S-N curve

A plot of stress (S) against the number of cycles to failure (N). The stress can be the maximum stress (Smax) or the alternating stress amplitude (Sa). The stress values are usually nominal stress; i.e., there is no adjustment for stress concentration. The diagram indicates the S-N relationship for a specified value of the mean stress (Sm) or the stress ratio (A or R) and a specified probability of survival. For N a log scale is almost always used. For S a linear scale is used most often, but a log scale is sometimes used. Also known as S-N diagram.
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