Study Of Fatigue Dispersion Of Notched Parts Considering Stress Gradients

Mechanical components use many types of notches in order to meet diverse functional or inter-component assembly requirements.For notched components,factors such as notch size,load state and microstructure can directly affect fatigue damage behaviour,leading to fatigue dispersion.Combining crystal plasticity theory with fatigue indicative factors to study fatigue dispersion under different notched conditions can describe the crack sprouting driving force at the microscopic level and quantitatively analyse the fatigue dispersion law using polar statistical methods,which can not only provide a scientific basis for the design of notched structures to reduce the occurrence of accidents,but also reduce test costs.In this paper,a total of 1900 statistical volume cells with random grain orientation were created and 3420 computational analyses were performed using a combination of crystal plasticity finite element and extreme value statistics for a rectangular section notched part of 6061-T6 aluminium alloy material.The factors affecting the distribution of the extreme values of the fatigue indicator factor are firstly investigated;then the shortcomings of the conventional fatigue indicator factor are discussed,and the conventional fatigue indicator factor is improved by considering the influence of stress gradients;a comparison method is used for verification;finally the fatigue indicator factor is analysed for grain deformation inhomogeneity,and the connection between fatigue dispersion and grain inhomogeneity is discussed.The main research work in this paper is as follows:1.A fatigue indicator that takes into account the stress gradient at the notch is improved on the basis of the existing fatigue indicator by considering the stress gradient,the equivalent force and the longitudinal strain.In order to verify the applicability of the fatigue indicator to the fatigue dispersion problem of notched parts,the improved fatigue indicator is compared with the existing fatigue indicator from three perspectives: different displacement load amplitude,load ratio and notch diameter size,and it is found that the improved fatigue indicator is more applicable to the fatigue dispersion problem of notched parts.2.In order to improve the accuracy of the fatigue dispersion prediction results,the key factors influencing the extreme value distribution of the fatigue indicator factor were investigated.As a result,the higher the number of grains contained in a representative volume cell of 6061-T6 aluminium alloy,or the higher the number of statistical volume cells analysed,i.e.the more statistical information available,the better the fit of the Gumbel linear fit line to the extreme values of the fatigue indicator,and the better the fatigue dispersion of the notched part can be characterised.3.The applicability of the improved fatigue indicator factor to the problem of grain inhomogeneity in notched parts and the effect of grain inhomogeneity on fatigue dispersion is investigated.It is found that it is feasible to use the improved fatigue indicator to describe the inhomogeneity;in addition,as the grain deformation inhomogeneity increases,it affects the longitudinal stresses and longitudinal strains within the grain,increasing the crack initiation drive,and thus fatigue dispersion decreases.