From: "Ellen Mackenzie" 
To: 
Reply-To: "Ellen Mackenzie" 


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      I'm not sure how the formula will look in text so I'll try and express
it technically first:

    The elongation of a material is equal to the force applied axially times
the length of the test piece divided by the cross-sectional area of the
test piece times the modulus of elasticity ( Young's modulus )

elongation =                   Force x length
                 modulus of elasticity x cross-sectional area

The modulus of elasticity is usually denoted by a capital  E "E"
is the representation of Hookes law which states " stress is
proportional to strain " or:

    E = stress
          strain

A graphical representation of the stress to strain ratio in terms of  E
would be a straight line rising from lower left bottom to upper right.
Stress is the vertical ordinate and strain is the horizontal ordinate. This
graph is different for every material and the upper portion of the graph
beyond the straight line slope is called the  " plastic " region.
A material subjected to a force with enough strength to bring the material
into the plastic area will not return to it's original size. E for steel
was at one time 30,000 psi but is now generally published to be 29,000 psi.
The modulus of elasticity for steel is the same for compression, tension,
and shear is the same. Concrete has a different modulus in compression than
in tension, wood has a different value according to the direction of the
force relative to the grain. Many materials such as plastic will fail (
rupture ) under forces much lower than the published figure for E over a
period of time; this is called creep or cold flow; teflon is notorious for
this as is natural rubber. Fortunately RTV Silicones have not, to my
knowledge, exhibited this property. Many materials have properties which
change according to their dimensions, glass being a good example.
    RTV Silicone does not have a very large linear portion relative to it's
stress strain range when used in thick layers but as the cross-sectional
area decreases it's elastic properties improve particularly in shear, and
it's strength increases as well. I have seen wood that had been glued
together with silicone tear apart without the silicone failing. It is
something to remember when mounting your mirror with it, as it is wise to
place a small piece of toothpick beneath the mounting pads as shims so
there is enough distance between the pad and the mirror that abnormal
stresses won't distort the mirror as the temperature varies. Another thing
of note is the time it takes for silicone to cure. Although there are
silicones on the market that "skin" over in a few minutes and
appear to set in a day, it takes quite a while for a coin of silicone of a
diameter of 2 inches between two impervious layers of material to reach
full cure. As an experiment, place a 1 inch blob of silicone on an
impervious surface and leave it in the open and see how long it takes to
"gas off " the acetic acid smell. That acetic acid can be very
destructive to some materials so don't be in a hurry to get your mirror
into the unventilated tube of your scope. I feel the best Silicone for
telescope use is the type mechanics use for gasket seals, it is expensive
compared to builder's silicone and when used to mount a mirror would take
about a month to come to full cure, but it does not give of any toxic fumes
nor does it contain any corrosive ingredients.
    My handbook gives a relative E of 50.


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I'm not sure = how the=20
formula will look in text so I'll try and express it technically=20
first:
 
   =20
The elongation of a material is equal to the force applied=20
axially times the length of the test piece divided by the =
cross-sectional=20
area of the test piece times the modulus of elasticity ( Young's modulus =

)
 
elongation=20
=3D           &=
nbsp;      =20
Force x length
       &nbs=
p;         modulus=20
of elasticity x cross-sectional area
 
The =
modulus of=20
elasticity is usually denoted by a capital 
E
"E" is =
the=20
representation of Hookes law which states " stress is
proportional = to=20
strain " or:
 
    E=20
=3D stress
       &nbs=
p; =20
strain
 
A =
graphical=20
representation of the stress to strain ratio in terms of  E would = be a=20
straight line rising from lower left bottom to upper right. Stress is = the=20
vertical ordinate and strain is the horizontal ordinate. This graph is = different=20
for every material and the upper portion of the graph beyond the = straight line=20
slope is called the  " plastic " region. A material
subjected to a = force=20
with enough strength to bring the material into the plastic area will = not return=20
to it's original size. E for steel was at one time 30,000 psi but is now =

generally published to be 29,000 psi. The modulus of elasticity for = steel is the=20
same for compression, tension, and shear is the same. Concrete has a = different=20
modulus in compression than in tension, wood has a different value = according to=20
the direction of the force relative to the grain. Many materials such as = plastic=20
will fail ( rupture ) under forces much lower than the published figure = for E=20
over a period of time; this is called creep or cold flow; teflon is = notorious=20
for this as is natural rubber. Fortunately RTV Silicones have not, to my =

knowledge, exhibited
this property. Many materials = have=20
properties which change according to their dimensions, glass being a = good=20
example. 
   =20
RTV Silicone does not have a very large linear portion relative to it's = stress=20
strain range when used in thick layers but as the cross-sectional area = decreases=20
it's elastic properties improve particularly in shear, and it's strength =

increases as well. I have seen wood that had been glued together with = silicone=20
tear apart without the silicone failing. It is something to remember = when=20
mounting your mirror with it, as it is wise to place a small piece of = toothpick=20
beneath the mounting pads as shims so there is enough distance between = the pad=20
and the mirror that abnormal stresses won't distort the mirror as the=20
temperature varies. Another thing of note is the time it takes for =
silicone to=20
cure. Although there are silicones on the market that "skin" over
in a = few=20
minutes and appear to set in a day, it takes quite a while for a coin of =

silicone of a diameter of 2 inches between two impervious layers of =

material to reach full cure. As an experiment, place a 1 inch blob of = silicone=20
on an impervious surface and leave it in the open and see how long it = takes to=20
"gas off " the acetic acid smell. That acetic acid can be very =
destructive to=20
some materials so don't be in a hurry to get your mirror into the = unventilated=20
tube of your scope. I feel the best Silicone for telescope use is the = type=20
mechanics use for gasket seals, it is expensive compared to builder's = silicone=20
and when used to mount a mirror would take about a month to come to full = cure,=20
but it does not give of any toxic fumes nor does it contain any = corrosive=20
ingredients.
   =20
My handbook gives a relative E of 50.
   =20

 

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