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RE: How can Rupture Allowable Stress be Less than Creep Allowable?

RE: How can Rupture Allowable Stress be Less than Creep Allowable?

(OP)
Additional info for (closed) thread367-305865: How can Rupture Allowable Stress be Less than Creep Allowable?
Original Poster: PSSC

A likely reason for the discrepancy/confusion could be that rupture tests and creep tests are carried out in a fundamentally different way-- namely, the strain rates employed. Creep testing is done with sensitive strain equipment because we are explicitly interested in the strain rate throughout the testing, and these tests are long (up to 10k to 30k hours). Rupture testing is done at much higher strain rates (shorter tests-- ~1k hours) with less sensitive strain equipment. The devil in the detail is that some of the defined parameters that illustrate the process depend on strain rate. So, when we extrapolate the data from rupture tests to, say, 10k hours, the illustrative parameters are not comparable to those borne of the creep test conditions. Furthermore, some extrapolations can be up to 30% different from experimental data, and there are some mechanical phenomena that occur only under long-life conditions (e.g. after 40k+ hours). Therefore, it is important to bear this in mind when considering the loading profile for a design.

It might be important to find the strain rates used in the rupture experiments.
That being said, according to an article by Euro Inox on BSSA's website:
"It is usually satisfactory if creep does not exceed 1% deformation in 10,000 hours.... A design stress figure commonly used for uniformly-heated parts not subjected to thermal or mechanical shock is 50% of the stress to produce 1% creep in 10K hours. However, this should be used carefully and verified with the supplier. A[nother] common criterion is for the load to cause failure at 100K hours, with an added safety factor of 1.5 (see the ASME code, for example)." (article link, page 8)

more detail on creep and rupture differences: http://www.tech.plym.ac.uk/sme/mats340/cpintro.pdf


Cheers,
Jesse

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