Same thermal conditions | stresses increase | primary plus secondary

[shiraz883]

Hi,

I am doing an evaluation with the thermal conditions (expansion coefficient) on - same temperature for the whole assembly - and other without thermal conditions - meaning that a constant temperature is given in both load cases, but thermal expansions are off (using APDL snippet) in the second load case to evaluate primary stresses. The first load case will give primary plus secondary stress since we have the thermal load (expansions coefficients on).

When I see the equivalent stress for the thermal load case and compare it with primary load case, the stresses increase for the primary plus secondary load case even though the assembly is at same temperature i.e. all parts have same temperature and same material properties.

If everything is at the same temperature and same material properties this should not happen. or is it because of bonded contact in the model and bolts modeled as beams, some boundary conditions constraints. Note that boundary conditions are same in both load cases.



Shiraz
Sr. Engineer

 

[TGS4]

I'm not sure what the question is here? If this is a question about stress classification/categorization, then all that I can recommend is to refer to ASME Section VIII, Division 2, Part 5, paragraph 5.2.1.2.

Also, note that for the so-called "primary plus secondary load case", you are to be using a stress range, based on the operating conditions. That's because the specific failure mode for those stress ranges is ratcheting, whereas the failure mode for the primary stresses is plastic collapse.

 

[shiraz883]

Let me rephrase:

There are two load cases acting in an assembly (with sub parts of same material) which I am analyzing. The FE model is same in both cases - no change in loads, boundary conditions etc.:

Load case 1. P+D (Pressure + deadweight) - temperature is applied so that material properties are captured at that temperature in ANSYS but thermal expansion coefficient (alpha) is turned "OFF" - so that this load case will give primary stresses

Load case 2. P+D+T (Pressure + deadweight + temperature) - Thermal expansion coefficient (alpha) is turned "ON" - so this load case will give secondary stresses or stress range which is the stress at the operating conditions for a cycle (start at room temperature where the equipment is not operating and goes to operating condition)

Now since the assembly has same temperature for all the parts and same material properties (since all materials are same) so the stresses should be same in both load cases. There will be no thermal stresses since the assembly is free to expand and there is nothing which restraints it.

But I am getting high stresses at some of the locations in the second load case which should not happen.

Shiraz
Sr. Engineer

 

[TGS4]

There's something certainly amiss about your overall stress situation. That's for you to troubleshoot.

Now, I have some issues with several of your statements, though:

Load case 1. P+D (Pressure + deadweight) - temperature is applied so that material properties are captured at that temperature in ANSYS but thermal expansion coefficient (alpha) is turned "OFF" - so that this load case will give primary stresses

No, you will still have primary and secondary stresses. Your primary stresses will be those necessary to satisfy equilibrium, and the secondary stresses will be those necessary to satisfy compatibility.

Load case 2. P+D+T (Pressure + deadweight + temperature) - Thermal expansion coefficient (alpha) is turned "ON" - so this load case will give secondary stresses or stress range which is the stress at the operating conditions for a cycle (start at room temperature where the equipment is not operating and goes to operating condition)

Not necessarily will this load case only give you secondary stresses. Again, you need to put some additional focus on properly categorizing your stresses.

 

[tigny]

Hello,

The following is only about the "high stress" results.

if you have continuity of geometry but with materials of different Coefficients of Thermal Expansion (specifically at a weld or even at a bolted connexion), you may experience very high stresses with the same temperature throughout your model.

Also, if you have a fixed constraint boundary limit condition, you will develop high stress.

Tigny