Effect of Static and Intermittent Shear Stress on the Fatigue Strength of Notched Components under Combined Rotating Bending and Torsion

In power transmission machinery or crane industries, shafts are designed using specific standards that may not correctly cover the large panel of materials and loadings involved in these sectors, making it necessary to use safety factors leading to over-conservative life predictions and non-optimized designs.Shafts are key components, usually experiencing alternated normal stresses combined with static or fluctuating (intermittent) shear stresses. In high cycle multiaxial fatigue, it is admitted that mean shear stress has no significant effect on the fatigue strength of smooth specimens if the maximum shear stress does not exceed about 80% of the material shear yield limit. Under combined rotating bending and torsion the situation is unclear, especially in presence of notches.In this paper the effect of a static and intermittent shear stress on the fatigue strength of two quenched and tempered steel grades, 30NiCrMo8 and 42CrMo4, is studied on notched specimens with bending theoretical stress concentration factors Kt = 1.7 and Kt=2.7. It is shown that mean shear stress has little effect on the median rotating bending endurance limit at 3×106 cycles, the maximum decrease is about 5%, whereas this decrease is more pronounced for intermittent shear stress (varying in blocks), reaching up to 30% for the sharpest notch.