Creep and Stress Concentration Factors

[mikepagnotta]

I am working on a project with some fairly exotic materials at high temperatures 9Molybdenum, Hastelloy, Inconel 718, all at about 800 degrees C). We have been able to get some creep data for these materials, but the data does not seem to distinguish between notched and unnotched specimens. The basic question that I have is: When calculating a stress to compare to creep allowables, do you need to calculate the notched stress (at a hole in a plate for example), or does the creep allowable pertain to the far field (gross cross-section) stress? Any insight would be greatly appreciated.

 

[aybee]

my guess(note that word)is that the creep stress data available is not notch related. I would factor in a stress intensifier to allow for the notched plate effect and use that to set the stress for us in the calc...be interested to see what some more learned guessers have to say

 

[Maui]

I assume that you are working to some customer specification that stipulates the maximum allowable creep rate. In your question you mentioned that you were uncertain about which calculated stress to use in comparing to creep allowables. How are your creep allowables stated? Exactly what are you comparing your calculated values to?


Maui

 

[stanweld]

Whether or not notch-rupture creep data is required depends on your application, service and design requirements. If your application is bolting or the design is notch sensitive {high thermal/applied load cycling, sharp dimensional changes of the item(s), etc.}, notched creep data is recommended.

 

[mikepagnotta]

The creep allowable specified by the customer is the allowable for 1% creep at 100,000 hours. We can calculate both the gross (or net) cross-section stress or the stress due to a local concentration (hole, sharp radius, etc.). I would have thought that the effects of a local stress riser would be washed out in a creep environment since the material exhibits permanent deformation in the creep environment, but I'm not sure, which is why I've raised the question. Thanks to all that have responded.