S-N Curve Data |
 
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S-N Curve Data |
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S-N curves are generally defined in the form 
where S denotes stress range, N the number of cycles to failure at this range, and m and K are constants. Taking logarithms of both sides and rearranging gives:
(1)
which is the equation of a straight line when log S is plotted against log N. In this case m is the inverse slope and K is a function of the line intercept. These are the parameters input above. The specification of an Endurance Limit is optional, and by default there is no endurance limit.
A piecewise-linear S-N curve is one which plots as a series of straight line segments when log S is plotted against log N. In this case m and K vary between line segments; the N1 and N2 values above specify the segment of the S-N curve X (number of cycles to failure) axis where each m and K combination apply.
It is also possible to define an arbitrary S-N curve, in terms of pairs of S and N values.
•*S-N CURVE is used to define fatigue analysis S-N curves.
•*FATIGUE DATA is used to assign fatigue data to hot spot sets.