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Strength of Materials: S-N Diagrams

To help determine when a specimen will fail S-N diagrams are constructed to compare the fatigue strength (Sf) to the life cycle of the specimen N. For some materials, such as steel and iron, the S-N diagram will become horizontal at a certain point, this point is known as the endurance point S'e. Refer to the figure below for UNS G41300 steel.

S-N diagram

S-N diagrams are constructed in of data obtained off of specimens in a controlled laboratory environment. The specimens will have no geometric stress concentrations, and the surface will be highly polished to eliminate the potential of surface stresses. The test is normally a simple controlled bending test where the specimen is rotated at a specific speed so that the loading will fluctuate from a compressive load to a tensional load. Refer to the figure below to see the shape of a typical specimen that might be used.

Fatigue bending test specimen

Due to the fact that this is an experiment that is controlled it is possible that even if a part uses that exact material it most likely will not fail at the same time as specified by the S-N diagram. Instead the S-N diagram should be used as a guide. This is why it is important as an engineer to perform tests on a newly developed products to make sure everything is performing as expected.

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