In: Civil Engineering
Find the planar density on the (1 1 1) plane in atoms per millimeter squared for BCC tungsten. R=0.137 nm. b.Describe how a S-N curve is constructed and how it can be used to predict if a component will or will not fail due to fatigue.
Fatigue properties of materials are often described using the fatigue limit or the S-N curve (fatigue curve, Wohler curve). The S-N curve describes the relationship between cyclic stress amplitude and the number of cycles to failure. The fig shown above shows a typical S-N curve. On the horizontal axis, the number of cycles to failure is given in the logarithmic scale. On the vertical axis, the stress amplitude of the cycle is given.
The S-n curves are derived from fatigue tests. Tests are performed by applying cyclic stress with constant amplitude on specimens until failure of the specimen. In some cases, the test is stopped after a very large number of cycles (N>10^6). The results is then interpreted as infinite life.
Fatigue curves are often given for Kt=1 (unnotched specimens). Those curves describe the fatigue properties of a material. Actual structures are better described with S-N curves for Kt>1 (notched specimens).
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