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ANSYS Thermal-Structural Analysis

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Old   January 16, 2020, 20:37
Default ANSYS Thermal-Structural Analysis
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Hello,

I am fairly new to ANSYS and am trying to use it to solve an assigned thermal analysis problem. I have a cylindrical plug of stainless steel with a smaller cylinder of ceramic coaxial inside it. I am to study thermal expansion of this assembly where one end of the cylinder is at STP and the other is heated to 200 C.

I have meshed the geometry, executed a static thermal simulation with those temperature conditions, and fed the results into a static structural analysis in workbench. I applied a fixed support BC to the round face of the cylinder, while the flats are free to expand outward.

The stress magnitudes I am getting for this set-up seem excessive--my maximum principle stress reaches hundreds of MPa, exceeding the yield stress of either material. The Von Mises stress is even greater.

I have consulted tutorials online for similar problems and I have seen similar stress magnitudes without comment from anyone else.

It just doesn't at first glance make sense--on the face of it, both steel and alumina should be able to take a measly 200 C without failing. I checked my material properties and they seem consistent with MATWEB and other sources. Is my BC (the fixed support) not the right one to use? Or am I misinterpreting my results somehow, and I am not actually breaking the material?

Thank you.
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Old   January 16, 2020, 22:40
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Svetlana Tkachenko
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Do you have any reference data to compare your results against? For example, from past literature.
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Old   January 21, 2020, 13:18
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Why are you using a fixed support? Is that where your stresses are high? Why not let the assembly expand freely? (Using weak springs)

If you want to fully constrain the model, you have 6 degrees of freedom you must constrain. Figure out how to constrain the model, while still allowing free thermal expansion or shape change.

HINT: constrain 3 points in a plane, call it an XY plane.
Constrain all 3 points in Z,

2 in Y,

and one in X.

Figure out which 3 and which 1 would allow free thermal expansion
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Old   January 22, 2020, 20:31
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Quote:
Originally Posted by evcelica View Post
Why are you using a fixed support? Is that where your stresses are high? Why not let the assembly expand freely? (Using weak springs)

If you want to fully constrain the model, you have 6 degrees of freedom you must constrain. Figure out how to constrain the model, while still allowing free thermal expansion or shape change.

HINT: constrain 3 points in a plane, call it an XY plane.
Constrain all 3 points in Z,

2 in Y,

and one in X.

Figure out which 3 and which 1 would allow free thermal expansion
I have been attempting this with some success. The problem is that, the way the problem is set up, there's no "obvious" place to constrain anything--thermal expansion can, after all, take place in all three dimensions when you have a part that's, for the purposes of the problem, floating freely in space (in truth, we're talking about a piece of steel embedded in a cantilever beam so that the fixed end is far away from the part we're interested in).

But still, I applied a frictionless BC to one side to prevent motion in one axis (call it Z, so that X is the axis of the cylinder and Y is the other orthogonal direction).

I have never tried weak springs before--how do they relate to free thermal expansion?

There is an additional complication I've encountered this week, and which explains a great deal of my problems--the default "bonded" contact is unrealistic. In truth, since steel expands faster than ceramic, there should be *no* stress on the ceramic cylinder because a gap should form between it and the steel around it. But that requires me to switch to a "rough" contact condition--which requires more constraining to converge the simulation (still no luck there).

Regarding your six points...I'm not sure where to start with that. Again, defining X as my cylinder axis, Z as the direction toward the distant fixed support, and Y as the other orthogonal direction, I just don't see which points should be constrained. Shouldn't all points be able to move freely in all three directions in free thermal expansion?

I suppose I could constrain three points on the "hot" face of the cylinder, at 0 degrees, 90 degrees, and 180 degrees--don't allow them to expand toward the heat source. That's one axis taken care of (X).

The assembly is not totally radially symmetric, though--there are other cylinders bored into the assembly, not radially symmetrically. So I don't think I can just take my 0-degree and 180-degree points and constrain them in Z for symmetry. So I'm at a loss here.
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