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It is known that when metals are subjected to fluctuating load, the failure occurs at a stress level much lower than the fracture stress corresponding to a monotonic tension load.1 With the development of the railway in the nineteenth century, the fatigue failure of railway axles became a problem and much attention was given to the understanding of the fatigue failure phenomenon. To understand fatigue failure mechanism induced by repeated loading, full-scale as well as small-scale fatigue tests were conducted in the laboratory. In 1852, the German railway engineer August Wohler (Director of Imperial Railways in Germany from 1847 to 1889), conducted several constant amplitude fatigue tests on full and small-scale railway axles. The results of this work [1] were presented in the form of plots of the failure stress as a function of the number of cycles to failure. This plot is a useful tool for the total life prediction of a part subjected to constant amplitude cyclic loading and is known as the Wohler S—N diagram. The Wohler approach was extended to other areas of concern, such as bridges, ships, and machinery equipment that undergo repeated loading. The S—N approach is still a useful tool to assess fatigue failure of many modern structures that are subjected to repeated loading, where the applied stress is under the elastic limit of the material and the number of cycles to failure is large. When material failure occurs under a relatively large number of cycles, and stresses and strains are within the elastic range of the material, the failure mechanism is called high-cycle fatigue. |
