Experimental And Numerical Study On The Effect Of Residual Stress On Fatigue Strength Of The Ship Typical Welded Joints

Large ship above water and under water structure have been paid more and more attentions in engineering field. The working conditions of these large ship structures are often very complex and these structures are subject to many kinds of periodic loads, so they can be damaged easily. In order to improve the strength, large ship above water and under water structures are usually builted by the high strength steel. Fatigue strength decreases at the same time of ensuring the structure’s strength, so fatigue life of the structure must be checken. The effect of welding residual must be taken into account at the assessment of fatigue life because welding residual stresses exist in welded structures. So the effect of welding residual stress on fatigue strength of ship typical welded joints is a very important work to study.Typical welded joint of FPSO and cone cylinder pressure structure were selected as the research objects. In this paper, the initial residual stress and released residual stress of the typical welded joints are studied with finite element method and experiment method firstly, and then, the influence of the residual stress on fatigue life of the typical welded joints is researched with numerical simulation method and experiment method. The main research work and important conclusions are listed as follows:(1) Typical welded joint of FPSO and cone cylinder pressure structure of high strength steel were selected as the research object. The welding simulation program was improved by APDL language of the large commercial finite element software ANSYS. Initial welding residual stress and release of the residual stress of typical welded joints was studied with sumilation method.(2) The numerical simulation of the initial welding residual stress of typical welded joint of FPSO was done. The results show that the residual stress on the surface of the weld toe area was largest and it was tensile stress; the residual stress decreased gradually, and part of the stress translated into the compressive stress far from the weld toe. Then, initial residual stress was measured with X-ray nondestructive detection metod, and it shows that the results are consistent with simulative results.(3) The numerical simulation of the initial welding residual stress of typical welded joint of cone cylinder structure was done. The results show that the tensile residual stress at the weld fusion line was largest on the surface of the concave cone, and the compressive residual stress at the weld center line was largest on the surface of the convex cone. The residual stresses decrease rapidly far from the welding area. Then, initial residual stress was measured with X-ray nondestructive detection method. The results are consistent with simulative results. It proves that the numerical simulation of the welding residual stress is reasonable and reliable.(4) The numerical simulation of the released residual stress was done, and the effects of the external load on the release of the residual stress was studied. It shows that value of residual stress reduce after release and the trend of distribution keeps constant. The release of the residual stress is not affected by the number of cycles of the external load or the sequence of external load, and it is affected by the amplitude of the external load. Then, released residual stress was measured with X-ray nondestructive detection method, and the results are consistent with simulative results.(5) The hot spot stress of FPSO typical welded joint was calculated and the fatigue life was simulated. Fatigue life with consideration of welding residual stress was compared with fatigue life without considering the residual stress based on the S-N curve method. It shows that fatigue life with consideration of welding residual stress is obviously smaller than fatigue life without considering residual stress.(6) Fatigue life with residual stress and without residual stress of typical welded joint of cone cylinder structure was simulated based on the local stress-strian method. It shows that fatigue strength of the typical welded joint of cone cylinder structure is decreased with residual stress and the effect of residual stress is crucial to fatigue life. The test of fatigue strength of typical welded joints of the cone cylinder was taken. The results are close to the sumilation results. It proves that the simulation method of fatigue life is reasonable and reliable.