Thermal stress in Dissimilar joint

[Payyan1234]

Hi,

I have a question regarding joint strength of an equipment , which is placed on a honeycomb core and backed by a doubler plate. Please see the attached picture.

The equipment sees high temperature around 102 degrees. I have analysed the joint by calculating the delta displacement from thermal expansion of two metal plates and assumed them to act as parallel spring K= K1 + K2 and found the The total force at the Joint using F= K. delta displacement.

This force is used for checking the fastener shear strength and joint bearing strength.

Is this the right approach or am i missing something??

Thanks

 

[RPstress]

If the 2024 plates are pushing on the bolt as they expand what is restraining the bolt? A free-body diagram would help; what is pushing against what as what tries to expand? Your 'F' seems to come from the core; not possible with normal cores. Is F from the honeycomb skins? But then why would they not be subject to the same temperature? Most confusing.

 

[Payyan1234]

RPStress,

Thanks for the reply. I assuming the sandwich has zero TEC, therefore top and bottom doubler plates expands and restricted by the sandwich panel in bearing.

 

[RPstress]

When you say 'restricted by the sandwich panel in bearing' I assume you mean restricted by the sandwich panel skins, which for some reason are not subject to the temperature change or maybe are carbon with low CTE. Are the thick 2024 plates bonded to the sandwich panel skins? If their only connection to the sandwich panel is the bolt then they will just expand about that position in the sandwich panel.

 

[Payyan1234]

RPstress,

I have attached picture for more clarification.

The equipment (Aluminium alloy (0.125" thick base plate) attached to the sandwich mounting plate via 4 steel screws (#10) and an aluminium doubler plate and sealed/bonded at the edges.

To assess the strength due to high temperature (The equipment getting heated upto 102 deg).
I assume the metallic components expands & no expansion in sandwich panel and all the loads taken by the bolts ( no effect on bonding).

Using the above assumptions I have so far checked the bolt shear strength and doubler plate bearing strength?

 

[RPstress]

The carbon if woven will have a TEC of about 2e-6 / deg C if you need a little (~10%) bit of relief. Remember to add in the expansion of the Al across the panel. This is pretty rough first pass estimation. You'll need a pretty safe M.S. (I'd want at least 25%). If it's aerospace a fitting factor should be in there unless it's going to be tested.