ASME viii div 2 fatigue failure probability?

[mats_lindqvist]

Where in ASME viii div 2 can I find for what failure probability the S-N curves in Annex 3-F are specified? Can those S-N curves be modified for a different failure probability?

 

[TGS4]

For non-welded (smooth bar) curves, the "failure probability" is not provided and the curves may not be modified for use in constructing ASME pressure vessels.

The welded curves for use with the Structural Stress Method do have confidence intervals about the mean presented. The engineer is directed to use the -3σ curve unless otherwise negotiated with the user (and likely the jurisdiction).

 

[mats_lindqvist]

Thanks TGS4 for a prompt, informative reply. I have a case with a customer requesting us to use this pressure vessel norm, but the application is not a pressure vessel. I argue that this norm is not suitable for regular machine components (forged, machined steel), where you might want to know for which failure probability your S-N curve is defined. The customer wants us to guarantee a certain fatigue life using this norm, ASME VIII Div2. I'm trying to locate a norm or similar documentation, that we can use instead and that is better suited for regular machine components, where the failure probability is well defined.

Having this info, I might be able to issue a guaratee of a life of X years at load level Y, 2X years at load level 0.8Y etc. We can adjust the load level by using a different hydraulic relief valve pressure setpoint. Then I can leave it to the customer to choose combination of load level and fatigue life. It's a large 50 tonne forging, so we can't do destructive testing to get the actual S-N curve.

 

[TGS4]

The smooth bar S-N curve has a nominal factor of 20 on mean life, but is generally accepted to just be a lower-bound on the minimum of the test data.

Personally, I would shy away from any guarantee on life for any fatigue analysis. But I play in the pressure vessel field, where other factors such as corrosion and corrosion-fatigue come into play.