<< Click to Display Table of Contents >> Navigation:  Flexcom > Data Inputs > $LIFETIME FATIGUE >
*S-N Curve

Purpose

To define fatigue analysis S-N curves.

Theory

Refer to Fatigue Analysis (Mode 1) for further information on this feature.

Keyword Format

Blocks of lines that define an S-N curve repeated as often as necessary. Each block begins with a line defining the S-N curve name and type. This is then followed by either of three types of lines to define the S-N curve.

Line to define S-N curve name and type:

CURVE=S-N Curve Name, TYPE=S-N Curve Type

The format of subsequent lines depends on the type of S-N curve being defined.

Line to define a log-linear (single sloped) curve, with or without an endurance limit:

m, K [, Endurance Limit]

Lines to define a piecewise log-linear curve (the line is repeated as often as necessary):

m, K, N1, N2

Lines to define a curve directly in terms of (S, N) data pairs (the line is repeated as often as necessary):

S, N

S-N Curve Type can be LOG-LINEAR or DATA PAIRS. LOG-LINEAR in this context refers also to piecewise log-linear. Endurance Limit defaults to 1, that is, no endurance limit.

s-n Curve - linear

Purpose

To specify linear S-N curves.

Table Input

Notes

(a)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:

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.

(b)The specification of an Endurance Limit is optional, and by default there is no endurance limit.

s-n Curve - piecewise linear

Purpose

To specify piecewise linear S-N curves.

Table Input

Notes

(a)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.

(b)Use as many lines as you need to completely define a particular S-N curve. Simply leave Column 1 blank for second and subsequent lines. For subsequent curves, put the curve name in Column 1 and specify the S-N data in the same way.

(c)For stress ranges which are within the bounds of the specified S-N data, log-linear interpolation to find the number of cycles to failure.

(d)The smallest specified stress range is assumed to correspond to the endurance limit of the material, hence no fatigue is deemed to occur for stress ranges which are lower than the smallest specified stress range, regardless of the number of cycles experienced.

(e)For stress ranges which are higher than the largest specified stress range, the number of cycles to failure is assumed equal to the N value corresponding to the largest specified stress range.

s-n Curve - data pairs

Purpose

To specify S-N curves directly in terms of S and N data pairs.

Table Input

Notes

(a)Use as many lines as you need to completely define a particular S-N curve. Simply leave Column 1 blank for second and subsequent lines. For subsequent curves, put the curve name in Column 1 and specify the S-N data in the same way.

(b)For stress ranges which are within the bounds of the specified S-N data, log-linear interpolation to find the number of cycles to failure.

(c)The smallest specified stress range is assumed to correspond to the endurance limit of the material, hence no fatigue is deemed to occur for stress ranges which are lower than the smallest specified stress range, regardless of the number of cycles experienced.

(d)For stress ranges which are higher than the largest specified stress range, the number of cycles to failure is assumed equal to the N value corresponding to the largest specified stress range.