How to understand the affection of TN and TS in Wöehler Curve?

[XiongYuZeng]

Hi Pylife group:
Need some help to understand TN and TS although I have read the help document about them.
I realize that if I do not provide TN, TS, it will use the default value 1. It seems that if TN and TS is 1, then I am not able to change the Wöehler curve to other failure probability. the code: woehler_curve_data.woehler.transform_to_failure_probability(new_failure_probability).to_pandas() will give me the same data no matter which value I assign to new_failure_probaility. Why?

In the help text, it stands beneath text, which I got few questions to ask.

TN: The scatter in cycle direction, (N_90/N_10)
If the key is missing it is assumed to be 1.0 or calculated from TS if given.

TS: The scatter in cycle direction, (S_90/S_10)
If the key is missing it is assumed to be 1.0 or calculated from TN if given.

Questions:

1. what does (N90/N_10) and (S_90/S_10) mean?
2. what is the reasonable value range of TS and TN I should use if I only have values for SD, ND, k_1 and k_2?

[johannes-mueller]

A Wöhler curve is always valid for a certain failure probability. A 50% Wöhler curve means for example, that left and underneath it you'd expect less than 50% of the specimens to fail, right and above it it more than 50%.

So for every failure probability P there is a Wöhler curve which has its values ND_P and SD_P. The TS and TN values are the ratio of N_90/N_10, TS the ratio of SD_90/SD_10. So for example in this picture the TN value is the distance between the 10% Wöhler curve and the 90% Wöhler curve in life time direction.

As for reasonable values, they tend to vary a lot between different material groups.

Edit

There is indeed one detail inaccurate in the documentation. The TS value is the ratio SD_90/SD_10 whereas TN is the reatio between N_90 and N_10 (without the D). Where N_xx is the endurance limit in cycle direction for a certain load level above SD_xx

We will make the docs more accurate.

[johannes-mueller]

Please check #94