The Effect of Stress Sequence on the Fatigue Strength under Program Loading

Abstract

The annealed S35C carbon stetel was tested under the many-fold multiple repeated stresses in four stess levels in rotating bending. The effect of the stress sequence on fatigue lives was investigated in association with the effect of the initial stress level and the magnitude of stress levels as a whole. The number of cycles in one program block, n₀, was chosen at 3600. The results show : (1) The effects of the initial stress level and the stress sequence on the fatigue lives can not be seen, because the value of n₀ in this experiment is very small in comparison with the total number of cycles to failure. (2) When the maximum stress σ₄ is high and all the stresses are overstresses, the actual fatigue lives coincide nearly with the theoretical ones which are calculated by using the linear damage law. When σ₄ becomes low, the actual fatigue life Nf becomes samller than the theoretical one Nₜₕ, and Nf reaches the smallest value at a certain stress level. When the all stresses except σ₄ are understresses, Nf becomes equal to or larger than Nₜₕ.