Engineer Properties of Material
Engineering or more often called mechanical properties deals with material's response to tension, compression, shear, and repeated combined actions of these three. Testing machines have been developed to provide data for engineering applications, and the data are usually listed in various handbooks. Some engineering concepts are required for understanding these properties, and it is helpful for a general reader to acquire some of these concepts in order to understand the material world. On the other hand, engineers have to consider physical and chemical properties in their designs and applications.
As an introduction, consider the following properties for engineering applications.
|The Mohs Scale |
Ability to abrade or indent one another is referred to as hardness. There are several scales of hardness, and the oldest one is a Mohs hardness scale, which assigned a number consecutively from 1 to 10 for talc, gypsum, calcite, fluorite, aptite, orthoclase, quartz, topaz, corundum and diamond, respectively. Diamond is the hardest of all substances. This scale is not linear, but relative. The hardness is assigned by a scratching test. A modified Mohs scale gives 15 grade of hardness between talc and diamond, whereas the Knoop values of hardness range from 32 for gypsum to 7000 for diamond. Hardness should be prime considerations in all tools and machine parts. Harder substances are used for cutting tools. Due to their hardness, the most important applications of small synthetic diamonds are in cutting tools. In these applications, their colors and sizes are not important.
Hooke's Law states that the strain produced in an elastic body is proportional to the stress which causes it. The stress may be in the form of tensile or shear.
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 . If the yield load is not removed, the rod lengthening continues until it breaks. The tensile strength is the yield load divided by area of the cross section of a rod or bar. Thus, tensile strengths have the units of force per unit area. The yield load of a cable or rod is equal to the tensile strength multiplied by the cross section area.
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.
Shear strengths are important for bolts, rivets, drive key, cutting, and polishing applications.
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.
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.
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.
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.
Why is diamond the hardest substance? Explain the reason based on its crystal structure.
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?
Chung (Peter) Chieh (Chemistry, University of Waterloo)