Controling yield strength of spring steel AISI 1075

[valmeidan]

I am designing a backplate for a graphics card and am trying to use a single sheet of stamped spring steel with tab cutouts as my force to maintain pressure on the gpu and various other components needing cooling. There would also be a thin piece of plastic underneath this like nylon, to get the deflection required on the tabs. I am using AISI 1075 steel sheet, but am having trouble inputting the tensile strength into Solidworks to do a stress analysis to ensure the tabs do not yield and provide constant force on the card. treating the steel can vary the strength significantly. what kind of spec can I use to ensure my results in my simulation is accurate. I am assuming it is the yield strength that will determine whether or not I would get permanent deflection, as the modulus of elasticity is relatively the same with most steel?

 

[valmeidan]

Here is a pic of one simulation I ran using 73000 psi yield 29,000,000 psi elastic modulus

 

[BiPolarMoment]

I'm not sure if I will be able to help much but I wanted to get some clarity on the application:

In your FEA are you using an enforced displacement or a force? If I read your description correctly you should be dictating the displacement (thickness of nylon?) of the tab and "backing out" the resultant stresses. Also, your output is in [very small] displacement rather than VM Stress, is that intentional?

You mention things "needing cooling"--is this metal sheet intended to perform heat dissipation or is it intended to "push" things into contact with what will ultimately be sinking the heat? Steel is not a particularly good conductor.

 

[TheTick]

Your assumption about yield strength is correct. Other assumptions are not.

Part of the beauty of doing FEA with steel is that the elastic modulus is the same for all grades of steel. This does not change significantly with steel grade or yield strength. Unless you are running a non-linear material with yield, you don't actually need to input the yield strength to get your stress values. Just run it linear and see where the stresses end up. It doesn't take calculus to see if the stresses are below yield.

 

[valmeidan]

the backplate is not for dissipation it is strictly used for applying force to sandwich the pcb in between the top copper coldplate and the bottom backplate. I just want to be clear on yield strength as it relates to permanent deflection. If I bend a piece of 1mm thick spring steel sheet it will eventually bend permanently. I want to make sure that the force I require (around 16in lbs) per tab is maintained without the metal bending or yielding. so when I input my material as spring steel and see that the yield strength is exceeded does that mean that the metal will bend and provide no force at that point ?

 

[TheTick]

It will still provide some force, but it will be permanently deformed, returning to an intermediate position determined by the yield curve. Usually spring steel does not have much in the way of plastic deformation before breaking, so the part is likely to break.

Inch-lbs is not a unit of force. Something doesn't add up with your description.

 

[valmeidan]

in the following I am using AISI 1045 steel, and you can see on the left about 20lbf. On the right you can see the yield strength exceeded on the bending moment arm, so am I to assume this will bend permanently? Will it atleast provide the 20lbf force continous or will this yield affect the force on the tabs?

 

[TheTick]

The force would stay at roughly whatever it was when yield started.

 

[valmeidan]

but once you take off the screws would the metal then be bent lower because it yielded on the previous assembly?

 

[TheTick]

yes

 

[valmeidan]

thanks TheTick

 

[tbuelna]

Looking at the images of your FEA results, one question I have is what were the boundary conditions used to constrain the steel plate? It appears as though the steel plate lower face is rigidly fixed and all of the bending strain is taken out in the tab struts. If your FEM used a constraint for the steel plate face that had a structural stiffness similar to the graphics card itself, rather than an infinitely stiff structure, the peak bending stress in the tabs would likely be lower for a given displacement.

I also have some questions regarding how you apply the forces to the tabs. If the applied force is constant and acting along a fixed axis, then the moment on the tab will change as it bends. If the applied force remains normal to the tab surface as it bends, then the moment on the tab will not change so much.