Research On Fatigue Life Prediction Of Welds With Undercut Imperfection Based On Zero-Point Characteristic Structural Stress/Strain Method

Welding structures are widely used in machinery,electric power,railway,aviation,and other industrial fields due to its good workmanship and economy.Due to the stress concentration caused by geometric mutation in the structure and welding imperfections during welding,the fatigue bearing capacity of welding structures is often lower than that of the base metal.Among these welding imperfections,undercut,as a common imperfection form at the weld toe,usually leads to more complex stress concentration,which seriously affects the fatigue life of the weld.There are generally two methods for the fatigue assessment of undercut imperfections in engineering,namely qualitative analysis based on the definition of quality level by undercut depth,and quantitative analysis based on the relationship between imperfection quality level and FAT fatigue level to approximately predict the fatigue life of welds.Due to insufficient parameter considerations and the limitations of traditional fatigue stress,both methods are unable to accurately identify the impact of undercut imperfections on the fatigue performance of welded structures.Meanwhile,both methods are based on fatigue assessment under linear elastic conditions,so the coupling effect of notch effect and plastic effect are not considered.Based on the limitations of fatigue assessment for undercut with mentioned above,this paper conducts in-depth research on the identification parameters of notch effects and fatigue life prediction methods for welds with undercut imperfection.The main content is as follows:（1）Both the effective notch stress method and the structural stress method have the ability to identify stress concentrations at the notch.Therefore,fatigue life prediction for welds with and without undercut is performed based on two methods,respectively.In the effective notch stress method,the improved method based on virtual radius can be used for imperfection-free welds,and the original method based on average stress length for undercut welds.The calculation results show that the fatigue life prediction accuracy of the two methods is low,because the selection of virtual radius and average stress length has a great impact on the accuracy of the results.The structural stress method can obtain more accurate life prediction for imperfection-free welds,but for undercut welds,the simulation method and formula method are used to obtain overestimated and underestimated life prediction results respectively,because these two methods can not accurately identify the stress concentration caused by notch effect at the undercut root.（2）The limitations of identifying notch effects in structural stress were explored,and the zero-point characteristic structural stress method was proposed for identifying notch effect and stress concentration of undercut welds.According to the combination of extreme values of geometric parameters of weld undercut imperfections in engineering,16 kinds of limit undercut models are obtained,and the structural stress of each model is calculated.The results show that the structural stress can only identify the influence of undercut depth on the notch effect,but cannot identify the influence of other geometric parameters.Based on the basic theory of structural stress method,combined with the concept of zero-point length reflecting the range of notch effect,and a two-stage crack propagation model for nonlinear stress equivalence,the zero-point characteristic structural stress method is proposed.The geometric parameters of undercut are analyzed by controlling variables,and the influence trend and sensitivity of different parameters on zero length x_zpand characteristic structural stressσ_zpin the method are explored.The results show that x_zpcan reflect the range of the notch effect,andσ_zpcan reflect the influence of different undercut parameters on the notch effect;For undercut geometric parameters,undercut depth d has the greatest influence on the notch effect,depth ratio R_n/d takes second place,and weld side angleθand undercut opening angleβhave less influence.Due to the accurate identification of the notch effect by characteristic structural stress,this method can also be used to calculate the fatigue notch factor.（3）In order to consider the coupling effects of plasticity and notch effects,the zero-point characteristic structural strain method was proposed.Firstly,the possible stress states of undercut weld and the stress and strain distribution along its ideal path are analyzed and discussed.Strain was used instead of stress to improve the plasticity of the zero-point characteristic structure stress method.Based on the driving strain equivalent method,the structural strain was obtained and the zero-point characteristic structural strain parameter were obtained.An accuracy analysis was conducted on the results of the driving equivalent method for calculating structural stress,and a formula for calculating the critical element size was obtained.The simulation calculation of existing undercut weld specimens shows that compared to the zero-point characteristic structural stress,the zero-point characteristic structural strain can effectively consider the plastic effect of the undercut weld and is applicable to both linear elastic and elastic-plastic conditions.（4）The fatigue life prediction method based on zero-point characteristic structural stress parameters-equivalent characteristic structural stress method-was studied and derived.Based on the integral form of the Paris crack growth model,the shape parameter of the stress intensity factor and the correction parameter of the cyclic plastic zone are introduced.By defining the I_nintegral and the equivalent characteristic structural stressΔS_zpin the formula,the equivalent characteristic structural stress method based on the zero-point characteristic structure parameters is obtained.The characteristic structural strain can also be transformed into pseudo-elastic characteristic structural stress for life prediction by this method.The calculation formula of modified equivalent initial flaw size（EIFS）is proposed to calculate the initial crack size in I_nintegral.The simulation analysis of undercut test datas verifies the method’s effectiveness in predicting the fatigue life of undercut welds.（5）The fatigue tests using standard undercut specimens and engineering examples have verified the engineering application ability of the equivalent characteristic structural stress method.Firstly,the fatigue life prediction of butt weld specimens with undercut defects was conducted,and the comparison with experimental data shows that this method has high prediction accuracy.Then,the imperfection evaluation standards commonly used in engineering are compared and analyzed,and the standard evaluation of the assumed undercut imperfection in a shock absorber seat is carried out.The results show that the fatigue life prediction based on the imperfection quality levels cannot consider the influence of the transverse length of the imperfection,the relative position with the overall stress concentration of the structure,and the opening direction.Finally,considering the problem of mesh size differences in large welded structures,an engineering application method combining submodel technology and equivalent characteristic structural stress method is proposed.The fatigue failure case’s simulation calculation and life prediction of an electric locomotive bogie frame are carried out,and the results show that the equivalent characteristic structural stress method combined with the sub-model technology can be used to predict the fatigue life of undercut welds in large welded structures.