How can Rupture Allowable Stress be Less than Creep Allowable?
How can Rupture Allowable Stress be Less than Creep Allowable?
(OP)
Hi guys,
I hope the collective knowledge around here can raise me up a little.
I have come across some cut sheets for different high temp casting materials (specifically A351 HK40) where the rupture strength is listed as less than the creep strength.
I thought that rupture testing is to failure at a constant temp, and creep was a measurement of strain (elongation) at a controlled temp for a controlled time.
If this is true, then the specimen would break before it bends.
Which is fine if it is very brittle.
But the literature does provide a higher creep strength at the same temp that the piece has supposedly already failed at a lower stress value.
So I am confused and would greatly appreciate some clarity on this.
Thanks
I hope the collective knowledge around here can raise me up a little.
I have come across some cut sheets for different high temp casting materials (specifically A351 HK40) where the rupture strength is listed as less than the creep strength.
I thought that rupture testing is to failure at a constant temp, and creep was a measurement of strain (elongation) at a controlled temp for a controlled time.
If this is true, then the specimen would break before it bends.
Which is fine if it is very brittle.
But the literature does provide a higher creep strength at the same temp that the piece has supposedly already failed at a lower stress value.
So I am confused and would greatly appreciate some clarity on this.
Thanks





RE: How can Rupture Allowable Stress be Less than Creep Allowable?
The rate of creep deformation is a defined test method where strain is measured as a function of time based on a pre-selected temperature and stress (which could be higher or lower than the stress to rupture in x hours). Typically, stress to failure is not reported, only x% of deformation and or x%/hr of creep rate.
RE: How can Rupture Allowable Stress be Less than Creep Allowable?
I think I understand the testing procedures, what I had thought happened is inline with what you explained.
Here is an example of my confusion.
Per Kubota Metals for A351 HK40
Rupture at 1700F for 10,000 hours occurs at 2.67ksi
1% Creep at 1700F for 10,000 hours occurs at 3.35ksi.
So in a test for 10,000 hours the material went to rupture (failure) at 2.67 ksi, but only elongated 1% at 3.35ksi?
This seems to me to be like saying the Tensile strength of a material is less than the Yield strength.
I am sorry if I am missing something obvious, but I am definetely missing something.
RE: How can Rupture Allowable Stress be Less than Creep Allowable?
I would contact Kubota and inquire as to their published test values. Normally, the stress rupture value will be much higher at the same temperature in comparison to creep rate. The stress to rupture may be a misprint because of unit conversions - you never know.
Here some information I have on HK40 from another publication
Cast Heat Resistant Alloys from NiDI
1800 deg F
Stress (psi) Rupture Time (hr)
4800 100
2800 1,000
1700 10,000
RE: How can Rupture Allowable Stress be Less than Creep Allowable?
At 1800 Deg F, the creep rate for HK alloys is
2500 psi for 0.0001%/hr. This does not mean failure, only an established creep rate between 1,000 and 10,000 hours as indicated in the post above.
RE: How can Rupture Allowable Stress be Less than Creep Allowable?
And in Cr:Mo type materials , the amount of strain at rupture decreases at longer times.
RE: How can Rupture Allowable Stress be Less than Creep Allowable?
If you have a sample piece of material, and you hold it to 1800F for 10,000 hours and the most stress you can put on it before it fractures is 1700psi, then how could you ever put 2500psi on it?
It would break at 1700psi.
You can't bend/elongate that which is already broken.
It broke at 1700psi, or is what you guys are saying is rupture does not equal "broken".
It may not seem like it, but I am really trying to wrap my head around this.
RE: How can Rupture Allowable Stress be Less than Creep Allowable?
Look at the data I provided again
You have rupture at 10,000 hours at 1800 deg F using a fixed stress of 1700 psi. During this time the material has been subjected to increased deformation rate because of accelerated creep damage in this type of test.
You now increase the stress level, knowing the rupture time will be lower than 10,000 hours, but instead you decide to measure the actual rate of strain (deformation) over a defined period of time prior to rupture (which in the above case would be between 1,000 and 10,000 hours.
As mentioned above, some designers use creep deformation rate to decide on failure versus stress rupture.
This is all it is.
RE: How can Rupture Allowable Stress be Less than Creep Allowable?
After the strain is recorded at a lower time span, then the stress is extrapolated out to 10,000 hours?
RE: How can Rupture Allowable Stress be Less than Creep Allowable?
Perhaps this is where the confusion lies. No, these are two independent tests (not combined). Also, you are not dealing with linear functions, these are power functions used for predicting creep behavior regarding time, as a function of stress and temperature.
RE: How can Rupture Allowable Stress be Less than Creep Allowable?
RE: How can Rupture Allowable Stress be Less than Creep Allowable?
For extrapolating, The Larsen-Miller parameter was pretty good; with one stress rupture point , you could make reasonable estimate for other conditions.
RE: How can Rupture Allowable Stress be Less than Creep Allowable?
This I can visualize.
The creep stress level is recorded on a test specimen that is kept in the second phase of creep.
The final rupture tension is recorded right before it breaks, necking has occurred so it takes less tension to break it than it took to get it to the point of failure.
I remember this from college, but completely forgot about it when looking at this data.
I just knew it looked odd to me.
If I understand, I would like to thank you all for helping me.
If I don't please don't give up on me.
RE: How can Rupture Allowable Stress be Less than Creep Allowable?
RE: How can Rupture Allowable Stress be Less than Creep Allowable?