Fatigue Exponent Q & A

Question:

What is meant exactly by “fatigue exponent”. I generally input a value between 3 (for notched parts) and 6 (for (unnotched ones). Is it right?…I’ve found confusing and contrasting definitions about b and m exponent (among MIL-STD-810 G and various other documents)

Answer:

The fatigue exponent is the slope of the SN curve in log-log format. It is also the greatest unncertainty factor.

The slope can be affected by many factors including stress concentration, notches, mean stress, residual stress, surface roughness, temperature, corrosion, etc.

Steel with a welded joint has a slope of 3 to 3.5.

Steinberg’s text recommends 6.4 for electronic components.

For bare aluminum, I would use 9 or 10.

A good approach would be to run three cases using lower, nominal, and upper estimates for the fatigue exponent.

Damping is another uncertainty factor. So you should probably also vary the Q value.

So now you will have permutations for both the fatigue exponent and Q.

Best wishes,
Tom Irvine

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