thermal stress on Truss 2

[HaoZ]

Hi,
I used Ansys Workbench to analyze the truss structure. The structure basically looks like the figure in following link.

http://img585.imageshack.us/img585/512/solidstructure.jpg

The thermal strain is about 650 microstrain applying 50 degrees Fahrenheit change.

But for fatigue, I need the stress. Ansys doesn't show high stress based on the thermal load.

The questions:
1. Will termal strain generate certain thermal stress?
2. Can thermal stress calculated as strain times elastic modulus?
3. If there is thermal stress, can I add up this with equivalent stress for overall stress?

Thanks.

 

[BAretired]

1. Will thermal strain generate certain thermal stress?

Not always, but in your case, it will. A beam with a hinge and roller can strain without creating stress. In your case, strain in the horizontal truss will lead to bending in the columns.

2. Can thermal stress be calculated as strain times elastic modulus?

No. Strain in the columns does not lead to any stresses at all. You must consider the change in length of the truss and determine how that affects the various members.

3. If there is thermal stress, can I add up this with equivalent stress for overall stress?

Yes. You can combine thermal stress with other stresses using the principle of superposition.

BA

 

[HaoZ]

Thanks.

 

[HaoZ]

One more question,

Is thermal stress already considered in Ansys equivalent stress?

I checked the experimental results and Ansys results. The strain values are close. But for stress, I am not sure if it is included.

 

[HaoZ]

I ask this because I notice that the equivalent strain or equivalent total strain is much smaller than the thermal strain in ANSYS, which means that they don't count thermal strain. I am not sure if it is so for thermal stress.

 

[BAretired]

I am not familiar with ANSYS software. To find the answer to your question, try a run with only temperature change, a separate run with applied loads and another with combined loads and temperature change. Then compare results to see if temperature stress was included or not.

Alternatively, you could visit the ANSYS forum on Eng-Tips.

BA

 

[HaoZ]

Thanks.

 

[Ron]

HaoZ...unless there is restraint, thermal strain doesn't typically induce stress. As BAretired noted, a roller condition creates no strain (well, actually it does in a truss because of joint restraints, but generally that's true).

Excessive repetitive strain can create a fatigue condition; however, thermal stresses are usually not high enough to induce this, unless restraint creates a true stress condition.

If thermal stresses are induced, they are additive to other stresses.