Fatigue life at high temperatures

[corus]

The SN fatigue curves commonly quoted generally seem to apply at room temperatures. What is the effect of high temperatures (say about 400 C) on the fatigue life of an unwelded carbon steel component?

corus

 

[flamby]

http://www.springerlink.com/content/v40w44776vp2q748/

might interest you. I am not an expert in the topic but keeping with the traditions of eng-tips, I googled it for you. ;-)

Ciao.

 

[corus]

Thanks flamby, I too had tried googling but found nothing. I'm not sure that reference is relevant though as this material is just ordinary steel plate and not cast iron. I'd be interested on the effect on the SN surve (if any) that is published in current design standards for fatigue assessments.

corus

 

[israelkk]

If you know the mechanical properties UTS and Yield strength at the said temperature you can scale down the SN curve linearly to the UTS and Yield strength at room temperature.

 

[corus]

isrealkk,
I've noticed in various software packages that they make a correction for temperature, but don't say how. In design standards on fatigue I've seen a correction for the change in Young's modulus at different temperatures, but not for yield strength with temperature. Do you have a reference for your suggestion of scaling for yield strength?

corus

 

[israelkk]

No, I do not have a reference. But look at it a stress concentration that raises the stress level with reference to Yield or UTS. The high temperature does the opposite it lowers UTS and Yield. The change in modulus of elesticity is usually minor.

 

[flamby]

The temperature of 400C is quite high and I suspect that the material in question can still be considered as structural steel.

If that is some serious business, go for lab tests otherwise consider special steels like fired pressure vessel quality steel for which some temperature related information is available in standards.

Ciao.

 

[rb1957]

there is a very limited amount of data (fatigue at elevated temperature) in AR-MMPDS-01 or Mil Hdbk 5 for 4340.

there is also a limited amount of data on the effect of high temperature exposure on Fty and Ftu, this is highly dependent on the duration. you could use this, like israelkk's suggestion, to reform your goodman diagram.

but i think fatigue testing at elevated temps is very difficult.