The fatigue behavior of the SAE 4140 steel under combined stress load

This study examines the fatigue behavior of the SAE 4140 steel under combined stress loads. Machine elements work in their operational environments generally under combined stress loads. Identification of fatigue behavior of elements under combined stress loads is extremely important for the designers. In order to examine fatigue behavior under combined stress, initially a fatigue test device producing combined stress loads on test specimen was designed and manufactured. The specimens were produced employing four different production methods to examine effects of the production methods on the test specimen. It was observed that the best method in specimen production was to produce the specimen in CNC turning machine between double chuck. For the calibration of the fatigue test machine, theoretical calculations have been made and combined stress load calibrations were made using the test specimens. It was seen that the data obtained from theoretical calculations and the data obtained from experimental tests largely overlap and so fatigue test machine calibration was completed. The behavior of the fatigue test specimen under combined stresses (bending+tensile, bending+compression) were examined with the calibrated rotary bending fatigue machine. Based on the data obtained from the tests, Wohler fatigue diagrams, Goodman Master and Gerber Master diagrams were generated. With the examination of the obtained data, it was observed that during rotary bending fatigue tests the tensile stress reduces and compressive stress increases the fatigue strength of the specimen. Keywords: Fatigue under combined stress, SAE 4140 steel, rotary bending, fatigue test