The 2018 Structures Congress (Structures18) Songdo Convensia, Incheon, Korea, August 27 - 31, 2018 Experimental and numerical investigation of strengthened cruciform welded joints using FRP *Elyamani Abdallh Arabi 1) , Tamer Hanafy Sayed 2) , Sahour Nabil Sayed 3) , Nahla Kamal Hassan 4) and Kamal Gad Sharobim 5) 1),5) Department of Civil Engineering, Suez Canal University, Ismailia 41511, Egypt 2),4) Department of Structural Engineering, Ain Shams University, Cairo 11511, Egypt 3) Department of Mech. Design and Production, Cairo University, Giza 12613, Egypt 1) yamani_arabi@yahoo.co.uk ABSTRACT This paper presents an experimental and finite element study on the fatigue strength using fiber reinforced polymers (FRP) to repair load-carrying cruciform full penetration welded joints. Fatigue tests were carried out on a total of 15 specimens. The stress intensity factor (SIF) were determined for welded joint at weld toe failure. Test results showed that, the patching of FRP materials prolongs fatigue life with sufficient values and at the same time debonding was prevented in case of bearing plate and bolts. The fatigue life was then predicted using fracture mechanics approach. The results showed that the resulting fatigue enhancement agreed well with experimental specimens. The effects of different parameters on fatigue life and SIF at crack tip were also presented. By comparing S-N curves of different parameters with that of AWS2002, it was found that S-N curve of cruciform welded joint shifted from category D to a curve located between category B' – B. 1. INTRODUCTION Fatigue cracking in existing steel structures such as bridges is becoming an increasingly important due to the natural aging of these structures. In the welded joints, areas of high stress concentration are the most adopted points for fatigue cracks initiation and propagation which are the main reasons of catastrophic failure. These welded joints are weak points for fatigue resistance. Structural retrofitting with CFRP materials in comparison with conventional retrofitting techniques provides an environmentally friendly and economic way to save and utilize the infrastructures, and, therefore, it became one of the primary interests in civil 1) Graduate Student 2), 3) Assistant Professor 4), 5) Professor