Experimental analysis of bending fatigue strength of plain and notched case-hardened gear steels

Abstract Fatigue tests were performed on two commercial case-hardened steels using plain and notched specimens and results were compared with data available in the literature for similar steels. The purpose of the paper was to compare performances of different materials under the same testing conditions. Moreover, the results obtained may form the basis to apply ISO Standard methods mentioned in the Annex A, which estimate the fatigue life of gears starting from experimental data generated from specimens. Even though ISO Standard suggests primarily the use of experimental results generated from testing gears, specimens’ testing is faster and cheaper to evaluate different materials during the design process. From this point of view, specimens’ based approaches seem promising. Plane bending as well as axial fatigue tests were carried out, along with static tensile tests. Fatigue design coefficients relevant to the notch support effect, surface finish, mean stress effect in bending and load type (axial or bending) were derived from the experimental test results and, whenever possible, were compared with ISO Standard recommendations. Microstructures, hardness profiles, residual stresses and fracture surfaces were analyzed as well. To evaluate the accuracy of specimens-based approaches mentioned in the Annex of ISO Standard, additional work is needed in order to compare theoretical estimations with experimental results obtained from gears.