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Thermal Set

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ULBsha:
Hello,

I was wondering if there is a difference between the two following load cases:

1) Only a mechanical set (without any thermal set) for which the temperature for material properties is set to 295 K.

2) The same mechanical set (295 K for material properties) PLUS a uniform thermal set of 295 K everywhere.


Actually, I realized that I get 2 different results when optimizing these 2 studies but I do not understand why. Do you have any explanation for that ?


Thanks a lot.
Regards

Christian W.:
The temperature will extra stresses in the structure. Typically, in a thermal analysis you apply not T but dT.

James:
Christian W. is right, the extra thermal load will cause thermal stresses proportional to (alpha * delta T).

The reference temperature, defined for each load set on the project setup form/load sets tab, defines the reference temperature for the material properties. This is not a load, so there is no additional element forces calculated based on the thermal case.

If you create the model and apply a thermal case applied, there will be additional loads calculated based on the thermal expansion coefficient (alpha). For composites, HyperSizer handles the calculation of residual ply strains locally by computing the difference in the free thermal growth of the entire laminate vs. the thermal growth of the local plies.
See: http://hypersizer.com/download.php?type=analysis&file=HyperSizer_Stiffness_Method_-_Thermoelastic_In-Plane_Stiffness_Formulation.HME.pdf, section 4.

You should also be aware that HyperSizer will use different factors for a thermal set vs. a mechanical set. The thermal help/hurt factors, applied on the design-to loads tab, will factor up/down the thermally-induced loads based on if the thermal load increases or decreases the margins of safety. As a best practice, we recommend separating thermal and mechanical loads into unique FEA subcases, and creating the load case combinations inside HyperSizer.

To understand the thermal help/hurt factors, see: http://hypersizer.com/help/#Loads/load-factors.php

I hope this is helpful

-James


 

ULBsha:
Thanks Christian and James.

Indeed, I understand that thermal stresses are proportional to (alhpa*delta T).
But, do you mean that if we only have a mechanical load set with a reference temperature of 295 K, using Ti-6Al-4V let say, HyperSizer doesn't consider the thermal expansion of this material, which is alpha=8.82 at 295 K ?

If it doesn't consider it at all when we only have a reference temperature, I understand the thing (in this case I could ask what would be the usefulness of mentioning/setting a reference temperature then?);
But if it does, why should there be a difference between
- no thermal load, but only reference temperature of 295K, and
- uniform thermal load of 295K everywhere ?
I mean, in this particular case where the reference temperature and the thermal load have the same magnitude, the "delta T" is the same in both cases, isn't it ?


I am asking this because my optimized structure weights almost 50 tons under only a mechanical load set (and 295K reference temperature), but I get 800 tons (!!!) if I add a uniform temperature load set of 295 K (the whole structure is made of Ti-6Al-4V) !? Note also that if I remove the mechanical subcase and I only impose my uniform thermal load of 295 K, the structure weight drops to 3 tons (which seems logical since there is almost no stress in the structure).


Kind regards.

James:
If you specify a reference temperature on the loads sets tab, HyperSizer does not calculate thermal strains. Since an initial temperature is not specified there is no way to calculate a delta T. You must define a thermal load set for HyperSizer to include the thermal stresses.

The usefulness of defining a reference temperature for mechanical cases on the load sets tab is to set the reference temperature for the material properties. So when HyperSizer calculates strength and buckling margins for the mechanical load set it uses the appropriate material properties.

If a thermal set is paired with a mechanical set, HyperSizer will compute the material reference temperature for each component based on the average element temperature.

-James

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