A Study of Crack Propagation at Stress Amplitude below Fatigue Limit

Abstract

To clarify the effect of stress amplitude below the fatigue limit on fatigue lives, two step and multi-fold program fatigue tests were carried out and the crack propagation behavior of cyclic stresses was investigated after pre-loading.
The experimental results are summarized as follows.
(1) The relation between the crack propagation rate and the crack length can be represented by
dl/dn=10^{-(7.99-0.116σ)}l^{(2.26-0.042σ)}
(2) If the crack propagation rate at the stress amplitude below the fatigue limit were estimated by the above relation from the crack propagation rate at the stress amplitude above the fatigue limit and the crack length, the fatigue lives of program fatigue tests at various stress amplitudes above and below the fatigue limit could be calculated. The experimental fatigue lives were compared with the calculated fatigue lives. The fatigue lives calculated by this way agreed with the experimental fatigue lives more exactly than those calculated by Miner's law or modified Miner's law.
(3) The crack propagation rate in the initial period of the repetition of σ₂ was faster than the estimated value, but with increasing the number of cycles of that stress, it became slower than the value estimated by the above equation.
(4) The crack propagation rate of σ₁ was not influenced by pre-loading σ₂ when the fatigue crack was propagating at cyclic stress σ₂. But when the fatigue crack became the non-propagating crack with icreasing the cyclic numbers of σ₂, the crack propagation rate of σ₁ was influenced by the preloading and became slower than the estimated value by the above equation.