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Properties Of Glass
Mechanical
The properties of glass can be varied and regulated over an extensive
range by modifying the composition, production techniques, or both. In
any glass, the mechanical, chemical, optical, and thermal properties
cannot occur separately. Instead, any glass represents a combination of
properties. And in selecting an individual glass for a product, it is
this combination that is important. Usually one property cannot be changed
without causing a change in the other properties. It is the art of the
glass scientist to produce the most favorable combination possible.
Mechanical properties deal with the action of forces on a material and
the effects that these forces produce within the material.
Push, pull or twist a piece of glass hard enough, and it will bend or
stretch. Not very much, admittedly, but some bending or stretching is
possible. Watch the reflections in a large window when a strong wind is
blowing on it and you can observe the way the window bends from the
force of the wind. Glass is an unusual material in this respect, not
because it bends or stretches (most materials do) but because it returns
exactly to its original shape when the bending or stretching force is
removed. This characteristic of glass classifies it as a perfectly elastic
material. If you apply an increasing force, the glass breaks when the
force reaches the ultimate strength of the glass. But at any point short of
breakage, the glass will not deform permanently.
To be precise, glass must be classified as nearly perfect elastic
material because under some conditions permanent deformation, or plastic
flow, and to be produced.
There are three types of forces to be considered:
A tensile force exerts a pull on the material (a mild tensile force
exerts a pull on the material; a severe tensile force pulls the material
apart).
A compressive force acts to squeeze the material.
A shear force acts on the material in a manner similar to a pair of
shears to slide one part of the material in one direction and another in
the opposite direction.
Tensile forces are the most important in glass because they give rise
to tensile strain within the glass and glass breaks only from tensile
tension. (The terms stress and strain are sometimes confused. In fact,
they are used interchangeably at times as if both represent the same
phenomenon.)
Stress produces strain and strain cannot exist without stress. However,
the two terms represent different physical qualities.
Strength of glass is only slightly affected by composition but is
highly dependent on surface condition. Commercially produced glass ware can
acquire small nicks and scratches in the course of manufacture and
later in use. Any applied stress will be concentrated at these points of
damage with the result that the stress at these points will be increased
above the amount of the applied stress.
The strength of tempered commercial glass runs about 20,000 pounds per
square inch.
When new products are being tested for strength, they are usually
tested with all surfaces thoroughly abraded so test results will reflect the
strength of the product after it has experienced the worst treatment it
is likely to encounter in use. Also when checking out a new product the
test level has to be adjusted to take into account the time-load effect
in glass, or how long a time stress applied.
(If a glass item must withstand a load for 1000 hours or more, this
load can be only approximately one-third the load that the same item could
withstand for one second. After 1000 hours the strength of the glass
does not decrease further.)
Strength is measured in the laboratory by applying a bending load to a
bar of glass. This stretches the lower surface of the bar so that it is
in tension and squeezes the top surface so it is in compression. The
load is increased until the bar breaks. The break originates in the lower
surface since glass always fails from tension.
Glass never disintegrates or explodes; inside, a crack is started at a
specific point and progresses to failure. After separating, distinctive
contours on the fracture surface record the point of origin, the
direction of crack propagation, and other factors present during crack
initiation and fracture. The reconstruction of the failure events from these
features is known as fractography.
What happens when we pre-stress glass i.e., put all of its surfaces
under compression? When pre-stress is applied there will be an equal
amount of tension somewhere to maintain the balance. This tension is buried
in the middle of the glass where it is safe from damage protected by
the outer skin of compressed glass.
Now, when a bending load is applied to this pre-stressed bar, the load
must first overcome the built-in compression before it can put the
lower surface in tension. The result is that the strength of the lower
surface is approximately equal to the sum of its strength when stress-free
plus the amount of built-in compression.
Pre-stressing is often done by thermal tempering. First the glass is
heated to the point of almost sagging under its own weight, then it is
chilled suddenly. A large temperature difference exists between the
surfaces and interior because of low thermal conductivity. Negligible stress
is generated because the glass is too fluid, both at the surface and in
the interior. As cooling continues to room temperature, the large
temperature difference between the interior and surfaces diminishes and
eventually disappears. This means that the interior must shrink
considerably more, which in turn forces the surfaces into a state of compression
with a state of tension in the interior for balance.
Glass can also be strengthened by chemical means. There are several
such methods. One of the most commonly used requires an exchange of ions
in the glass surface. The glass is immersed in a molten salt bath and
large ions migrate into the surface, replacing smaller ions. This action
crowds the surface and produces compression.
Laminate strengthening is another process. In this method the product
is made of a "e;sandwich"e; of glasses. The inner glass shrinks more on
cooling, causing the outer layer to be put into compression.
Mechanical hardness can be measured by three methods: scratch,
penetration and abrasion. Mechanical hardness of glass is a complex phenomenon
that is not completely understood. When a product is required to
withstand wear or abrasion it is best to evaluate it under actual operating
conditions rather than to rely on laboratory hardness tests.
Optical
The properties of glass can be varied and regulated over an extensive
range by modifying the composition, production techniques, or both. In
any glass, the mechanical, chemical, optical, and thermal properties
cannot occur separately. Instead, any glass represents a combination of
properties. And in selecting an individual glass for a product, it is
this combination that is important. Usually one property cannot be changed
without causing a change in the other properties. It is the art of the
glass scientist to produce the most favorable combination possible.
When a beam of light falls on a piece of glass, some of the light is
reflected from the glass surface, some of the light passes through the
glass, and some is absorbed in the glass.
The measure of the proportion of light reflected light from the surface
is called reflectance.
The measure of the proportion absorbed is the absorptance.
The measure of the proportion transmitted is the transmittance.
Each quality is expressed as a fraction of the total quantity of light
in the beam. If the intensity of the beam is represented by the
numerical 1, reflectance by R, absorptance by A and transmittance by T,
intensity may be expressed as follows: R + A + T = 1.
Optical properties are concerned with the behavior of glass toward
light, the visible spectrum that extends like the rainbow from violet on
one end to red on the other. However, as the term is usually employed,
optical refers also to behavior towards the infrared and ultraviolet
regions of spectrum. The infrared region lies next to the red end of the
visible spectrum and the ultraviolet is on the opposite end of the
visible region next to the blue.
The greatest physical difference between these bands of energy spectrum
is in the wavelength. Ultraviolet waves are shorter than visible waves
and visible waves are shorter than infrared. All of them are so short
that extremely small units are used in measuring them.
(Wave lengths are expressed in millimicrons, microns and Angstroms.
There are 25,400,000 millimicrons, 25,400 microns, and 254,000 Angstroms
in an inch. Or, in the metric system, a millimicron is one-thousandth of
a micron, a micron is one-millionth of a meter and an Angstrom one
tenth of a millimicron.)
Most glass is transparent, or, to be more accurate, partially
transparent. Complete transparency would imply no reflection and no absorption.
No glass achieves this uncompromised state but most glass transmits
most of the light that lands on it. For this reason it is easy and
convenient to classify glass loosely as a transparent material.
A number of glasses are selectively transparent. They transmit light of
one wavelength or color more efficiently than any other. A green
traffic light is an example. The lamp behind the green lens supplies white
light, or light that contains some light of all colors. The green lens
absorbs all colors except green and green is all we see coming through
the lens.
This selectivity carries over into the ultraviolet and infrared
regions. A number of special-purpose compositions have been designed to
transmit either ultraviolet or infrared while absorbing visible light. These
glasses are black in appearance. Also some glasses are designed to
absorb infrared and transmit visible-the heat-absorbing filters such as are
found on film projectors. The purpose of these filters is to get as
much light as possible on the screen while keeping the slide or film as
cool as possible so the film doesn't melt.
The bending (or refraction) of light when it passes through glass is
the phenomenon that makes lenses possible. In a lens all the rays that
pass through the glass are refracted by the lens and brought to focus at
a single point. A measure of the amount of bending is the refractive
index. The higher the refractive index, the greater the bending.
But not all colors of light are refracted the same. Blue light bends
more sharply than red light in the same glass and the intermediate shades
(green, yellow) take a middle course. This difference results in some
dispersion and prevents all rays that go through a lens from focusing at
exactly the same point. The measure of this difference in refractive
index is the dispersion coefficient.
Several hundreds of glass compositions are produced for optical uses
alone. These glasses comprise a range of refractive indices, dispersion
coefficients and combinations of these two properties to provide the
lens designer with a wide array of materials. A simple hand magnifier or a
pair of eye glasses may employ only single-element lenses but it is not
unusual for a sophisticated camera lens to employ as many as seven
elements. Each element may be made of a different glass.
Reflectance from a glass surface can be regulated by coatings applied
to the surface. A metallic coating will produce the maximum
reflectance- a front-surface mirror for instance.
Other coatings show selective reflectance, such as the heat-shielding
glass that reflects a high proportion of infrared but transmits a high
proportion of visible light. Still other coatings eliminate reflectance
almost completely such nonreflective coatings are commonly used on
lenses.
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