Investigation on Fatigue Damage by Means of Many Fold Multiple Repeated Stresses

Abstract

For the purpose of studying the progressive fatigue damage accumulation normalized V notched steel (0.36%C) specimens were subjected to experiments under the repeated stress of multifolded and multiple loading.
The mechanical properties of the material of the specimens are given as follows; tensile strength σ_B=58.0kg/mm², lower yield point σ_S=30.4kg/mm², actual breaking point σ_T=91.5kg/mm².
The first experiment consisted of test of fatigue under the stress of multifolded and multiple loading of the following patterns, and the experiment was repeated until fracture was made.
(1) σ₁→σ₂→σ₃→σ₂→σ₁→σ₂→……
(2) σ₂→σ₃→σ₂→σ₁→₂→σ₃→……
(3) σ₃→σ₂→σ₁→σ₂→σ₃→σ₂→……
where σ₁=15kg/mm², σ₂=18kg/mm², σ₃=21kg/mm².
It is found that the larger is the number of loading cycle by decreasing the frequency of repetition on each stress level the smaller the total sum of the cycle ratio will become, descending ultimately to approximately 0.6.
Furthermore, in order to investigate the above mentioned fact, fatigue tests were conducted under the same stress pattern with only one loading cycle at the beginning of the loading, followed by constant stress amplitude. It appears that the total sum of the cycle ratios is much affected by the stress change at the beginning of the loading.