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
Ng 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-
Ng curve was expressed as follows:
σ
aw=σ
w-σ
rwhere, 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
Ng 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=N
1/α
1+α
2N
1/N
2+α
3N
1/N
3+…+α
kN
1/N
kwhere, α
1, α
2, …, α
k are the cycle ratio of stresses σ
1, σ
2…, σ
k in one load spectrum, and
N1,
N2, …,
Nk are the number of cycles to failure for the stresses σ
1, σ
2, …, 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
Ng in the spectrum fatigue. tests were found in safety side in almost all experiments.
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