Research On High-Cycle Fatigue Strength Assessment Of Notched Components Based On The Critical Distance Method

In engineering structures,the existence of notch is unavoidable and can significantly reduce the fatigue strength of the structure.Therefore,an accurate assessment of the notch effect on the fatigue strength of a structure is fundamental to the fatigue reliability design of structures.The critical distance method is one of the widely used methods for high-cycle fatigue strength assessment of notched components.In the traditional critical distance method,the critical distance is taken as an inherent material parameter.However,it has been shown that the critical distance is also related to the notch shape and size.In this paper,a new way of determining the critical distance is proposed from the perspective of the effect of notch on stress distribution,and the Zero-point Critical Distance Method(ZCDM)for fatigue assessment of notched components is further developed.Based on the developed ZCDM,the fatigue limit prediction of notched components and the high-cycle fatigue strength assessment of welded joints is carried out.The main research results are as follows:(1)The development and application of the critical distance method in the fatigue assessment of notched components are summarized and analyzed.Taking the fatigue limit assessment of the plate with the central hole as the research object,the accuracy of different critical distance methods is systematically evaluated.Results show that the line method(LM)of the Theory of Critical Distances(TCD)has the highest prediction accuracy and the simplest form.Besides,the limitation of using fixed critical distance in the traditional methods is demonstrated by analyzing the notch size effect.(2)Based on the stress classification method,the ZCDM is proposed to assess the fatigue limit of notched components.In the ZCDM,the critical distance rises with the increase of notch radius.By comparing the fatigue test results of 68 sets of specimens with different materials and geometries,it is concluded that the prediction error of the proposed ZCDM is within ±15%.Besides,the comparison against the LM shows that the ZCDM has the advantages of higher accuracy and fewer parameters needed.(3)A unified mean stress correction model of the fatigue limit of metallic materials is proposed from the perspective of energy dissipation during fatigue damage process.The accuracy and applicability of the model are verified by comparing 145 sets of fatigue data of different materials under various stress ratios.On this basis,the mean stress correction model of the fatigue crack propagation threshold is also derived based on K-T theory.The proposed mean stress correction model can be used to solve the mean stress effect on the fatigue limit of notched specimen on the condition of elastic deformation.(4)The nominal stress method,the structural hot-spot stress method,the effective notch stress method and the ZCDM are used to assess fatigue property of the same cruciform joints,with the S-N curve dispersion band index Tσ of 2.29,2.16,2.09 and 2.02,respectively,proving that the ZCDM can effectively assess the local shape of the welded joint on fatigue performance.Besides,the ZCDM is also used to explore the optimal process parameters when adopting high frequency mechanical impact(HFMI)treatment to improve the fatigue performance of welded structures.The results show that a minimum zero-point effective notch stress can be obtained with a needle diameter of 3-4 mm.In addition,as the impact depth increases,the zero-point effective notch stress also rises.As a result,for actual welded structures,excessive treatment should be avoided after eliminating the sharp notches in the original weld toe.