S-N Diagram

Plot of stress (S) against the number of cycles (N) required to cause failure of similar specimens in a fatigue test.

Data for each curve on an S-N diagram are obtained by determining fatigue life of a number of specimens subjected to various amounts of fluctuating stress. The stress axis can represent stress amplitude, maximum stress or minimum stress.

A log scale is almost always used for the N scale and sometimes for the S scale.