Beta results from fatigue testing

[Geoff Lee]

I am performing fatigue testing of a spring. The resulting beta values, at a given stress level (above endurance limit), are around 0.4. This indicates an infant mortality characteristic.

Is this:
a) expected for fatigue testing; i.e. 'weak' items will fail early and 'strong' items will last much longer without tending to wear out, or

b) an indication of excessively variable quality of my test specimens, or

c) neither of the above - provide your own explanation.

Your opinions would be appreciated.

[Pantelis RS]

Well, (a) can be implied by the data, but (a) could be caused by (b) or due to “(c) inconsistent testing schemes”, or...

In general with fatigue issues one would expect a much higher beta.

[Pravin Kadekodi]

Do you know the material, and have the S/N curve?
Here are couple of check points:
- For the stress level is the number of cycles to failure in the same order as in the S/N curve
- If you have the S/N curve showing the times to failure at a stress level, you can plot weibull plot to get the 'expected slope'. Your test slope should be close to this slope.

Try this out and let me know if it helps.