R. Muzzolini, Alcan Aerospace, Kirkland, WA; J. C. Ehrström, S. Arsene, Alcan - Pechiney CRV, Voreppe, France; S. Van der Veen, Alcan Aerospace, Issoire, France

Summary:

Lower wing panels are usually loaded in tension and therefore driven by their damage tolerance capability. Advanced aluminum alloys with increased toughness and fatigue properties have been recently developed to help reduce the weight of such tension-driven components. However, the large cost and weight savings requested by aircraft manufacturers can more easily be achieved by introducing new advanced concepts combined with these improved materials.

An earlier study [i] for instance showed that adding simple rectangular Damage Containment Features (DCF) to the skin could increase the number of loading cycles required for the crack to become critical by 70 %. The influence of the number and geometry of these DCF has been subsequently studied, all configurations having the same total weight. DCF solutions were surprisingly not systematically better than the standard without DCF. The initial crack length has also been found to be a key parameter. An optimum panel shape therefore exists for any given configuration.

Crack turning has been investigated as a way to enhance the Residual Strength of integrally stiffened panels. Numerical modeling has been performed to maximize the amount of T-stresses, which are believed to control the crack turning. Test panels have then been built using an advanced lower wing skin alloy 2027 T351 and a 7XXX. Two configurations were tested: one with classical parallel stringers and another one where stringers were slanted. Crack turning was only observed in the 2027-slanted stringers configurations, but an interesting phenomenon occurred in the 7XXX: the crack did not grow up in the stringer but stayed in the skin, leading to higher residual strength than theoretically calculated.

Bonding stringers or doublers to the skin also greatly improves the damage tolerance of a stiffened panel. However, conventional aerospace bonding practice requires careful preparation of surfaces and expensive bonding procedures. An inexpensive bonding practice derived from the automotive industry was used in the evaluation of crack retardation in a stiffened panel. Testing showed promising improvement of the damage tolerance.