Study On Sifs Calculation And Propagation Path Of Cracks At Fatigue Hot Spots In Ships And Offshore Structures

Ships and offshore platforms are a kind of huge welding structures under complex wave loading. The stress distribution around its fatigue hot spot is very complex. In order to predict the fatigue life of a cracked detail, the crack propagation path and the stress intensity factor of crack in complex stress field must be precisely calculated. In this paper, the structure details of fatigue hot spots are studied, and the calculation methods of SIF are summarized. Crack propagation path in complex stress field are studied, and the effects of welding residual stress and structural redundancy on fatigue crack propagation are studied. The main works of this paper are as follows:(1) Fatigue assessment details defined in CCS Standard are divided into four categories: weld toe of butt-joints and T-joints, bottom plate of bracket toe, bracket plate of bracket toe, and intersection point of three perpendicular components. The SIFs of surface cracks at the first three categories are analyzed with 3D FEA and compared with the results of SIF magnification factors calculated with equations recommended in BS7910.The results show that the SIF magnification factors of surface crack at first category is close to that of BS7910, while the results of second and third categories are quite different from BS7910. Correction formulas are proposed respectively for second and third categories, and the results are well agreement with that from FEA.(2) The coordinate systems of tubular Y joint are defined in this paper. The formulas of intersecting line in different coordinate systems are calculated. The formulas of intersecting line considering weld toe are also calculated. Based on these formulas, the coordinate transformation equations are calculated. The FE models of main tube, secondary tube and the crack piece are created according to the coordinate transformation equations.(3) Based on the numerical simulation method of crack propagation and the criterion of crack propagation, a crack propagation program is developed using Abaqus Python script. Fatigue crack propagation path in test sample is simulated and the results are in good agreement with the experimental results. Two ship components(hatch corner and bracket) with initial cracks under different loading ratios are chosen to study the fatigue crack propagation path and the growth life of crack under complex stress field. Stress amplitudes on attachment structure in two main directions are used as external loads. The results of crack growth path calculated under different load ratio indicate that the cracks in hatch corner are not sensitive to load ratio while cracks in bracket are very sensitive to load ratio.(4) Based on the calculation of SIFs of type I and II cracks in welding residual stress fields, the calculation technique of SIF induced by welding residual stress is integrated in the program. Fatigue crack propagation paths and propagation life of cracks in the welding residual stress field are predicted, the predicted results are relatively close to the experimental results. In order to apply the crack propagation simulation program in ship structures, both sub-model and substructure technique are used in considering the effect of structural redundancy. The results show that both sub-model and substructure technique can deal with structural redundancy and load shedding and the substructure technique is more accurate and efficient.