To reducing emissions of pollutants, automotive and aerospace industries are seeking new solutions to create lighter structures. Extreme lightweight structures can today be obtained by using high-performance composites based on continuous fibers and polymeric matrices. Assembling composite parts is however still a challenge that often jeopardizes the energy efficiency (e.g. bolting or riveting). Integral adhesive bonding is not used for primary structure today, because of its extreme sensitivity to the quality of the substrate preparation that can largely modify the intrinsic performance of the joint. More important for us, the failure of adhesive joints is often unstable: the joint behaves well until the development of a catastrophic crack that would propagate throughout the whole joint.
Our objective is here to introduce new strategies to equip by design adhesive interfaces with crack arrest features. From a practical point of view, we are manipulating the R-curve of the interface by introducing non-local dissipative mechanisms, such as bridging, that will add to the classical cohesive energy of the adhesive.
Two different technologies are introduced to realize this toughening objective.
In the first approach [1-3], we are introducing heterogeneous interfacial properties (strength and toughness) between the adhesive layer and the substrate. The introduction of inclusions with higher strength results in separating the main crack into two sub-cracks that are propagating simultaneously and are increasing the effective toughness. In a second approach , bridging ligaments are triggered by introducing a non-symmetric thermoplastic inclusion inside the thermoset based adhesive layer. The progressive stretch of the thermoplastic ligaments results in an extra dissipation that participates in toughening the joints (see figure 1b).
For each approach, this presentation will cover the fundamental concept via simulation of the joint failure. Guided by these, an extensive experimental campaign has been performed in which we successfully demonstrated that controlling ligament bridging is possible via simple manufacturing techniques or structuration of the interface. These strategies open new directions towards more trustable adhesive bonding-based solutions.
 R. Tao, M. Alfano and G. Lubineau (2018). Laser-based surface patterning of composite plates for improved secondary adhesive bonding. Composites Part A: Applied Science and Manufacturing, v. 109, pp. 84-94.
 R. Tao, M. Alfano and G. Lubineau (2019). In situ analysis of interfacial damage in adhesively bonded composite joints subjected to various surface pretreatments. Composites Part A, v. 116, pp. 216-223.
 R. Tao, X. Li, A. Yudhanto, M. Alfano and G. Lubineau. On controlling interfacial heterogeneity to trigger bridging in secondary bonded composites: an efficient strategy to introduce crack-arrest features. Submitted.
 A. Yudhanto, M. Almulhim, L. Fatta, O. Alqahtani, R. Tao, M. Alfano and G. Lubineau. Enhancement of fracture toughness using 3D-printed thermoplastic carriers in secondary bonding of CFRP laminate. In preparation.
KAUST, Saudi Arabia