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Fracture toughness of architected joints involving crack instabilities

par Van Innis, Charline ;Budzik, Michal K.;Pardoen, Thomas
Référence CECAM 3D cracks and crack instability(14 to 16 june 2023: Lausanne, Switzerland)
Publication Publié, 2023-06-14

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Résumé :

Many complex structures such as airplanes are made of dissimilar materials, combining for instance Carbon Fibre Reinforced Polymer (CFRP) Composites and metal sheets. Manufacturing these multimaterial structures requires an efficient bonding process, and the joints must be characterized by a high fracture toughness [1]. However, adhesive joints often suffer from time and labour-intensive processes and low to moderate fracture toughness [2]. Joint with designed stop holes or meshes is one of the strategies already investigated to improve the joint fracture toughness [3]. An attractive option to improve the process efficiency is to perform bonding and composite curing simultaneously. As a first step on the road to the integrated bonding of hybrid metallic composite structures, potential toughening strategies relying on joint architecturing are investigated in this work while co-curing two composites parts. For this purpose, a tough prepatterned thermoplastic film is inserted between the composite parts. This results in a bondline made of the composite resin and the thermoplastic film that replaces conventional adhesives. The mode I fracture toughness of these joints is investigated through Double Cantilever Beam tests. A stick-slip behaviour involving three fracture toughness regimes is produced by the internal architecture associated to different crack paths and instability issues. The first regime corresponds to a moderate fracture toughness at the order of aeronautic adhesives. The second regime is characterized by an intermediate fracture toughness while the crack arrest mechanism of the third regime results in unexpected high values. The micromechanical origin of the high toughening potential is unravelled through fracture surface analysis and finite element models including the details of the joint architecture. Although, offering unexpected high fracture toughness, the remaining drawback of these joints are the crack instabilities taking place when going from a tougher to a weaker regime, resulting in large crack jumps.