Strengthening-By-Stiffening (SBS) is a technique for delaying buckling of thin-walled web plates of steel girders using bonded GFRP stiffeners. Tests have shown that SBS can achieve shear strength gains in excess of 51%. The tests also showed that SBS failure is controlled by debonding of the GFRP stiffener, which is a complex fracture phenomenon. Understanding the fracture characteristics of bonding two dissimilar materials (steel web and GFRP stiffener) is crucial for further development of SBS design procedures. This paper presents results from single leg bending (SLB) experiments conducted to determine the fracture properties of the adhesive layer. Two adhesive types are investigated with substantially different mechanical properties. A digital image correlation (DIC) technique was used to capture the relative separations in normal and tangential directions, and rotations between the substrates at the artificially induced crack tip. A finite element (FE) model was then used to simulate the full-scale beam tests for the purpose of validating a cohesive zone model (CZM) developed based on the SLB test results. Results from the FE model are in good agreement with the experimental results.