Experimental study on the mixed mode debonding by using a modified CFRP-to-steel double strap joint

Abstract

The bond response of CFRP-to-steel joints in mixed mode-I/ΙΙ is a big concern since the loads consistent with mode-I may decrease the shear load capacity of the joints. For this motive, the current work introduces a modified CFRP-to-steel double strap bonded joint in which different initial peeling angles can be created. The specimens were monotonically loaded with a tensile machine until failure that subjected the CFRP-to-steel joint to a pull-pull condition. Although the specimens were geometrically symmetric throughout their length, the results showed that the initial debonding only occurred on one side of the two bonded CFRP strips. The side that initially debonded showed an interfacial failure between the CFRP strip and the adhesive, while the other side showed a CFRP delamination. The increase of the peeling angle significantly affected the initial shear load capacity, which decreased from the baseline 0° by 50% and 70% when 2° and 3° were adopted, respectively. Moreover, a peeling angle of 4º was found to be the critical peeling angle for the adopted CFRP-to-steel joints, i.e. the angle beyond which the specimens debond when subjected to pure mode-I loading even before any loads consistent with pure mode-II were applied to the bonded joints. The initial debonding induced a quick increase in the global slip and lower stiffness in the load-slip relationship. For the bonded region near the CFRP-loaded end, the maximum bond shear stress decreased from 20 to 7.5 MPa when the initial peeling angle varied from 0º-3º. To represent these effects on the local bond behavior, a new nonlinear bond-slip relationship is proposed and adjusted with the Integrate Absolute Errors (IAE) to the experimental results, which was achieved with IAE values lower than 4%. The results show, therefore, a high precision and versatility of the model to locally represent the bond behavior of the present CFRP-to-steel joints with different initial peeling angles.