Influence of weld toe radius and steel grade on the fatigue life of fillet welds analysed by a strain based approach

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Insinööritieteiden korkeakoulu | Master's thesis
Nordic Master Programme in Maritime Engineering
Degree programme
Degree Programme in Mechanical Engineering
Nowadays the marine industry plays an important role in our everyday life, offering a cost effective way for international freight transfer. From a financial point of view the more cargo can be transferred, the better. For a vessel with a set dimensions this can be achieved by increasing the deadweight, hence decreasing the lightweight. To that end, there is an increasing trend of application of HSS material, thin plate structures or both. Key challenges with these new structure is the consideration of their fatigue strengths. The Thesis project deals with the fatigue life of fully penetrated fillet welds and the effects of the weld toe radius in HSS and LSS. A localised strain based approach was used for the fatigue strength assessment. This approach applies a step by step discrete crack growth simulation for the total fatigue damage process (crack initiation and propagation) within the LCF (Low Cycle Fatigue) and HCF (High Cycle Fatigue) range, respectively. The model considers the effects of the microstructure, i.e. hardness and grain size for the two material, and the toe radius (R= 1 mm and 0.01 mm). Using Abaqus, the FEM analysis was carried out on a simplified parametric model of the fillet welds created with Matlab. The resulting stress and strain values were averaged using a characteristic length of the material, and the summation of the cycles, estimated by the Coffin-Manson relation for each discrete step, gave the overall fatigue life of the welded structure. The predicted fatigue strength and the shape of the S-N curve for both materials are in line with expectations. However, due to the material model used, where the strain vs life curves of the two materials differ significantly in the LCF range but seem to be similar at HCF one, the resulting S-N curves differ. The smooth-weld, R= 1 mm without any initial cracks seem to have a higher FAT class for both materials, whereas the weld with R= 0.01 mm gave a lower FAT class than the IIW recommended values. Also, the long crack propagation rate is reasonably in line with the recommendations for fillet welds. The differences of the short crack initiation and propagation period are significant for the two materials, whereas the long crack propagation and final fracture showed fairly similar behaviour. The initiation period in the presence of a larger toe radius, R= 1 mm lasts longer and is more dominant for the HSS material than for the LSS material. Hence, for this geometry, the HSS material could offer a better fatigue resistance. On the contrary, a smaller toe radius, R= 0.01 mm, or the presence of initial cracks of the size of 0.1 or 0.2 mm, seems to significantly lower the fatigue strength more for the HSS, than for the LSS weldments. This result shows the high dependency on the geometry, related to the actual material and implies that, due to the higher notch sensitivity of HSS, the fatigue strength will be lower for the R= 0.01 mm case than for the R = 1 mm. This tendency is not prominent in the LSS material.
Remes, Heikki
Thesis advisor
Liinalampi, Sami
fatigue, strain-based approach, short fatigue crack, propagation, fillet weld, weld radius
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