Prediction of the Effects of Phase Difference and Mean Stress on the Fatigue Strength of Specimens Containing Small Holes under Combined Loading

Abstract

A criterion is presented that allows us to predict the effects of phase difference and mean stress on the fatigue strength for hole-containing components subjected to combined load. Combined axial and torsional fatigue tests were conducted using annealed S35C and quenched/tempered SCM435H specimens containing a small hole of either 100 μm or 500 μm in diameter. The phase differences between axial and torsional loads were 0 and 90°. Effects of mean or static stresses were also investigated. The non-propagating cracks emanating in the radial direction from the holes were observed at the fatigue limit. Under the same loading condition, they were present on a plane inclined at the same angle to specimen axis. This suggests that a crack on this specific plane, a critical plane, would play an important role in the determination of fatigue strength. A criterion was proposed on the basis of the assumption that at the threshold level, the time-variation in the mode I stress intensity factor of a crack under multiaxial cylic loading is equal to that under uniaxial cyclic loading. Using this criterion, a prediction method of fatigue strength was also presented. This method is useful since no fatigue test is necessary in making predictions. Good agreement between predictions and experimental results was obtained.