This thesis investigates the use of Tow-Based Discontinuous Composite (TBDC) interleaves to enhance the mode I fracture toughness of adhesively bonded joints with Carbon Fiber Reinforced Polymer (CFRP) substrates. Aiming to improve joint safety by slowing crack propagation and facilitating less sudden failure, this study focuses on integrating interlaminar-toughened substrates to resist crack growth and enhance the fracture behavior in the joint's substrate. Two main research questions are addressed: the influence of TBDC interleaves on mode I fracture toughness of CFRP laminates and their subsequent effect when used in CFRP-based bonded joints.

For CFRP laminates, Double-Cantilever Beam (DCB) samples were tested across three configurations: a non-toughened baseline and two TBDC-toughened variants. Based on previous research, three DCB configurations identified as the most promising for leveraging TBDC toughening in adhesive joints were tested. The [90/45/-45/TBDC/0]s and [90/60/90/-60/TBDC/0]s laminate substrates were bonded with the low-toughness adhesive Araldite 2015-1, while the [0/TBDC/90_2/0]s substrate was bonded with AF 163-2U, a high-toughness adhesive.

TBDC-toughened CFRP laminates demonstrated up to 130% higher fracture toughness compared to non-toughened counterparts. This was due to TBDC material crack propagation mechanisms such as crack branching, deflection, and fiber bridging.

In adhesively bonded joints, TBDC interleaves in CFRP substrates enhanced the decay of fracture toughness in specimens where cracks deflected from the bond line into the substrate, leading to a less abrupt reduction after reaching peak values. Joints with low-toughness adhesive exhibited more than a 100% increase in crack length from peak fracture toughness to the final value compared to non-TBDC-toughened substrate joints. Meanwhile, joints with high-toughness adhesive demonstrated toughness values 150% to 750% greater than those observed in non-toughened configurations at comparable crack lengths.

These findings highlight the potential of TBDC interleaves to enhance joint toughness, presenting new pathways to improve the safety of composite bonded structures.