how does thickness affect the design
how does thickness affect the design
(OP)
I put a sheet of following dimensions on floor. Assume the material is glass.
1) thickness 1.2 mm diameter 2"
2) thickness 1.5 mm diameter 2"
3) thickness 1.8 mm diameter 2"
4) thickness 2 mm and diameter 2"
And I apply same compressive load on all these sheets
Let us assume that i transfer the load with a rod of 2" OD diameter and ID of 1".
The compressive stresses on these would be load/(pi*(2*2-1*1)/4)
Since compressive stress is same. As i start increasing the load i know the glass sheet with minimim thickness will break first.
Assuming that this is not a bucking case, how do i deterimine at which load the 1.2 mm (minimim thickness) sheet will break
In the calculation of the stress that includes thickness what is the area i need to use for calculation.
Is it the area i need to consider for calculating the stress is (D*t) or (pi*D*t). what should be the value of D
Is it the outer diameter of the rod or MEan diameter of the ID of the rod.
1) thickness 1.2 mm diameter 2"
2) thickness 1.5 mm diameter 2"
3) thickness 1.8 mm diameter 2"
4) thickness 2 mm and diameter 2"
And I apply same compressive load on all these sheets
Let us assume that i transfer the load with a rod of 2" OD diameter and ID of 1".
The compressive stresses on these would be load/(pi*(2*2-1*1)/4)
Since compressive stress is same. As i start increasing the load i know the glass sheet with minimim thickness will break first.
Assuming that this is not a bucking case, how do i deterimine at which load the 1.2 mm (minimim thickness) sheet will break
In the calculation of the stress that includes thickness what is the area i need to use for calculation.
Is it the area i need to consider for calculating the stress is (D*t) or (pi*D*t). what should be the value of D
Is it the outer diameter of the rod or MEan diameter of the ID of the rod.





RE: how does thickness affect the design
I would therefore take a the surface area of a cone with a slope of 45ยบ and a height equal to the thickness of the glass and calcuate the shear load as the point load divided by that area. The glass will tend to break when the shear stress equals 1/2 the ultimate tensile strength of the glass.
It is also conceivable to use the lateral area of a truncated cone for shear analysis, with the top diameter of the cone = diameter of your rod. The bottom diameter would = the top diameter + 2 * glass_thickness * 1.4142
I don't think the stress in the immediate vicinity directly under the point load is important, unless you need to consider a local crushing failure. I believe punching shear would occur first, unless the glass thickness is relatively great.
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RE: how does thickness affect the design
RE: how does thickness affect the design
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RE: how does thickness affect the design
the glass will also tend to deform elastically and the stress will not be uniform as your calculation requires.
failures of windows almost always occur from edge defects and not from the location where a simple model predicts the maximum stress.
google search under John Pepi, Keith B. Doyle and the website of Schott glass (look for a documet with TIE-31)
Corning also has some usefull information on mechanics of glass
RE: how does thickness affect the design
You can't ask us to consider glass as the material and then apply the replies you get to sheet metal. The two materials will behave in entirely different ways! If I read your original post correctly, you are fully supporting the sheet behind the rod. If the material is glass it will shatter violently at some point. There is a defined failure criteria. With sheet metal there is no such point. What do you consider failure? Any plastic deformation at all? Or shearing through the material altogether?
RE: how does thickness affect the design
RE: how does thickness affect the design
I may be wrong, but the way I read the OP is that a disk of 2" OD is placed on a solid surface and is compressed by a rod of 2" OD and 1" ID. That's why a clear definition of failure is needed. This wouldn't be a case of shear, but ultimately some sort of crushing, ripping failure.
RE: how does thickness affect the design
I would think the cone effect will play
a part at the far end of each thickness.
I would expect that you would have a different
size burr or amount of extra material deformed
on each of the different thicknesses.
If you stacked them in the reverse manner as
above, I think you would see even great distortions
at the far ends.
Most of the parts we have with stamped holes in them
are ss and probably less than 1mm thick and even
they have a burr or distorted material on the
far side. I do not know if they muti stack these
or not. We use them for shims. The entry side
is always smooth and probably slightly indented
while the other side is rough or burred typically.
RE: how does thickness affect the design