Boundary Of Fatigue Crack Initiation And Propagation And Its Application In Cruciform Joint

The boundary between the crack initial initiation and the propagation of fatigue is still in the stage of qualitative analysis,which hinders the quantitative calculation of fatigue life.All the research related to the crack initial initiation and the propagation can only avoid this problem.Therefore,this paper proposes the definition of the boundary between the initial initiation and the propagation,which can solve the above problems and carry out related calculations on the fatigue life,which is also the main content of this paper.In the process of proposing the definition of the initiation and propagation stages,two concepts are proposed: the crack initiation rate and the notch stress ratio,respectively.The former can derive the unity of the Manson-Coffin formula and the Paris formula.The fatigue calculation system can fully derive the relationship between the crack growth rate and the stress intensity factor from the Manson-Coffin formula,and each parameter in the formula has physical meaning,rather than an empirical fit such as the Paris formula.The latter can provide the meaning of the threshold region.Explain that the traditional fatigue theory considers that the threshold region is the intrinsic area of the material and is a material property.This paper denies this statement and explains the meaning of the threshold region based on theory and experiment.The final conclusion is: threshold region is not a material property,and the reduction of the notch stress ratio is a necessary condition for the occurrence of the threshold region.After the abovementioned paving content,a complete definition of the crack initiation and the propagation phase is finally given.The definition consists of two parts,a cracked body and a non-cracked body,which expands the concept and application range of the crack initiation stage.At the same time,based on the above viewpoint,the revised Miner criterion is given,which also raises another point: in the process of crack propagation,the "re-growth" of cracks still occurs.Aiming at the welding cruciform joint,a simplified method of welding residual stress based on shrinkage strain method is proposed.This method can ignore the randomness of residual stress,such as random factors,welding process and environment,while maintaining the outstanding characteristics of welding residual stress.The ABAQUS extended finite element method establishes the calculation system of the residual stress intensity factor.For the cruciform joint with a thickness of 10 mm,the maximum position of the residual stress intensity factor is not at the weld toe position,but at a position 2 mm from the weld toe.The influence of residual stress on the quantification of the initiation and the propagation of fatigue can be used as the basis for the calculation of the full fatigue life under the influence of residual stress.The effects of the notch radius,eccentricity and external load of the weld toe on the crack the initiation and propagation phases of the non-cracked body are investigated.The results show that the most important factors affecting the initial life are the residual stress and the external load,notch radius and eccentricity.The most important factors affecting the propagation life are residual stress,external load and eccentricity.The factors affecting the boundary between crack initiation and propagation are analyzed.The results show the boundary dimensions between crack initiation and propagation are not a constant,but a variable that varies with structure,load,residual stress,and so on.The effects of initial crack size and external load on the crack initiation life and propagation life of cracked bodies were analyzed.The full fatigue life of the welded cruciform joint was calculated,and the influence of the notch radius and joint eccentricity on the full-life S-N curve was analyzed.The key problems of welding fatigue were solved:(1)Whether the welded joint has crack initiation stage(2)Causes of the thickness effect of the welded plate.The results show that:(1)The cruciform joint with residual stress does not have the initial stage of crack,because the residual stress reduces the initial life and propagated life of the crack,but the residual stress reduces the initial life of the crack more.The proportion of the initial life is only 8% at the condition that radius is 0.5mm,and in the case of the irregular height of the actual weld toe,the proportion of the initial life of the crack is basically zero;(2)The reason for the thickness effect of the welded joint is that the stress intensity factor generated by the welding residual stress field makes the magnitude of the initial crack insensitive to the effect of the life.