Numerical Analysis of Stiffener Runout Sections
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Numerical Analysis of Stiffener Runout Sections Andrea Faggiani & Brian G. Falzon
Received: 29 March 2007 / Accepted: 3 July 2007 / Published online: 3 August 2007 # Springer Science + Business Media B.V. 2007
Abstract The recent trend of incorporating more composite material in primary aircraft structures has highlighted the vulnerability of stiffened aerostructures to through-thickness stresses, which may lead to delamination and debonding at the skin–stiffener interface, leading to collapse. Stiffener runout regions are particularly susceptible to this problem and cannot be avoided due to the necessity to terminate stiffeners at rib intersections or at cutouts, interrupting the stiffener load path. In this paper, experimental tests relating to two different stiffener runout specimens are presented and the failure modes of both specimens are discussed in detail. A thinner-skinned specimen showed sudden and unstable crack propagation, while a thicker-skinned specimen showed initially unstable but subsequent stable crack growth. Detailed finite element models of the two specimens are developed, and it is shown how such models can explain and predict the behaviour and failure mode of stiffener runouts. The models contain continuum shell elements to model the skin and stiffener, while cohesive elements using a traction-separation law are placed at the skin– stiffener interface to effectively model the debonding which promotes structural failure. Keywords Composites . Delamination . Debonding . Stiffener runout . Finite elements
1 Introduction The design of aircraft and spacecraft structures is being revolutionized by the efficient use of composite materials, which are ideal for structures requiring high strength-to-weight and stiffness-to-weight ratios. Composites also offer superior fatigue performance as well as lower life cycle costs achievable by taking advantage of their potential low maintenance.
A. Faggiani : B. G. Falzon (*) Department of Aeronautics, South Kensington Campus, Imperial College London, London SW7 2AZ, UK e-mail: [email protected]
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Appl Compos Mater (2007) 14:145–158
However, one major factor which has hampered the widespread use of composites in aircraft primary structures is the relatively low through-thickness strength. Delamination in composites may initiate in regions of high shear or peeling stresses which can lead to failure by unstable crack propagation. The principal form of airframe construction is characterized by a thin skin acting as a membrane and forming an aerodynamic surface which is stabilized in compression by the use of stiffeners. A lot of experimental and analytical work has been undertaken for such structures, emphasizing the vulnerability of co-cured and co-bonded stiffened structures to through-thickness stresses [1–4]. Recent trends have led to the development of thickerskinned stiffened structures to be used on the heavily loaded regions of the wing’s primary structure, and the problem of through-thickness stresses is even more substantial in critical re
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