The strengthening of steel bridges using CFRP laminates has become a commonly used technique because of its numerous advantages compared to conventional repairs which involve welding or bolting of new steel plates. These structures will experience some sort of irregular cyclic loading during their lifetime and to analyze these complex loading cases, small scale testing is used to evaluate the fatigue performance between the steel substrate, adhesive layer and the CFRP laminate. In this research, double-strap joints (DSJ) were fabricated consisting of two high-modulus CFRP laminates and ASTM A36 steel plates bonded using a two-part epoxy adhesive. Two types of steel surface conditions were considered to evaluate the fatigue behavior under constant force amplitudes. Roughness on the steel substrate was achieved by ½ in (13 mm) diameter pits approximately 1/8 in (3.18 mm) deep to simulate an irregular surface. The results show that the surface condition has marginal influence on the total life of the specimens. To assess the damage accumulation in the DSJ, phenomenological methods like the nonlinear strength wearout Model (NLSW) and stiffness degradation were used. It was found that residual strength and stiffness decreased in a non-linear fashion. A non-linear model was used that agrees well with the experimental results and can be used to predict the residual strength of the specimens under variable amplitude fatigue (VAF). / Master of Science / With an aging and deteriorating infrastructure potentially being subjected to heavier loads than initially designed for, bridge engineers are increasingly looking for innovative, yet cost-effective solutions for repairing and maintaining the existing bridge inventory with sufficient capacity to carry legal loads. One alternative is to enhance the flexural strength of deteriorated steel members using carbon fiber-reinforced polymers (CFRP). This research project will investigate whether the use of CFRP retrofits is feasible for fatigue strengthening. The research will serve as insight to help guide bridge engineers or researchers on when and where the use of CFRP retrofits is suitable for the strengthening of steel bridges.

Carbon fiber reinforced polymers (CFRP), which can be used to strengthen and repair damaged steel structures, have gained popularity in recent years. On the one hand, CFRP has demonstrated substantial advantages over conventional reinforcing techniques like welding and bolting, such as light weight, high strength, and corrosion resistance. Additionally, the CFRP application process is relatively easy, rapid, and labor-intensive. On the other hand, failure is more likely to happen at the bond interface due to the high strength characteristics of steel and CFRP. Thus, studying the bond behavior and failure mechanism of CFRP strengthened steel structures as well as the variables that are crucial to the bond quality. Prior to implementing these elements in an actual construction, it is necessary to thoroughly study the factors affecting this bond strength. Despite the fact that some theoretical predictive modeling for the strength between steel/CFRP joints under various loading situations has been published, in this work, by using finite element modelling, one may compute the failure loads and effective length of the steel/CFRP specimens quickly, simply and accurately. Additionally, factors affecting these parameters are also investigated in this study. / Master of Science / Structural Steel deteriorates over time. Due to this, engineers are constantly on the look-out for cheap and easy ways to repair and maintain these structures. One of the methods is the use of carbon fibred polymer or CFRP. In the literature, it has been frequently documented that CFRPs can make existing structures stronger. Additionally, CFRP has the advantages of not corroding and prevents the structure from becoming significantly heavier. Due to this high strength of CFRP, the failure occurs at the steel-CFRP interface and thus this bond and the factors affecting this bond needs to be studied. One way to do this is experimental testing and another way is finite element modelling which can give you data that is harder to get using experimental testing. Thus, this study focuses on finite element modelling of these joints and how it can be used for studying these joints.

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