| Cast steel joint has a good figure,and can be designed into complex shapes.The welding area can be removed from the joint to a less stressed and easier accessible area.Residual stresses can be prevented from the joint.In addition,the cast steel joint allows flowing forms without any sharp edges.Due to the above,it has already been widely used in offshore platform structures,bridge structures,tall building structures and large span spatial structures.In these applications,they may bear reciprocating loads which may cause fatigue problems.The fatigue problem of the circular butt-weld on cast steel joint is the weakest part.Therefore,experiment and finite element studies for circular butt welds on cast steel joints were conducted in the paper.(1)Fatigue tests of cast steel were carried out in order to study its fatigue performance.Fatigue tests were conducted on the standard specimens of G20Mn5 QT cast steel at strain ratio of-1,the strain-life(e-N)curve was obtained.Compared with other commonly used steels,G20Mn5 QT cast steel had a worse fatigue behaviour.It was because the pore like defects caused by the casting process which can not be eliminated,and the mechanism of fatigue failure of cast steel was revealed.(2)Fatigue tests were also conducted on the standard specimens of G20Mn5 QT cast steel at strain ratio of 0.1.The e-N curve was obtained and changed into the stress-life(S-N)curve.Compared with the method of reduction factor according to DNV,Basquin formula provides a more reliable reference on the modification of the S-N curve based on the mean stress.In comparison with formulas of Goodman and Geber,Solonberg formula shows an appropriate range of fatigue limits.(3)The e-N curve was obtained from the fatigue tests of the welded standard specimens.Nearly all the welded standard specimens were not fractured in the weld,but the heat affected zone of the welds at the cast steel side.The common fatigue sources were also casting defects,so the fatigue behavior of the welded specimens was similar to the G20Mn5 QT cast steel.The sizes of grain were not homogeneous because of welding,which led to a little worse fatigue behavior compared with the cast steel.(4)A new form of welding on the circular butt-weld was proposed,and the S-N curve was obtained.The new welding form exhibited a better fatigue behavior compared with the other two kinds of welding forms,while it was much worse than the welded standard specimens.In order to assure the welding quality,the welding forms provided a backing ring which caused a natural notch.The notch in the weld root gave rise to the stress concentration which dramatically accelerated the process of the fatigue.(5)Different welding forms and details led to different fatigue performance,thus finite element analyses were carried out.The curves of hot spot stress and fatigue life were proposed based on these three kind of welding forms.Fatigue lives analysed by effective notch stress approach were dangerous for engineering,while the modified one provided safe but imprecise fatigue lives.Local notch strain approach based on the base metal was complex and unsafe,while the local notch strain approach based on the weld specimens can provide a more accurate fatigue life,which had a good practicability and applicability.(6)According to the local strain approach based on the welded specimens,parametric analyses were carried out on the notch factor K_f.Within the regular range,K_f ranged from 1.5~ 3.0 and increased linearly with the tube's thickness t and the ratio of diameter to thickness d/t.As the increase of ratio of the thickness T/t,K_f increased firstly and then decreased,and reached the peak value when T/t=2.0.Based on the above,an improved local strain approach was proposed.Therefore,S-N curve can be simplified into a line,and can be expanded to different welding forms and parameters.In addition,the approach was proved available. |
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