Fatigue strength of steel girders strengthened with carbon fiber reinforced polymer patch

Fatigue sensitive details in aging steel girders is one of the common problems that structural engineers are facing today. The design characteristics of steel members can be enhanced significantly by epoxy bonding carbon fiber reinforced polymers (CFRP) laminates to the critically stressed tension areas. This paper presents the results of a study on the retrofitting of notched steel beams with CFRP patches for medium cycle fatigue loading (R = 0.1). A total of 21 specimens made of S127 X 4.5 A36 steel beams were prepared and tested. Unretrofitted beams were also tested as control specimens. The steel beams were tested under four point bending with the loading rate of between 5 and 10 Hz. Different constant stress ranges between 69 and 379 MPa were considered. The length and thickness of the patch were kept the same for all the retrofitted specimens. In addition to the number of cycles to failure, changes in the stiffness and crack initiation and growth were monitored during each experiment. The results showed that the CFRP patch not only tends to extend the fatigue life of a detail more than three times, but also decreases the crack growth rate significantly.