S-N curves of structural materials are obtained through fatigue tests. These tests are often performed using five different stress levels, with fifteen test specimens for each stress level. This kind of test plan provides estimates that are less precise compared to other experimental plans, for example, the ones called optimum plan or compromise plan. The main reason for this drawback is the use of the same number of specimens for all stress levels. It has been observed that less precise results are obtained for lower stress levels because failure occurs less frequently. That is why more specimens should be used for lower stress levels as compared to higher stress levels. As long as the number of specimens to be tested at low stress levels is increased, the total number of failures will also increase, which allows one to develop a more precise data analysis. The objective of this work is to present an alternative experimental plan to obtain S-N curves, which intends to provide accurate estimators. A practical application is done for planning a fatigue test, in a flex-rotating machine, to obtain the S-N curve of SAE 8620 steel.
Residual stresses are present in materials or structural component in the absence of external loads or changes in temperatures. The most common causes of residual stresses being present are the manufacturing or assembling processes. All manufacturing processes, such as casting, welding, machining, moulding, heat treatment, etc, introduces residual stresses into the manufactured object. The residual stresses effects could be beneficial or detrimental, depending on its distribution related to the component or structure, its load service and if they are compressive or tensile. In order to do the studies of residual stresses inside pipes, where the fatigue cracks normally initiate, an equipment that allows applying the Hole-Drilling Method was developed. The equipment efficacy was confirmed in this work by using it to detect residual strains inside a Mock-up that simulates the relief and security nozzle of Angra 1 Nuclear Power Plant (NPP) pressuriser.
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