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NA_119618873-Newcomer
3-Newcomer
February 19, 2026
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How to plot logarithmic S-N curve in MathCad prime 11

  • February 19, 2026
  • 1 reply
  • 109 views

Hello!

In my case, I would like to plot an Sā€“N curve for fatigue life.
I need the initial slope to be m = 3 or m = 5, and after the knee point, the slope should change to m = 22, specifically for the high-cycle fatigue region.

On the Y-axis, a stress range up to 300 MPa is sufficient.
On the X-axis, I need the range from 0 to 4 Ɨ 10ā· cycles, and importantly, the X-axis must be in logarithmic scale.

I have attached an example Sā€“N curve below for better understanding.
I noticed that there is a logarithmic scaling function available in Mathcad plots, but I am not able to apply it correctly for my specific case.

 

sn-curve-s355-en-lg.png

Br

NLAAL

 

 

Best answer by Werner_E

Obviously the y-axis in the plot you show is logarithmically scaled as well.

The function therefore must be of the form y=a*x^b to look line a straight line in a log-log plot.

That's not a linear function so it does not have such a thing like a constant 'slope'.

So you would have to define what you would like to be seen as the 'slope', why its positive (+3, +22) for a declining curve and larger (22) for the part of the plot which looks flatter in the log-log plot.

grafik.png

 

Prime's native plots unfortunately only allow to show full decades in a log plot, so we have to show the range from 100 to 10000 at the y-axis even though we would only need the range from 400 to 1600.

 

Prime 11 sheet attached

1 reply

W
Werner_E25-Diamond IAnswer
25-Diamond I
February 20, 2026

Obviously the y-axis in the plot you show is logarithmically scaled as well.

The function therefore must be of the form y=a*x^b to look line a straight line in a log-log plot.

That's not a linear function so it does not have such a thing like a constant 'slope'.

So you would have to define what you would like to be seen as the 'slope', why its positive (+3, +22) for a declining curve and larger (22) for the part of the plot which looks flatter in the log-log plot.

grafik.png

 

Prime's native plots unfortunately only allow to show full decades in a log plot, so we have to show the range from 100 to 10000 at the y-axis even though we would only need the range from 400 to 1600.

 

Prime 11 sheet attached

    N
    NA_119618873-NewcomerAuthor
    3-Newcomer
    February 26, 2026

    Thank you for your time and effort. This helps a lot šŸ™

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