Thermal Expansion help

[RBMECH123]

So I was reading a client guideline and noticed something that wasnt correct from my studies, but perhaps it can be true. If anyone can share some light, it would be a big help.

If a piece of pipe installed at ambient temperature and noted that at times the pipe can cool from a higher than ambient temperature to a lower than ambient temperature. My problem is that the client wants me, the stress engineer to show results of a thermal expansion from the higher temp to the lower temp, does this make sense?

Thanks,
Richard.

 

[metengr]

Yes, it is called thermal stress, and should be at least evaluated based on delta T,and pipe configuration.

 

[TVP]

The following shows some information on simple thermal stress calculations:

http://physics.uwstout.edu/statstr/Strength/Stress/strs38.htm

 

[RBMECH123]

The link only shows one change in temperature, what about anything on the difference of two different delta T's, once in tension to a compressed state of elasticity?

 

[RBMECH123]

I would think if a pipe cooled down from hot to cold, ambient in between, that the stress calcs would have to cases for thermal expansion, not one...if anyone can show me an example, that would help tremendously?
Thanks

 

[desertfox]

Hi RBMECH123

I see your situation like this:-

Imagine you connect a piece of pipe with flanges at room temperature to two immovable flanges, now if you heat the pipe up, the pipe tries to expand and can't so it under goes compressive stresses.Conversly if the pipe is now cooled below room temperature the pipe tries to contract but can't so tensile stresses are induced.
So if this is the case you can use the formula:-

change in length= original length*coeff expansion* (T1-T2)

all you need to do is use 20 degrees for room temp and substitute the temp rise or fall in the equation above to get the expansion or contraction.
Assuming the pipe is in the elastic region you can calculate the stresses from the normal Hooks Law formula.

Regards

desertfox

 

[RBMECH123]

Then there are two cases...if the pipe does not exceed the material's elasticity, then it would cool down to AMB at it's original state....THEN, try to compress as the temperature drops below AMB, therefore, 2 cases. How is the material acting otherwise? Basically if the material cools down to its AMB state, then its like it never got hot. Again, the material would under a serperate thermal stress. Yes/No, how am I wrong? I'm not trying stubborn, I would just like to be clear on the matter. Thanks

 

[jedward]

some special cases may occur (not common)
consider that some "non typical" materials may undergo plastic deformation at "Hot" and will be subjected to the full temperature range stress when quickly cooled