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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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