Notch Fatigue Life Analysis Of Welded Joints Based On Multiaxial Criterion

Fatigue failure is one of the most important failure modes in welded structures,the welding components on the bogie of railway locomotives often bear the effect of multi-axle loads.Therefore,it is of great practical significance to develop the multi-axial fatigue theories and establish the multi-axial fatigue assessment methods which can be applied to engineering practice.Compared with uniaxial fatigue,multi-axial fatigue is more complicated in mechanics analysis and physical mechanism,it is still a difficult and hot issue in the field of international fatigue research at present.At present,the traditional nominal stress method and structural stress method(hot spot stress method)are widely used at home and abroad.The nominal stress method ignores the influence of structural weld details,therefore,it is difficult to determine the nominal stress parameters,and it is difficult to match the typical welding joints.The structural stress method takes into account the size of weld details,however,there is a strong mesh sensitivity at the weld toe and the weld root notch,which can easily lead to the local stress singularites,therefore,in many cases,the extrapolation method is used to estimate the stress state at the weld hot spot.Even so,the method also has difficulties in fatigue analysis at the root of weld.In order to overcome the defects of traditional anti fatigue design method,in this paper,the fatigue life of welded joints is studied by a more accurate and reliable notch stress method,so that the experimental data of different welding joints can be unified in the same fatigue life curve coordinate system by notch stress method.In view of that the fatigue assessment of welded joints is only applicable to uniaxial loading,in this paper,the multi-axial fatigue analysis of welded joints is carried out based on the notch stress method and the multi-axial criterion.In this paper,several common welded joints used in engineering are selected,and the notch stress concentration factor is calculated by using the nominal stress data and the finite element method,so that the notch stress of welded joints can be obtained;According to Von Mises??W and EESH these three multi-axial fatigue criteria,the S-N curve can be calculated;Then,we use the least square method to fit the different fatigue life curves;In the end,the new multi axle notch life curve is used to predict the fatigue life of the main frame of bogie frame.Compared with the uniaxial curve recommended by IIW standard,these multi-axial notch fatigue curves have lower inclination and higher fatigue rating.On account of that whether the stress concentration factor of welded joint can be determined accurately will have a direct impact on the calculation results of notch stress,therefore,we discussed the influence of welding parameters on notch stress in the fourth chapter.Through analysis,the effect of the decrease of the throat depth on the equivalent notch stress is very limited,and the effect of reducing the weld toe angle is more obvious;Grinding wheel grinding is very obvious to reduce the equivalent notch stress,compared with the original design parameters,when the weld toe is in the grinding radius of 6mm,the notch stress can be reduced by at least 20%.