ATM RE: Shear Load

["Arjan te Marvelde (ELN)" <Arjan.te.Marvelde@eln.ericsson.se>]

Richard,
This may also be of interest to the list, so i cc-ed it there.
  
> I think that if you have the silicone pads bonded to the 
> glass at least 2 mm
> thick, you will not need to worry.

I got the reply from Bison, giving me some clue about the shear force:
	E-modulus (100% cured) = 0.49x10^6 N/m^2
	Tensile strength       = 0.55x10^6 N/m^2
	all in accordance with ISO-8339 or DIN2-8339

My mechanics textbook gives:
	G = E / 2(1+n), 
	where n = rel. lat. shrink / rel. long. stretch
	G = shear modulus
	E = elasticity modulus
Since n must be larger than 0, the worst case for G is E/2

More from the textbook:
	T = alpha x G, 
	where alpha is the shear angle, approximated by displacement/height
	T is the shear load, in N/m^2
This reduces to:
	F = G x A x d/h, which is i believe the formula you gave.
	here F is the total force on the silicone pad.

Relating this to my 9.93" by 0.75" mirror and a six point cell: 
	area of 1cm^2, 
	thickness of 1mm, 
	displacement of 30micron
This gives a shear force per pair of support points of 0.75N (equiv. to 75 grams) over a 7.5 cm support separation.

For comparison, the mirror weighs appr. 2kg, i.e. 20N or 3N per support. The shear force however is not like gravity all in the same direction, but pinches the mirror between support pairs. 
Do you have any gut feeling how bad .75N is? My guts have no clue, and i have no time to put it in FlexPDE.

Of course, this is a worst case calculation, so real life is better. The silicone dab should be thicker (factor two) and i'm using plate glass which has a smaller CTE difference with Al (factor 3) so in my specific case things will be 6 times as good.

/Arjan te Marvelde