Stress combination

[pegus]

Good day,

I am currently developing a FEA analysis to a pipe line system, this system is under high temp (1200F) and high pressure (300 psig). This system has previous selected expansiom joints. I have run seperate analysis, the first consider the thermal expansion due to the temperature and the reactions that the expansion joints that this system has. The second analysis considers the stresses due to the internal pressure. I wonder if there is any method to obtain an equivalent stress from the 2 stresses of each FEA analysis.

Thanks!

 

[rb1957]

why can't you just add them together (superposition) ?

another day in paradise, or is paradise one day closer ?

 

[pegus]

Thanks for the response rb1957,

Do you mean add both stresses into one main stress? (Stresses due to Thermal reaction + stresses due to internal pressure?)

I do not run both conditions into 1 FEA analysis because the computation time is really big and also because the results seems strange.

Regarding superposition, I know that to add them we need to consider the direction of the stress, but as this geometry is really complex, I am not really sure if the hoop stresses are oriented in the direction that the bending, compressive and tension stresses are.

is the addition of the both principal stresses (Maximum stress due to Thermal reaction + Maximum stress due to Internal pressure) a valid approach?

Regards,

 

[rb1957]

the element results should add well enough, your post-processor should allow you to combine results.

don't add principal stresses, unless you prepared to accept the conservative approach. since the principals will (almost certainly) have different directions, adding magnitudes is conservative.

another day in paradise, or is paradise one day closer ?

 

[TGS4]

What's the purpose of "adding" the stresses?

 

[rb1957]

to combine the stress due to pressure with the stress due to temperature (presumably temperature and pressure are occurring at the same time in real life, both too difficult to apply together in the FE world).

another day in paradise, or is paradise one day closer ?

 

[TGS4]

rb1957 - I know that and you know that. What I want is for the OP to tell me that - particularly in the context of a particular failure mode.

 

[pegus]

TGS4 - I think rb1957 addressed my issue correctly, When I combine these conditions in my FEA analysis the results of my hole model overstresses dramatically (Does not seem coherent) and the computation time takes too long. That it is why I am wondering if I can obtain a conservative approach if I obtain the maximum principal stress (using Von-misses) for each load (Thermal reactions and Internal pressure stresses) and just add them (Stress thermal reaction + Stress Internal pressure). I know that we can add stresses if they go in the same direction, but as this is a complex geometry pipe system, I would like if this is valid.

Thanks!

 

[TGS4]

And what failure mode are you evaluating? Not all stresses are relevant in a particular failure mode. And, yes, superposition can be used to add stresses, provided that you are adding component stresses - because of the fact that stress is a tensor.

Piping analysis is pretty simple for a piping stress engineer. You seem to be making this much more difficult than it needs to be...

 

[TheTick]

The principle outputs of FEA are displacement and the resulting strain. Stress is derived from strain. Any combining of analyses needs to work on that level.