Fatigue Analysis of Threaded Holes : A project performed at ÅF using static structural analysis in ANSYS

Sammanfattning: The effect of a plain hole on the fatigue properties of a body undergoing uniaxial loading is a well-studied area. However, for the case of threaded holes that are not being used together with a bolt, little can be found in the literature. In this project, stress concentration factors, Ktn, for threaded holes, following the ISO metric standard, have been calculated using finite element models in ANSYS. Based on theser esults, estimations of the fatigue notch factor, Ktf , for the threaded holes were derived. The magnitude of Ktf depends on how the notch sensitivity index, q, is being determined. In this project, both Neuber’s and Peterson’s formulas were used as estimates of q and a comparison between the two methods was made. The stress concentration factors for the threaded holes were significantly higher than the corresponding stress concentration factors for plain holes. As the size of the hole and the surrounding body was increased, Ktn decreased. In the fatigue analysis, Peterson’s formula predicted higher values of the fatigue notch factor compared to Neuber’s and could therefore be useful in design purposes. Furthermore, the reduction in fatigue strength for the threaded holes, compared to the plain holes, was calculated. These results showed that the percentage decrease in fatigue strength for threaded holes was approximately the same, regardless if Neuber’s or Peterson’s estimation of q was used. The reduction in fatigue strength for threaded holes with a coarse pitch, compared to plain holes, varied from about 16 % for the smallest diameter tested(M6) to about 10 % for the biggest (M68). The corresponding reduction for threaded holes with a fine pitch varied from about 14 % for the smallest diameter (M6) to about 9 % for the biggest (M64). As the size of the hole and the surrounding body was increased, the threads’ effect on the fatigue strength decreased.