Designing for toughness
How substrate stiffness controls crack path and effective engagement of toughening layers in adhesively bonded CFRP joints
R. A.A. Lima (Universidade de Lisbon)
S. Teixeira de Freitas (Universidade de Lisbon, TU Delft - Group Teixeira De Freitas)
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Abstract
Tailoring the stacking sequence of composites bonded joints improves fracture toughness and damage tolerance of the joint by encouraging extrinsic toughening mechanisms, such as crack deflection and crack branching. Previous works show that in composite substrates with tailored laminates, each crack deflection into a new ply can increase the joint's toughness. Still, once a 0° layer is reached, toughness drops abruptly due to sudden delamination. To overcome this limitation, this work explores embedding a co-cured film-adhesive layer to prevent delamination in 0° plies. It examines how the substrate's bending stiffness influences the effectiveness of this toughening strategy. Quasi-static double cantilever beam tests on four different carbon fibre reinforced laminates, with and without the co-cured layer, revealed two regimes: (i) compliant substrates lead to high peel stresses, triggered crack deflection into ±45° plies, enabling bridging and rising R-curves—up to 200% toughness increase; (ii) stiffer substrates suppressed near-tip rotation, and promoted cleavage-like crack growth with minimal toughening.
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