16th International Conference on Composite Structures ICCS 16

A. J. M. Ferreira (Editor) © FEUP, Porto, 2011

THE DISCUSSION OF STRUCTURAL ROBUSTNESS ON THE EXAMPLE OF FIBER METAL LAMINATES

Th. Beumler Airbus

Hamburg, Germany e-mail: thomas.beumler@airbus.com

Key words: FML, Robustness, Structural Performance & Response, Maintenance, Repairs.

Summary. It is invited to participate on the discussion about the definition of “robustness” for stiffened aircraft panels. A wide range of variables can be imagined, from ductile and impact resistant materials, via multiple load path design principles, up to “inspection free” and easy repairable structures. In almost any case, weights and costs are either a target or a result of the level of “robustness”. This document will discuss the topic “robustness” on the example of Fiber Metal Laminate structures.

The content of the definition of structural “robustness” might be discussed, provided the minimum demands of the relevant Airworthiness Regulations are met or exceeded. Depending on the point of view, the performance of the original structure might be considered as robust, if for example it provides a high Large Damage Capability or high reserve factors on static and/or fatigue strength. Others may consider the dispatch reliability as priority item, bearing in mind that a structure should tolerate severe damages in order to avoid immediate repairs which would need to ground the aircraft and finally lead to significant costs (hotel rooms for stranded passengers, etc.). If one tries to summarize the “robustness” items to be addressed, he may end up with the following list:

• Damage Tolerant Material • Damage Tolerant Structure / Large Damage Capability • Minimum scheduled inspection tasks (ALI’s), quantity and quality • Simple damage detection, preferably visual • Easy reparability • Preferably permanent repairs, preferable inspection free

The present list, even if not exhaustive, includes the focus of all kind of failure modes and damage types, such as the well known fatigue damage, the impact, the lightning strike damage, the lightning strike damage or the unexpected but repeatedly occurring accidental damage.

Fiber Metal Laminates have been developed to satisfy all the above mentioned demands,

combining “the best of two worlds”. However, the laminate alone can contribute with some of

Th. Beumler

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its properties to a “robust” design, for example it can provide high burn through resistance and high impact resistance. On the higher level of structural integrity, however, the material needs to be teamed with a damage tolerant, multiple load path design.

This group of hybrid materials has some key features in common monolithic alloys, i.e. the potentials to yield, to absorb energy, to oxidize, to fatigue and the electrical conductivity . These characteristic properties made it obvious to qualify and certify FML structures as metallic structures. The composite part of the hybrid, i.e. the fibers embedded in the resin film, have the important task to control any kind of damage on both, material and structural level – with other words to contribute to Damage Tolerance. The presence of the fiber prepregs as joining medium for the metal sheets added some composite typical items to the metallic certification.

The design approach for the A380 fuselage shells made of Fiber Metal Laminates as well

as some damage cases which happened during manufacturing and in-service will be discussed with regard to structural “robustness”, along with some details about the material properties and the structural response on damages.

Figure 1: Typical impact damage on A380: Structural robustness required