PSI - Issue 39

J.C. Ehrström et al. / Procedia Structural Integrity 39 (2022) 98–103 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Fiber Metal Laminates (FML) are hybrid materials made of thin Aluminum layers and glass fiber reinforced epoxy intermediate layers. This material is certified and flying on Airbus A380 fuselage, see Beumler (2009). The aim of the present study is to assess the fatigue and damage tolerance of a FML lower wing skin. The novelty versus flying material is the metal sheet thickness: 0.8 mm, when 0.25 – 0.4 mm sheet is used in Glare®. Lower wings require higher skin pocket thicknesses (typically ~10 mm) than fuselage (~1.6 – 3 mm) due to higher load flows. The use of very thin metal sheet on wing skins would be too costly. It is however known that the Fatigue Crack Growth (FCG) is slower when sheet thickness is lower, due to a better transfer by shear of the tensile loads to the glass fiber reinforced epoxy layers (Şen 201 5). This thesis in reference deals in details with the optimization of FML for lower wing skins. The loading of the panel tested in the present study assumes a ~20% increase versus 2024 T3 bulk material in stress allowable. In addition, the test aims at demonstrating that a largely increased inspection interval can be reached with this increase of stress allowable. The test also aims at measuring the total fatigue life from a very severe “rogue” flaw as defined by Eastin (2009): 3.5 mm on each side of a 8 mm rivet i.e. ~15 mm in total, opening the way to an inspection free structure. The typical “rogue” flaw would rather be 1.27 mm in size. 2. Panel manufacture The test area of the panel, where the crack is initiated and propagates, contains 8 layers of 2024 T3 0.8 mm sheet and 7 layers of glass fiber reinforced epoxy of 0.25 mm thickness. This makes a panel thickness of 8.15 mm at the test location. Up to 11 layers of metal and 10 layers of composite are present in the grip area. The panel width is 800 mm. Five 2027 T3511 stringers are bonded onto the skin (Fig. 1).

Fig. 1. FML panel with the 5 stringers positioned prior to bonding.

A auxiliary rib is bolted on the panel to limit out-of-plane displacement. The fasteners are positioned through the stringer ears, the skin, and the rib foot. The initial defect is made on both sides of a fastener hole in the centre of panel (Fig. 2).