Failure modes of carbon fiber reinforced polymer plate - Steel joints with ductile or brittle adhesives

The failure modes of the CFRP-steel joints were critical determining the performances of the strengthened structures. Generally, cohesive failure was considered as an ideal failure mode, leading to ductility of the structures. In the present article, a series of single slap bond tests were conducted on CFRP-steel joints with three different adhesives with brittle or ductile properties. The CFRP-steel joints exhibited cohesive failure or CFRP delamination due to the varied properties of the adhesives. From the load-displacement curves tested with the single slap bond tests, the CFRP delamination failure can lead to large displacement, and thus the delamination failure is also acceptable compared to the cohesive failure. The failure mode of the joint depended on the relative strain value between CFRP and adhesive. When the ultimate strain of the adhesive is greater than the CFRP ultimate strain, the CFRP delamination failure tended to occur. Otherwise, the cohesive failure happens. From the bond-slip curves, the ductile adhesive has an approximately trapezoidal shape with an obvious plateau. However, the brittle adhesives have a bi-linear shape for the CFRP delamination failure mode. Finally, a prediction model of maximum shear stress is proposed for CFRP delamination failure mode.