Enhancing the quasi-static strength of co-cured bonded laminate joints via multiscale toughening

Composite laminate structures are widely used in load-carrying applications in various industries (e.g. aerospace and automotive) due to their lightweight and superior material properties (e.g. high specific stiffness and strength). Structural joining of load-carrying laminate components is of utmost importance as joints play a critical role in transferring loads between structural parts. In this perspective, the co-curing strategy (i.e. curing and bonding of the parts together in the same manufacturing step) can reduce manufacturing effort, cost and time. Furthermore, carefully incorporating different material combinations with co-curing can improve structural integrity. This study evaluates co-cured NCF/epoxy joints with different toughener combinations. An interlaminar toughener (polyphenylene sulfide (PPS) non-woven microfibre veil), a resin toughener (core-shell rubber (CSR) particles) and the multiscale toughening (the combination of both tougheners) are applied and compared against the baseline (untoughened) material system. Firstly, the fracture properties of the selected tougheners are measured in mode-I and mode-II fracture conditions. The largest improvement in mode-I fracture toughness is obtained with multiscale toughening (i.e. ~141% and ~190 in GI,C and GI,R, respectively), while thermoplastic veil interleaving-alone is outperformed both core-shell rubber particle toughening and the multiscale-toughening in mode-II fracture (i.e. ~84% and ~129% in GII,C and GII,R, respectively). Then, these tougheners are applied to co-cured joints of the same NCF/epoxy material system. The load-carrying capability of differently toughened co-cured joints is measured. In addition, co-cured joint specimens are instrumented with back-face strain gauges, and the full field strain/displacement measurements are carried out with a 3D-digital image correlation (DIC) system. 3D-DIC is validated against the back-face strain gauges. A good correlation is achieved. The results showed that the co-cured joint strength could be improved up to 50% with multiscale toughening. Surface strains obtained with 3D-DIC indicated that the toughened joints can arrest sub-critical damage and delay the final failure at the bondline. Similarly, the strain gauge measurements showed that the back-face strain at failure could be significantly improved. To sum up, this study shows that an enhanced toughening in mode-I and mode-II fracture toughness could be transferred to co-cured joint strength, leading to enhanced structural integrity. Furthermore, a balance between the interlaminar fracture toughness and co-cured joint strength can be obtained by the selective use of different toughening routes.