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Thermal Set

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ULBsha:
Thanks James.

I realized that I hadn't specified either the initial temperature, nor a reference temperature (in my MAT1 card). By adding them it solved my problem.

However, I have another question concerning the way HyperSizer calculates the material properties beside the indicated temperature range; How the material properties are calculated if the properties are given up to 700K for instance, but the applied load goes up to 750K ? It doesn't seem to be a simple extrapolation since my structure gets much much more heavier if the maximum applied temperature exceeds only "slightly" the material limit...

Thanks.

James:
HyperSizer will not extrapolate outside the stored temperatures to determine material properties. If a reference temperature is greater than the temperature assigned to the properties, then the properties from the highest reference temperature are used.

I'm not sure why your structure gets much much more heavier if the maximum applied temperature exceeds only "slightly" the material limit. Do you have material correction factors defined as a function of reference temperature?

-James

ULBsha:
Dear James,

It is very interesting to know that HyperSizer doesn't extrapolate outside the stored temperatures. So, does it mean that if the highest assigned temperature is 500K, there will be no difference between the case with a uniform temp load of 500K and the one of 700k ?

Regarding the material correction factors, I use isotropic materials (Ti-6AI-4V KMAT1%301). Therefore, if I am understanding well, the factors are already imposed by HyperSizer materials database and I am not allowed to change them.

Finally, I really do not understand what could cause this huge increase of mass if the applied temperature is higher than the assigned temperature... 

James:
If the maximum stored temperature is 500F then the HyperSizer analysis will not change if you increase the reference temperature to 700F. The same stiffness and allowable properties are used as if your reference temperature were set to 500F.

Check to see if the FEA solution is returning different loads. Compare the magnitude of the FEA loads for the thermal cases between the two sets of results.

-James

ULBsha:
Dear James,

I'm back to you regarding this issue again. Let me summarise the problem by describing a simplified case:

I'm trying to analyse an aircraft on which both mechanical and thermal loads are applied.
Let assume that the mechanical load is just a uniform pressure load over the structure, while the thermal load is a time-dependent function, as shown in the figure (see attachment):
1) 300K during the first 1500seconds;
2) An increase of temperature to 750K, performed in a period of 1500seconds;
3) 750K is maintained during the rest of the experiment (let say 9000seconds).

I want to size the aircraft structure with respect to its cruise phase, i.e. when the temperature is at 750K (in other words, in segment 3).

I use the following ordinary commands to setup my input Nastran file:

SOL SESTATICS
CEND
TEMP(INIT) = 1

SUBCASE 100
TEMP(LOAD)=101
BEGIN BULK

Now, the question is how to define the thermal set, regarding the initial temperature TEMP(INIT) ?
Should I put:
TEMPD,1,300
TEMPD,101,750

or:
TEMPD,1,750
TEMPD,101,750

Indeed, I want to have the effect of thermal stresses, so I think that my initial temperature (TEMP(INIT)) should not be equal to 750K, which is my (final) thermal load (TEMP(LOAD)). However, since the increase of temperature is done over a long period, is it realistic to put the initial temperature at 300K ?

The strange thing is that when my TEMP(INIT) value is equal to 750K, the HyperFEA calculation gives me a realistic increase of weight compared to the case without any temperature load. I guess this increase is just due to the change of the material properties because of higher temperatures that are applied.
But if I set TEMP(INIT) value to 300K, the calculation diverges and the weight goes infinitely high (thousands of tonnes !!) without converging. Note also that this divergence doesn't happen if I decrease the cruise temperature (TEMP(LOAD)) from 750 to 600K and maintain the TEMP(INIT) at 300K. It seems that the very high thermal stresses induce this divergence, but I don't know why! The used material is Titanium.

Thanks a lot.
Regards

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