Study On Fatigue Life Assessment And Dynamic Fracture Of Ship Structure

With the trend of ships dimension getting bigger and widely usage of high-tensile steel in ship structures, assessing the fatigue strength of ship structures becomes more and more important. Recently, the method based on S-N curve is carried out to evaluate the fatigue life of the ship structures, which is assumed that the material is continuum and without flaw. However, cracks or flaws of similar crack are existing in actual material, so applying fatigue crack propagation theory to forecast fatigue life becomes the development direction of fatigue research. Assessment methods based on fatigue crack propagation theory is adopted in this paper, providing reference for adding the fatigue crack growth theory into the ship structure fatigue strength specifications.Submarines often float and dive in service, so surface cracks are easily created under the changing alternating stress. In modern wars, submarines are inevitably affected by contactless shock wave from water, and brittle fractured is easily produced under impact loading. It will cause huge casualties and property losses. Therefore, researching the submarine structure with surface cracks under impact loading has become a very urgent task in dynamic fracture.The main researches are listed below:(1) Based on fatigue crack propagation theory and the result of FEM stress analysis, using the Growth module of MSC.Fatigue, fatigue life of connections details between bottom longitudinal, transverse bulkhead and transverse frame of a bulk carrier are analyzed. the effects of different crack shape ratios and bottom slamming load on fatigue life are discussed.(2) Using finite element analysis software ANSYS, the cone-cylinder shell model with surface crack is established, based on dynamic fracture mechanic, the dynamic stress intensity factor(DSIF) and the law of its maximum value changed along with the impulsive factor of the typical submarine structures with a initial surface crack are researched in this paper. Different depth of the submarine,different distance from the blast center and different orientation of blast point are considered when researching DSIF. Then taking 921A steel for an example, the unstable fractured of the submarine typical structure is preliminary judged under various conditions.The conclusions are as follows:(1) Crack growth rate is accelerated with the growth of load cycles, and it is lower in the early stage of crack propagation. Propagation life occupies the most part of entire life. The fatigue life of weld toe at transverse bulkhead is longer than at transverse frame.(2) The effect of different crack shape ratio on fatigue life is great. As the size of initial crack size is the same, the fatigue life increases with the a/c growth.(3) As slamming load has great influence on fatigue life, so it should be considered when analysing the fatigue life of ship structure.(4) Shock form of the dynamic stress intensity(DSIF) is similar for the same dive depth and action mode of impact loads, different distance from explosion heart to ship shell. Dynamic stress intensity factor of crack tip is bigger when the distance of explosion heart to ship shell is shorter.(5) The maximum value of the dynamic stress intensity factor increases with the rise of impulsive factor, and there is about linear relationship. The value of dynamic stress intensity is greater as submarine structure dives deeper under the same impulsive load and action mode of impact loads.(6) For the same dive depth, the shock brought by impact load on the circle is much stronger than on the axial direction, and the breadth is larger, it takes longer time to stabilize and the impact on the structure is greater. With the increase of impulsive factor, difference of the maximum dynamic stress intensity factor is also bigger under different action mode of impact load.(7) Under hydrostatic pressure action, submarine made by 921A steel doesn't occur brittle fracture suddenly in normal service. While under the impact load, the unstable fractured of the structure is preliminary judged when submarine dives to 100m(R=50m, 100m) or 300m.