As an introduction, consider the following properties for engineering applications.
| The Mohs Scale of Hardness | |
|---|---|
| 1 | talc |
| 2 | gypsum |
| 3 | calcite |
| 4 | fluorite |
| 5 | aptite |
| 6 | orthoclase |
| 7 | quartz |
| 8 | topaz |
| 9 | corundum |
| 10 | diamond |
A tensile stress pulls the two ends of a rod or cable apart.
When a small tensile stress is applied, the rod or cable lengthens
elastically and returns to its original length after the tensile
stress is removed. The ratio of tensile stress to lengthening (strain)
divided by the area of the cross section is called modulus of
elasticity. The (maximum) tensile stress or load causing permanent
or inelastic deformation is called yield load. At the yield load,
the lengthening will continue without adding more tensile stress
Shear strengths are important for bolts, rivets, drive key, cutting,
and polishing applications.
Properties of a material are important for any engineering applications,
and they are listed in handbooks. A general handbook for materials
science and engineering contains physical, chemical and engineering
properties. For example, bond lengths, bond angles, bond energies
of certain type of bonds can be found in the CRC Material Science
and Engineering Handbook. We can also find thermodynamic properties
such as energies of fusion, vaporization, sublimation, and
formation together with melting points, and boiling points.
Electric conductance, magnetic susceptibility, tensile strength,
yielding strength, hardness, etc. for some substances can also be found.
However, for special applications such as those used in nuclear reactor,
additional information might be found in specialized handbooks.
There are also handbooks for specialized class of material such
as various grades of steels.
In terms of color, we are interested in the property of a material
to filter and reflect lights. Our reaction to and perception of lights
depends on the wavelengths. For engineering, comparison, and
communication purposes, we must have standard measurements and
specifications of color. Four parameters, three for the
relative amounts of red, green, and blue components and one for
the brightness, are required to specify a color.
Specific concepts and terms must be developed for the description of
surface features. There are more than 20 mathematical parameters applied
to surface description and some of the terms are: roughness,
irregular features of wave, height, width, lay, and direction on the
surface; camber, deviation from straightness; out of flat,
measure of macroscopic deviations from flatness of a surface.
Furthermore, the surface properties not only affect visual perception,
they affect engineering applications as well. Rough surfaces in moving
contacts leads to wearing, whereas over smoothness is avoided because
of high cost for their manufacture. The importance of surface is
indicated by the many methods developed for its analysis. Optical
or laser technology can be used to measure large surface areas,
whereas visual examination detects features of the order of millimeters.
Finer surface features are examined by microscopy, which enlarge
the surface by a factor of 10 to 3000.
Still finer surface features are examined using scanning electron
microscopy (SEM), which enlarges a surface up to 10,000,000 times.
Some SEM machines are now equipped with an energy dispersive X-ray
analyzer (EDX), which identifies the chemical elements in the
bombarded area. The composition of the surface layers is often
determined by electron spectroscopy for chemical analysis (ESCA).
The surface is bombarded by low-energy X-rays to emit photoelectrons,
the analysis of which gives the atomic composition of the surface layers.
Another method yielding composition information is called secondary
ion mass spectroscopy (SIMS), which bombards the surface
with ions to remove some of the atoms. These atoms are ionized and
they pass through a mass spectrometer for further analysis.
Auger electron spectroscopy (AES) bombards the surface with an
electron beam, which causes the ejection of Auger electrons.
The Auger electrons is chemical element specific, and they reveal
also composition at the surface.
Dimensional properties deal with size and shape, which is also
related to quantities. In a modern society, material requirement
must be very carefully specified regarding the details such as the
surface texture, flatness, allowable defects, shape, dimensions,
camber, tolerance, etc. A slight misinterpretation or miscommunication
causes not only lots of money, but also a lot of frustration.
What are the desirable properties for cables of suspend bridge?
Gold and silver are better conductors than copper, why copper instead
of silver or gold is used for electrical wires? Is aluminum a suitable
metal for electrical wires?
Why is fatigue strength very difficult to define and measure?
How many parameters are required to specify a color? Explain why?
What methods are available for surface analysis?
What are dimensional properties?
Shear characteristics
Shear stress is a pair of force applied in opposite direction
but in a sliding fashion. In response to the shear stress, a material
deforms. Usually, shear stress causes adjacent laminar elements of a
solid body to slide over each other of. In the case of homogeneous
isotopic elastic medium, the ratio of shear strain to the shear stress
is called shear modulus. Shear strength is the stress, usually
expressed in force unit per unit area, required to produce fracture
when impressed perpendicular upon the cross-section of a material.
Fatigue strength
We all experience that repeated bending of a steel wire on the same
kink-point results in breaking it. Repeated loading and unloading of
a specific stress on a piece of material till it fails is called
fatigue strength or endurance limit. The stress can be tensile,
compressed, shear, bending, or a combination of these. Fatigue
strengths are hard to measure because of the varying and many
stress types. Usually some percentage of tensile and compression
strength are taken as a possible fatigue strength. Engineering designs
allow wide margins for safety to avoid failure due to fatigue.
Appearance and Dimensional Properties
Color and surface finish give an object an appearance, which is
a very important factor in engineering and commercial products.
Combining color and surface finish gives artistic designs appealing to
customers.
Activities
Why is diamond the hardest substance? Explain the reason based on its crystal structure.