Procedure for Estimating Bending and Torsional Fatigue Strength of Smooth Specimens on Criteria of Axial-Load Fatigue Strength Diagrams

(Based on the Concept of Equivalent Cyclic Stress Ratio R_EQ Followed by Dimension-Dependence Magnification Factor of Yield-Zone Partition Type in an Elastic-Completely Plastic Body)

Abstract

The equivalent cyclic stress ratio R_EQ followed by elastic-completely plastic yield zone partition type cross-section dimension dependence magnification factor, which is modified to fatigue strength criterion specification, is applied to estimation of plane bending fatigue strength of a smooth bar with rectangular, circular and rhombic cross-section, and rotating bending and torsion fatigue strength of a round smooth bar from an axial load fatigue strength diagram of a smooth specimen. This approach is expected to be effective in elucidating the size effect and its material dependence.

The main issues are as follows;

(1)R_EQ is calculated by using a plastic/elastic section modulus ratio (a shape factor) of elastic-completely plastic smooth specimen Z_Y/Z_E in place of the stress concentration K_t.

(2)Cross-section dimension dependence magnification factor F_S0 is derived as a yield strength criterion specification by means of the coordinate transformation of an equation of a yield zone growth curve, and the magnification factor F_S0^+- is modified to F_S0^kM as the fatigue strength criterion specification by using Schütz parameter M (a coefficient of a proportional term related to mean stress of a linear equation of fatigue strength).

(3)Rotating bending fatigue strength of a round bar specimen is simulated as completely reversed plane-bending fatigue strength of a smooth bar specimen with a cross-section of a pair of the isosceles triangles connected and fixed at the vertices.