Research On Fatigue Assessment Models Of Welded Joints Based On Notch Stress Approach

Welded structures are widely used in ships and marine structures.Fatigue failure is one of the main failure modes under the combined action of wind and wave loads.In the field of high-cycle fatigue assessment,the notch stress approach based on the microstructural concept,is an emerging fatigue assessment approach that can consider the influence of weld geometry,welding misalignment,and dimensional effects on fatigue strength.In addition,only one S-N curve is needed to evaluate all kinds of joints,so it has a wide range of engineering applications.Related research,however,is still in the exploratory stage.In view of this,work was carried out from two perspectives.On the one hand,based on the relevant specifications of the International Institute of Welding(IIW),fatigue assessment models including as weld and HFMI treated joints are systematically established by using the fictitious radius of 1mm.On the other hand,the support factor S is an important factor determining the value of the fictitious radius.The variation rule of S under different geometric parameters is analyzed,which makes a contribution to the improvement of the scheme of fictitious radius values.The main contents and conclusions are concluded as following:(1)The notch stress concentration factor K_t is an important parameter for analysis using the notch stress approach.Based on the linear elastic finite element theory,the stress concentration factor of typical welded joints with different weld geometry parameters under different loads is studied.The results show that the influence of plate thickness t and reinforcement angle?on stress concentration is more significant under the condition of small weld toe notch radius r.The dimensionless parameter t/r is recommended to evaluate the stress concentration coefficient of the joints.(2)Using the 1mm fictitious radius recommended in the relevant specifications of IIW for analysis,the safety of the two S-N curves recommended by IIW for as-welded joints was discussed.The results show that the S-N curve recommended by IIW(FAT=225MPa,m=3)are safe for uniaxial tensile or pure bending.The S-N curve(FAT=160MPa,m=5)recommended by IIW,however,is conservative for the pure torsion.In view of this,the slope of the curve m=5 can be kept unchanged,and FAT value can be increased to below 200MPa.For multiaxial fatigue,the assessment result is dangerous when the number of cycles is in the range of 1×10~4?1×10~5,and it is too conservative over 1×10~5 times when using the reference S-N curve(FAT=225MPa,m=3)recommended by IIW.The S-N curve with higher FAT and lower slope was obtained.For joints treated by HFMI,the SWT model can comprehensively consider the influence of stress ratio R and yield strength f_y.Based on it,the S-N curve(FAT=325MPa,m=6.5)for fatigue evaluation was obtained.(3)Based on the butt weld with different geometric parameters under plane strain,the variation rule of support factor S was discussed.It shows that S is related to plate thickness t,that is,S has"thickness effect".(4)It is found that the distribution of uniaxial and multiaxial fatigue tests data with unified fictitious radius of 1mm shows the characteristic related to plate thickness.Combined with the"thickness effect"of S,a new model that can be introduced into different fictitious radius for analysis according to different plate thickness was proposed,which can effectively reduce the scattered band of test data.