topic Re: How to plot logarithmic S-N curve in MathCad prime 11 in Mathcad

How to plot logarithmic S-N curve in MathCad prime 11

NA_11961887 — Thu, 19 Feb 2026 18:55:47 GMT

Hello!

In my case, I would like to plot an S–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 × 10⁷ cycles, and importantly, the X-axis must be in logarithmic scale.

I have attached an example S–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.

NLAAL

Re: How to plot logarithmic S-N curve in MathCad prime 11

Werner_E — Fri, 20 Feb 2026 13:30:06 GMT

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.

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

Re: How to plot logarithmic S-N curve in MathCad prime 11

NA_11961887 — Thu, 26 Feb 2026 14:56:21 GMT

Thank you for your time and effort. This helps a lot 🙏