Fatigue Life Prediction And Reliability Assessment Of High-strength Steel Welded Joints With Geometric Defects

With the developments of ship transportation,mechanical engineering and other structure industries,welding structures are rapidly developing for the direction of“high reinforcement”“lightweight”and“alloying”,and modern technologies have further demands of"digitalization"and"durability",refined analysis of fatigue life and reliability of welded structures has received widespread attention.However,the microstructures of the welded areas originate from the complex effect of heat,force and materials,which are the"hardest hit"areas for weld defects such as crack,undercut and misalignment,they are the significant reasons of fatigue crack generation and fatigue life reduction.Therefore,fatigue life prediction and reliability evaluation combined with welded characteristics and defects have great significance for the stable operation,health detection and maintenance of the integral structures.In this thesis,the fatigue life and reliability of defective Load-carrying Cruciform Welded Joints（including the undercut,axial and angular misalignments）were systematically studied.Firstly,the impact of welded characteristics and defects on the local stress state and fatigue life were analyzed with the fatigue experiment and finite element analysis method,the fatigue life and failure analysis model with defect correction method were established.Secondly,the differences and applicability of different methods in assessing the local stress state or the fatigue crack expansion were analyzed,then established and verified the fatigue reliability analysis models by combining with the various defect characteristics,the influencing factors and defect distribution characteristics were evaluated systematically.The analysis methods and main results are as follows:（1）Fatigue life assessment of defective welded joints was carried out by the Nominal Stress Method,Hot Spot Stress Method,Effective Notch Stress Method and Average Strain Energy Method.The analysis of the Nominal Stress Method showed that there was a 39.8%and 28.04%fatigue class reduction when the axial and angular misalignment within E/T=0.2 and A=2°threshold values.The Average Strain Energy Density Method has an advantage in fatigue life assessment,which takes into account factors such as welded characteristics and defects by considering the notch tip field energy,and it showed a smaller discrete interval in the S-N curve.In addition,the misalignments had a significant effect on the average strain energy density at the weld root and toe combined with the established analysis model,and with the increase of defect degree,the fatigue failure at the weld toe was more significant,the comparison with experimental results were in good agreement.（2）Comparison of the mechanical properties of basis material,heat-affected zone material,equal-match and low-match welding materials.The static tensile results showed that low-match welding regions of 921 and 907L were increased for the material flexural strength ratio and material ductility,and the yield strength decreased correspondingly.Crack expansion rate results showed that the different regions of resistance to crack expansion in the order of U-WM>HAZ>E-WM>BM,combined with the Nasgro equation fitting results,the crack expansion threshold:K_th,U-WM>K_th,HAZ>K_th,E-WM>K_th,BM.It means low-match materials have an improvement in crack expansion resistance,which may be attributed to the improvement in flexural strength ratio and ductility properties of the material.Taking the base material as the research object,the fatigue limits of axial and angular misalignment decreased 48%and 66.1%in the interval of 0 mm～1 mm and 0°～6°,respectively.（3）Considering the influence of multiple factors in the actual welding process,weld defects often follow the maximum distribution,lognormal distribution,gamma distribution,or the other types.The fatigue damage model showed that the defective welded joints fail first when compared with the non-defective welded joints with the fatigue failure conditions start at 3.5e4 and 3.6e5 cycles,respectively.Fatigue reliability analysis based on the local stress method and fracture mechanics method showed the results:The mean value of defects increases,the fatigue life and reliability decrease,and the fatigue reliability decreased from 97.9%to 33.95%when the specified critical damage conditions.With the dispersion increases,the fatigue life interval increases,and the fatigue damage probability increases under the same fatigue life.Therefore,the welding stability should be improved and the discrete state of defect parameters should be emphasized in the actual welding process,especially for the large dispersion of the welded joints,which is beneficial to the accurate evaluation of fatigue life.