  In this work, the mode I fracture toughness of dissimilar metal-composite
adhesive joints is experimentally investigated using the double cantilever beam
(DCB) test. The particular joint under study is resulted by the adhesive
joining of a thin titanium sheet with a thin carbon fiber reinforced plastic
(CFRP) laminate and is envisioned to be implemented in the hybrid laminar flow
control system of future aircraft. Four different industrial technologies for
the joining of the titanium and CFRP adherents are evaluated/compared;
co-bonding with and without adhesive and secondary bonding using either
thermoset or thermoplastic CFRP. The vacuum-assisted resin transfer molding
(VARTM) technique is employed for the manufacturing of the panels. After
manufacturing, the panels are cut into test specimens that, because they are
too thin (approximately 2.4 mm thick), needed to be stiffened from both
titanium and composite sides with two aluminum backing beams to ensure the
non-yielding of the titanium during the subsequent DCB tests. Towards the
determination of the fracture toughness of the joint from the experimental
data, an analytical model recently developed by the authors, that considers the
bending-extension coupling of both sub-laminates constituting the test specimen
as well as the manufacturing-induced residual thermal stresses, is applied. For
the four manufacturing options (MO) investigated, the load-displacement
behaviors, failure patterns, and fracture toughness performances are presented
and compared.
