Study on Fatigue Strength of 50HT Steel with Wrap-Around Weld

Fatigue tests were conducted on plate with wrap around weld. And the effect of stress ratio of the weldment on fatigue strengths was obtained experimentally. Despite cracking was observed in most of the compressive tests, no specimen reached complete failure. And the results for tensile (R=0) and compressive (R =-∞) fatigue strength for crack initiation life were compared. The fatigue strength for initiation life of compressive stressing at high cycle range exceeded that for the tensile (R =0) stressing by a factor of two.Analytical estimation for failure life was conducted for tensile specimens. Crack initiation, Nc of the tensile specimen was estimated using the relation between the crack initiation and the local stress range at weld toe. And the crack propagation life, Np utilizing the two dimensional stress distribution obtained from the FE solid model analysis. Comparison of the estimated life Nf=Nc+NP with the experiment showed a good agreement.A qualitative analysis for the fatigue crack arrest was conducted under compressive loading condition (R =∞). By obtaining the weld residual stress, the stress intensity factor due to the weld residual stress kres was calculated. Taking into account of the Kres, crack arrest conditions for different applied stress levels were formulated using the crack closure concept. A fair agreement for the length of the arrested crack was obtained between the analysis and the experiment. Application of the test results in evaluation of the side longitudinal element in a large ship structure is also discussed. It is stated that the stress at location of attached plate in L2 longitudinal is magnified by a considerable amount due to the out-of-plane deformation of the web. This was experimentally observed in the measured stress distribution on the face plate of the structure model.A reference stress St is proposed, which is a unified stress for evaluating the fatigue strength, for the element with a wrap-around weld in a structure with attachment. The proposed St is the stress at a distance t (plate thickness) from the weld toe.