Development of a Progressive Fracture Method to Predict Crack Arrest in Large Scale Composite Thermoplastic Fuselage Panels.

  • van Hoorn, Niels (Royal Netherlands Aerospace Centre)
  • van den Brink, Wouter (Royal Netherlands Aerospace Centre)
  • Tijs, Bas (Delft University of Technology)
  • Waleson, Jan (Fokker/GKN Aerospace)
  • Docter, Jan (Royal Netherlands Aerospace Centre)
  • van Ingen, Jaapwillem (Fokker/GKN Aerospace)

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Composite materials have become essential for modern aircraft. Recently, thermoplastic composite materials are on the rise and have been introduced in the design of large fuselage sections in the Multifunctional Fuselage Demonstrator of the European R&D program Clean Sky 2. This has led the STUNNING consortium to manufacture the largest thermoplastic aerospace structure to date. For such a fuselage structure it is common to show Structural Damage Capability (SDC) for a severed stiffening element and a "large notch" of two half-bays. It has to be shown that the notch in a pressurised fuselage under bending loads does not grow at limit load. In addition, as part of SDC, design measures should be in place that lead to crack arrest at adjacent stiffening elements. Crack arrest is achieved by preventing unstable growth of the notch past the stringer such that it can only grow by increasing the load. In the most recent design, interleaved tear straps are in place to ensure crack arrest at the stringers. Model predictions and experiments are required to evaluate the effectiveness of these crack arrest measures. Experiments and models are initiated at three scales: 1) double notched coupons, 2) notched three-stringer panel (3SP), and 3) large-scale notched fuselage panel. Specimens and panels are manufactured with AS4D/PEKK-FC and the test section (e.g., notched region) consists of two different layups. The base layup is quasi-isotropic and the second layup contains additional 0-degree plies (e.g., tear straps) around the stringers. These two layups are evaluated with the double notched coupons at four different coupon widths. Multiple widths are required to account for size effects [1,2]. With four samples per test this results in a total of 32 specimens that are tested at the TU Delft. The 3SP illustrated in Figure 1 is tested at Royal NLR. A displacement controlled uni-axial tension load is used for the coupons and 3SP. For the large-scale notched fuselage panel a different load case is applied using the ROTOP test setup at Royal NLR. A unique feature of this test setup is that realistic loading conditions are simulated by including internal pressure with a correct distribution of hoop loads to the skin and frames and an axial loading that corresponds to fuselage bending. In this work a Finite Element (FE) model is developed that is able to predict progressive fracture in large-scale composite thermoplastic fuselage panels. .....