Spectrum Fatigue Tests of Carbon Steels

Abstract

In the course of the development of the modern light construction, the spectrum fatigue tests have become more and more important. The authors made several spectrum fatigue tests by changing stress in normal distribution under rotary bending. The stress spectrum was obtained by changing the dead-loads. Two kinds of annealed mild steels were tested and the following results were obtained.
(1) The relation between the total number of stress repetition N_g and the meam amplitude of alternating stresses σ_a was plotted in normal S-N curve. Then the endurance limit σ_aw obtained from the σ_a-N_g curve was expressed as follows:
σ_aw=σ_w-σ_r
where, σ_w is the endurance limit of the original S-N curve and 2σ_r is the difference between the maximum and the minimum amplitude of stress on the normal distribution of stress.
(2) Total number of cycles to failure N_g is not affected by the order of stressing in load spectrum or by the load spectrum pattern, and calculated by the following formula:
N_g=N₁/α₁+α₂N₁/N₂+α₃N₁/N₃+…+α_kN₁/N_k
where, α₁, α₂, …, α_k are the cycle ratio of stresses σ₁, σ₂…, σ_k in one load spectrum, and N₁, N₂, …, N_k are the number of cycles to failure for the stresses σ₁, σ₂, …, σ_k in the original S-N curve.
(3) The results obtained by Corten-Dolan's method for calculating the total number of cycles to failure N_g in the spectrum fatigue. tests were found in safety side in almost all experiments.