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For a low-carbon steel part that will be put into a long lifetime use (10 years plus) what are recommended load cases to consider creep? The part I'm specifically working with will have a relatively low cycle count (20k load cycles over ten years) at about 10% of it's yield strength Loads are compression and tension loads with no load being it's 'normal' position.

Is there a percentage of yield strength that calculating creep worth considering? Are there material properties that are more critical? Is cycle count more critical?

And, how do you go about calculating creep? I vaguely remember doing this in university but can't find any simple questions.

Wasabi
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Diesel
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  • what temperature? – agentp Dec 01 '17 at 19:29
  • Can be assumed to be constant temp at 24 degrees C – Diesel Dec 01 '17 at 19:57
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    forget about it, you won't even find data for creep of steel below maybe 500c. – agentp Dec 01 '17 at 20:03
  • So creep below 500C never occurs at all? – Diesel Dec 01 '17 at 20:08
  • not at room temp at 10% of yield. If steel crept there all our bridges would be sagging. If you are concerned about incredibly small strains for some reason that also may be another matter. – agentp Dec 01 '17 at 20:17
  • A good analogy I suppose, under what circumstances then IS creep a case that's worthwhile considering? – Diesel Dec 01 '17 at 20:19
  • At that low stress ( less than 50% of tensile) fatigue will not occur. Relaxation can occur above 700 F at very high stresses , so also not a factor for this case. – blacksmith37 Dec 02 '17 at 00:49
  • creep is caused when applied stresses in a metal part can be relieved by movement of metal atoms via diffusion within the crystal lattice. diffusive transport only becomes important when the service temperature exceeds about 1/3rd the melt temperature (rough rule of thumb)- meaning steel doesn't creep at room temperature but lead/tin solder does. – niels nielsen Dec 03 '17 at 08:47
  • @nielsnielsen, I'd say that would answer my question exactly. Rule of thumb I think is sufficient, have you got a source on that if so, write it as an answer and I'll mark it as correct – Diesel Dec 04 '17 at 17:24
  • As other have pointed out and relating to your question, creep is not a consideration for structural steel. It is however a consideration with structural concrete. – Forward Ed Dec 04 '17 at 23:52

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creep is caused when applied stresses in a metal part can be relieved by movement of metal atoms via diffusion within the crystal lattice. diffusive transport only becomes important when the service temperature exceeds about 1/3 to 1/2 the melt temperature (rough rule of thumb, using degrees K)- meaning steel doesn't creep at room temperature but lead/tin solder does. the materials science text by Van Vlack covers this topic in more detail.

niels nielsen
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