Therefore, using a fracture mechanics approach, several models (different by the geometry and the types of reinforcement constraints) were simulated with the finite element solver ABAQUS. The bonding between skin and stiffener was taken either rigid or flexible due to the presence of adhesive. The possible rupture of the reinforcements was also considered. The stress intensity factor trend obtained numerically as a function of crack growth was used to determine the fatigue crack growth rate, obtaining a good approximation of the experimental crack propagation rate in the skin. Therefore, different solutions for improving the damage tolerance of aircraft reinforced panels can be virtually tested in this way before performing experiments. K EYWORDS . Aircraft stiffened panels; Fatigue crack growth; Damage tolerance analysis. I NTRODUCTION n aircraft fuselage, aluminum stiffeners are connected to panel in longitudinal and circumferential directions. A particularly significant application is the direct bonding between stringers and the surface of the fuselage skin. The main features of the reinforced bonded panels concern better damage tolerance and higher stability at different types of loads [1, 2]. The experiments on aluminum panels with bonded stiffeners show that a limit of the aluminum reinforcement is the premature rupture of the reinforcement caused by the load transfer from the skin to the stiffeners when the crack runs underneath it. To improve the tolerance to the fracture, the doublers or reinforcement cords should preferably be made of material resistant to fatigue, with high stiffness and static strength [3, 4]. Panels made of a thin metal skins stiffened with bonded reinforcements insensitive to fatigue, can ensure slow crack propagation if not its arrest, and the capability to withstand a large damage, combined with a low structural weight. The effects of this bonded reinforcements or doublers are very difficult to predict numerically or analytically, because of the complex mechanisms of failure: - separation at the interface between skin and reinforcement around the area of nucleation and propagation of the crack; - load redistribution between the damaged and undamaged reinforcement; - fatigue damage of the reinforcement which may cause his premature rupture; - crack bridging by the doublers thanks if they have a sufficiently high fatigue strength. I