Modern steel manufacturing techniques make it possible to produce steel with the nominal yield strength up to 1300 MPa for structural purposes. However, the application of very high strength steels is still limited in the civil engineering structures due to lack of knowledge about the effects of the manufacturing process and experimental results regarding the structural behaviour of the material. Moreover, in a fatigue loaded very high strength steel structure absolute and relative stress variations will be higher compared to stresses in structures made of lower steel grades. Accordingly, the fatigue issue will be one of the most important design criteria for very high strength steel structures. In this current study, V-shape welded specimens were manufactured from S690 and S890 rolled steels and cast steels with similar yield strengths. Fatigue cracks were created in the weld toe of the specimen under a fluctuated loading and subsequently the fatigue damaged specimens were repaired by the removal of the cracks with subsequent welding. The fatigue strength curves of repaired specimens are compared with the detail categories of EN 1993-1-9 [8] and the fatigue strength curves of the test results in the as-welded condition from literature. The fatigue strength of the fatigue damaged connections was completely recovered by the established repair procedure.@en

For cyclically loaded structures, fatigue design becomes one of the important design criteria. The state of art shows that with modification of the conventional structural design methodology, the use of very high strength steels may have a positive effect on fatigue strength of welded connections. However, there is little known about the repair of fatigue cracks in welded connections made of very high strength steels. In this study, the fatigue strength of the repaired base material and repaired fatigue damaged V-shape welded connections made of very high strength steels is investigated. This thesis consists of four parts. In Part I, the emphasis was put on the effects of the material imperfections on the fatigue strength of materials and this part consists of an extensive literature study and a microscopic examination of the fracture surfaces of the base material of very high strength steels. Part II presents a literature survey on the fatigue crack repair methods and the results of the experimental programme of the current study. The experimental programme comprises fatigue tests on the repaired base material and repaired V-shape welded connections made of very high strength steels. The V-shape welded specimens were made of rolled and cast steel plates. In Part III, fatigue strength prediction models were evaluated and a comparison was made between the fatigue strength curves from the prediction models and the fatigue strength curves of the test results. In the last part, conclusions and recommendations from the study are presented. The analysis of test results revealed that the fatigue strength of fatigue damaged V-shape welded connections made of very high strength steels can be recovered by an appropriate repair procedure. @en

Fatigue crack formation is an inevitable issue for welded steel structures subjected to cyclic loading. Accordingly, repair of the fatigue crack in welded steel structures is unavoidable to prolong fatigue life. However, there is limited knowledge available about the procedure to be adopted for the repair and the life extension to be expected after the repair. The current paper is focused on the effects of the repaired an artificial crack in the base material of S690 and S890 high strength rolled steels on the fatigue strength of the material. An artificial crack was created in the middle of the plate test specimens by spark machining and subsequently, the crack was repaired by using the FCAW (flux-cored arc welding) process. The repaired specimens were tested in a four point bending test rig with a constant amplitude loading for creating a uniform bending moment at the weld region such that the weld cap to be exposed to tensile stresses. The test results show that most of the fatigue cracks initiated at the start-stop points of the weld cap and the fatigue crack initiation life of the specimens occupy approximately 45% of the total fatigue life. The statistical analysis of the test results revealed that the characteristic fatigue strength of the repaired specimens is very close to the detail category 160 of EN 1993-1-9 [5].@en