Externally Bonded CFRP Reinforcement of Steel Structures: Mechanical Characterization of a Toughened Epoxy Adhesive

The use of toughened epoxy adhesives currently used in industrial applica­tions (crash-zone repair or bonding of wind turbine blades) was recently proposed for the structural strengthening of steel members where high impact energies or fatigue resistance are required. With reference to fatigue prone steel structures, many steel bridges around the world are beyond or dose to their fatigue lifetime. In these cases, the use of externally bonded carbon fiber-reinforced polymer (CFRP) plates is a valid alternative to traditional techniques (e.g., welded or bolted steel plates) for local rein­forcement. In (Kasper et al. (2021)), the potential of a toughened adhesive for the local reinforcement of fatigue-damaged steel components with non-prestressed or prestressed CFRP pultruded plates was investigated. Several aspects of the selected adhesive were covered, such as its basic material characterization, its impact wedge­peel resistance, its thermal characterization, its capability for the strengthening of fatigue damaged steel specimens, and its effectiveness for the fatigue repair of small­scale components. However, some features were not fully investigated, such as the adhesive-substrate bond-slip relationship. The bond-slip relationship is a key param­eter for the evaluation of the load bearing capacity of adhesive joints through cohesive models. Additionally, the capability of the adhesive to accommodate the steel plastic deformation without or with a moderate debonding of the CFRP reinforcement is fundamental since steel elements often exhibit local plastic deformation in regions of stress concentration. In this paper, the results of an experimental campaign aimed at evaluating the bond-slip relationship of CFRP-steel joints is presented. Single­lap reinforcement of a steel beam is considered and quasi-static tests are performed up to debonding of the composite reinforcement. Digital image correlation (DIC) is used to measure the displacement field and then evaluate the strain profile over the bonded area at different load levels. Numerical and analytical characterizations are then provided to reproduce the response of the tested adhesive joints through properly calibrated cohesive material laws.