Stress–strain behavior of CFRP-bond to steel in tension

Carbon fiber reinforced polymer (CFRP) sheets are used to externally reinforce structural elements. Compatibility is of major importance to transfer stresses and strains from the reinforced member to the CFRP through the bond. This bond is a contribution of three layers: the adhesive-to-structure, the adhesive-to-CFRP bond, and the properties of the adhesive-impregnated CFRP. While in modeling, the CFRP is assumed to be fully bonded; test results suggested that this assumption overestimated post-peak responses in particular. Defining accurate CFRP bond behavior is therefore obligatory in modeling. This research aimed to construct accurate stress–strain responses of CFRP bond layers. The study acquired this by investigating the strain-gauge responses at each layer as a function of incremental loading. CFRP sheets with a variation in length ranging from 40 to 120 mm were attached to a 300 mm steel plate subjected to flexural stresses. The CFRP was situated in the tensile zone. The steel plate was favored to ensure the failure mode occurred in the CFRP layer. It was concluded that bond length significantly influenced the transfer mechanism, concluding a minimum effective CFRP length of 100 mm. All stress–strain bond relationships are characterized by bilinear responses, with almost identical adhesive-to-CFRP and impregnated CFRP behavior. The adhesive-to-structural layer had a lower ultimate stress and post-peak response; initial stiffnesses were undifferentiated. An implementation of the obtained stress–strain response into a finite element analysis (FEA) demonstrated the accuracy of the results and the significant deviation when a full bond is assumed through the toughness of the strengthened member.