Creating and Formulating Master Diagrams for Fatigue Strength Estimation of Machine Parts and Structural Elements

(Derivation from Laboratory Axial Load Fatigue Test Data of Small Notched Specimens)

Abstract

In the present study, a practical methodology for creating and formulating a master diagram which is utilized for fatigue strength estimation of machine parts and structural elements is discussed. The consideration proceeds based on the correlation parameter between fatigue strength of smooth and notched specimens “Equivalent cyclic stress ratio R_EQ” that is followed by “Size-dependent magnification factor F_Sk^M1”. The master diagrams originally represent the axial load fatigue strength diagrams of smooth specimens and therefore they are determined as material-specific diagrams without depending on the shape and dimensions of the cross-section. However, they are mainly derived from axial load fatigue strength data of laboratory small notched specimens rather than smooth specimens and the derivation is done by the reverse direction procedure against the method of estimating the fatigue strength of the notched specimen from that of the smooth specimen. From a methodological point of view, it is more effective on the contrary. First, the notch fatigue strength estimation method by previous study is reviewed and then next improvements and new idea for the derivation of master diagrams are proposed. Finally, some examples of master diagrams are shown.