Due to high stress concentrations, welded joints represent the most common locations of fatigue crack initiation in steel structures that are prone to fatigue. Welding affects material properties by the process of heating, cooling, and combining basic and additional material. Since welding is the primary process of joining elements in steel structures, it is obvious that fatigue assessment during the design and maintenance process becomes inevitable. There are many fatigue assessment methods of welded joints, but their precision remains questionable. This paper represents a review of the most common fatigue assessment methods used for welded steel joints. As a result of this review, areas that require additional research are highlighted.

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This paper presents the fatigue life assessment of a welded detail in an existing steel railway bridge. The fatigue life is assessed based on strain measurements due to the passage of characteristic trains. Characteristic trains are an integral part of the load model formed by "Croatian railways" data. Strain gauges that measure deformations are set at specific locations in order to gain information on Nominal and Hot Spot stresses. Based on this measurement data, stress ranges and stress spectrum are determined and compared to a corresponding detail category. Obtained results in this paper are related to comparing fatigue life of the analysed detail based on local and global approaches according to Eurocode 3 and IIW recommendations.

Welded steel details are critical components from the aspect of fatigue. Additional fatigue resistance can be achieved by the High-Frequency Mechanical Impact (HFMI) treatment. This treatment increases the crack initiation period by improving the weld geometry, introducing compressive residual stresses, and increasing the weld toe’s hardness. The study presented in this paper is based on the development and calibration of an Initiation–Propagation-based Two-Stage Model (TSM), which is, by the combination of different methods, suitable to separately consider crack initiation and crack propagation. It is shown that a TSM is able to predict the fatigue life of as-welded and HFMI-treated welded steel details, which is proven by comparing the calculated results with the results of tests on similar details given in the literature. A parametric study of the TSM is conducted for different steel grades in order to investigate the influence of steel strength and HFMI parameters on fatigue lives of a welded steel detail with longitudinal attachment.

SažetakPrilikom procjene preostaloga životnog vijeka čeličnih detalja izloženih umoru najčešće se zbog jednostavnosti proračuna primjenjuje S-N metoda uz globalni pristup preko nominalnih naprezanja. Ta metoda na strani djelovanja zanemaruje lokalne utjecaje poput koncentracija naprezanja uzrokovanih lokalnom promjenom u geometriji detalja. Ovi se utjecaji indirektno uzimaju na strani otpornosti pomoću eksperimentalno dobivenih S-N krivulja. Ako se radi o geometrijski složenijemu detalju ili je detalj opterećen složenim stanjem naprezanja, primjenjuju se lokalni pristupi kojima se prilikom proračuna uzimaju u obzir lokalni utjecaji. Ovaj rad prikazuje procjenu životnoga vijeka zavarenoga čeličnog detalja dodatne lamele izloženog umoru. Detalj je sastavni dio čelične konstrukcije željezničkoga mosta, a procjena se provodi lokalnim pristupom preko hot spot naprezanja. Naprezanja se određuju mjerenjem deformacija na određe-nim mjestima u detalju zbog prolaza karakterističnih vlakova. Karakteristični vlakovi sastavni su dio modela opterećenja formiranoga prema podacima Hrvatskih željeznica. Na taj se način proračunava životni vijek zavarenoga detalja zbog umora, a rezultati se uspoređuju sa životnim vijekom istoga detalja dobivenog pristupom preko nominalnih naprezanja. Ključne riječi: životni vijek zbog umora, lokalni pristup Hot Spot naprezanja, zavareni detalj Fatigue life assessment by using local Hot Spot approach AbstractDuring the assessment of remaining fatigue life of the welded details that are prone to fatigue, S-N method with global nominal stress approach is the most common method due to its simplicity. This method neglects local action effects such as stress concentrations due to local geometry changes in detail. These effects are indirectly included on the resistance side trough experimentally obtained S-N curves. In cases of more complex detail or if the detail is loaded with a complex stress state, local approaches which take local effects into account during calculations are used. This paper presents a fatigue life assessment of fatigue prone cover plate welded steel detail. Detail is a part of the steel railway bridge structure and assessment is made by local hot spot stress approach. Stresses are determined based on strain measurements on specific locations in the detail, due to the passage of characteristic trains. Characteristic trains are an integral part of a load model formed by data from Croatian Railways. Accordingly, the fatigue life of the welded detail is calculated and the results are compared with fatigue life of the same detail which is determined based on the nominal stress approach.