Low-cycle fatigue in rotating cantilever under bending. III: Experimental investigations on notched specimens

Low-cycle fatigue tests of rotating cantilevered notched beams under deflection controlled conditions are conducted on four grades of high strength medium alloy steel of hardness 280, 310, 350 and 380 HB. The cyclic stress–strain response and strain-life fatigue curves are determined. The effects of hardness, prestraining and machine allowance after heat treatment, on the fatigue life have also been studied. A numerical methodology derived from a recent theoretical analysis is utilised which permits the determination of cyclic stress–strain response (n′ and k′) and strain-life curve of any material from its measured rotating bending data, namely: the nominal bending stress and both the vertical and horizontal deflections. The cyclic strain-hardening exponent, fatigue strength exponent and fatigue ductility exponent were found to be almost constants, being 0.104, 0.074 and 0.68, respectively, on the average. In notch fatigue tests under conditions of rotating bending, the actual measured lives in notched specimens for the four investigated materials were five times greater, on the average, than the predicted lives based on crack initiation model. Furthermore, abolishing the machine allowance after heat treatment for the notched specimens was found to lower fatigue life, in general, under rotating bending. This fact has serious implications in practice for through-hardened materials.