Allowable Stress in Creep Condition

[Paulettea]

Dear All

(b) At temperatures in the range where creep and stress rupture strength govern the selection of stresses, the maximum allowable stress value for all materials is
established by the Committee not to exceed the lowest
of the following:
(1) 100% of the average stress to produce a creep rate of 0.01%/1 000 h
(2) 100Favg% of the average stress to cause rupture at the end of 100 000 h
(3) 80% of the minimum stress to cause rupture at the end of 100 000 h

Can you please help me understand what the difference is between average and minimum stresses in cases 2 and 3 above?

 

[DSB123]

You get a scatter of results when testing material rupture strength NOT just a single value for multiple specimens. Maybe that gives you a clue.

 

[Paulettea]

So this is about the number of the specimens that are tested?
how many tests has to be done and based on which standard or spec those tests are performed?
Do they use extrapolation methods like Larson-Miller?

 

DSB123 is pointing to the correct answer. Creep curves (stress v log t) are often presented as bands, which incorporate the natural scatter in test results. These provide minimum and mean values.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[metengr]

When ASME B&PV code cases are developed, as part of the submittal there are three (3) heats required, which means multiple data points for creep and stress rupture tests. So, you need to determine the average as stated above for each test temperature and the scatter band for the lowest test values. This is basic engineering statistics.