A STUDY OF THE THICKNESS EFFECT IN FATIGUE DESIGN USING THE HOT SPOT STRESS METHOD Master of Science Thesis in the Master’s Programme Structural Engineering and Building Performance Design

The thickness effect is the phenomenon that the fatigue strength of a welded connection decreases when the thickness of load carrying plate increases. The thickness effect is observed in details where the fatigue crack initiation takes place at the weld toe. The hot spot stress approach is a fatigue design approach based on the structural stress obtained from Finite element Analysis (FEA) results. This method is more suitable when calculation of load effects is made using finite element method. Even though the method has been used for more than 20 years, it is yet not fully completed. One problem in this respect is how to account for the thickness effect, a problem which is not included in design codes and recommendations. Instead, the thickness effect conditions for the nominal stress method have been used with the hot spot stress. The thickness effect using the hot spot stress approach for stress determination for welded joints is studied in this thesis. A database containing fatigue test results with a number of selected welded joints is chosen for the analysis. Different types of modelling technique (according to the International Institute of Welding) have been applied to analyze the selected joints and the models were analysed using the FE program ABAQUS. The structural hot spot stresses are determined according to the IIW and other stress determination methods. Finally the result data obtained from the FEA are compared with the result from fatigue test data. The results show that the thickness effect correction factors in the hot spot stress approach are the same as the values obtained for the nominal stress approach for two of studied details. For the third detail, the thickness effect correction factor in the hot spot stress approach is larger than that for the nominal stress approach. There is no thickness effect when the so called “1 mm stress approach” is used for the cruciform joint.