Properties of Metals
More engineering components are made of metals and alloys than of any other class of solid. But increasingly, polymers are replacing metals because they offer a combination of properties which are more attractive to the designer and if you have been reading the newspaper, you will know that the new ceramics, at present under development world wide, are an emerging class of engineering material which may permit more efficient heat engines, sharper knives, and bearings with lower friction.
The engineer can combine the best properties of these materials to make composites which offer especially attractive packages of properties. And finally one should not ignore the natural materials like wood and leather which have properties which even with the innovations of today's materials scientists are hard to beat.
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| Materials used in Vehicle |
Lets look at an automobile as an example of a common product that uses a wide variety of materials. These materials were selected primarily because, among the materials that posses the desired properties and characteristics for the intended functions of specific parts of the automobile, these were the ones that can be manufactured at the lowest cost.
Steel was the chosen for much of the body because it is strong, easy to form, and inexpensive. Plastics were used in many components because of characteristics such as a wide choice of colors, light weight, and ease of manufacturing into various shapes at low cost.
Glass was chosen for the windows not only because it is transparent, but also because it is hard, easy to shape and clean, and resistant to scratching, Similar observations can be made for each components of an automobile, which typically is an assemblage of some 15,000 parts, ranging from thin wires to bumpers.
While designing a product, you need to select materials to create the product. For selecting materials, you must assess the properties of each material to ensure that the selected material is appropriate for manufacturing the desired product. The properties of materials can be categorized into the following types:
Properties of Materials
1) Physical
2) Mechanical
3) Electrical
4) Chemical
5) Thermal
Physical Properties
The physical properties of a material refer to the characteristics that describe the material. These properties include:
Physical Properties of a Material
1) Density
2) Melting point
3) Specific heat
4) Thermal expansion
5) Thermal conductivity
◆ Density
Density of material is defined as the mass per unit volume of the material. Density can be expressed as:
Density = Mass / Volume
Different materials have different density at the same temperature. For example, metals have higher density than ceramics. In addition, the density of materials can change. with a change in temperature. Therefore, materials can have different density at different temperatures.
When the density of a material is expressed with relation to water it is known as specific density.
Density plays a significant role in determining the strength ratio, which is the ratio of the strength of a material to its density.
◆ Melting point
Melting point is defined as the fixed or constant temperature at which pure metals or non-metals change from solid to liquid form.
The melting point of a material depends on the energy required to separate its atoms. In addition, the melting point of materials, such as alloys that are made up of two or more metals, varies on the basis of their composition.
Zinc, lead, and tin are examples of materials that have low melting points. Aluminum alloys have medium melting point, and copper alloys have a considerably high melting points. Ceramics have the highest melting points.
◆ Specific heat
The amount of heat required to raise the temperature of a material by 1℃ is called the heat capacity or specific heat of the material.
The specific heat of a material is the quantity of heat needed to raise the temperature of a unit mass of the substance by 1℃.
Metals have lower specific heat capacity than plastics.Therefore, they require less heat to reach a particular temperature than plastics.
◆ Thermal expansion
When thermal energy is added to a material, the change in its dimension is known as thermal expansion.
Most solid materials expand upon heating and contract when cooled. The change in length with temperature for a solid material may be expressed as follows:
lf - l0 / l0 = α1( Tf -T0 )
OR
∆l / l0 = α1 ∆T
Where l0 and lf represent, respectively, initial and final lengths with the temperature change from T0 to Tf. The parameter α1 is called the linear coefficient of thermal expansion. It is a material property that is indicative of the extent to which a material expands upon heating, and has units of reciprocal temperature [(℃)-1 or (°F)-1].
◆ Thermal conductivity
The thermal conductivity is nothing but the mode of transmission from one substance to the other in the direction of fall of temperature, but the substance remain fix at their original position.
Mostly the thermal conductivity is denoted by k.
