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Thermal Expansion Coefficient

Some products are assembled using a process known as shrink-fitting. Shrink fits use thermal expansion and contraction. One component is heated so that it expands. It is then assembled onto another component that is at room temperature. Once cooled, the first component contracts, or shrinks, and fits onto the second component. Most bearings are assembled onto their shafts using this method. For such applications, thermal expansion of the metal is an important property.

The thermal expansion coefficient, a, is expressed in per °C or per °F. We can obtain the amount of expansion undergone by a material by multiplying the original length by the expansion coefficient and temperature rise. That is,

                                                                                                               

where, DL is the amount of expansion, L is the original length and DT is  temperature rise.

In components that are constrained to move, thermal expansion can lead to thermal stresses. These stresses will add to the existing stresses caused by the external load and if the combined stresses increase beyond the yield strength of the material, failure will occur.

Thermal Conductivity

DID YOU KNOW:

Thermal Conductivity

The thermal conductivity of a metal or alloy refers to the rate at which heat flows within the material. Alloying elements usually have a significant effect on the value of thermal conductivity. Thermal conductivity is expressed in W/m K. Copper has a thermal conductivity of 393 W/m K, whereas steels have thermal conductivities between 15 to 52 W/m K.

In metals of high thermal conductivity, such as copper and aluminum, heat is conducted away quickly during plastic deformation. In materials of lower thermal conductivity, such as steel and lead, high thermal gradients can result during plastic deformation, causing non-uniform deformation.

DID YOU KNOW:Classification of Materials

DID YOU KNOW:Classification of Materials

We can generally classify materials into metallic and non-metallic. We can further classify metallic materials into ferrous and non-ferrous, while non-metallic materials can be classified into organic and inorganic. Examples of these materials are given in Table 2.1.

Table 2.1        Examples of Engineering Materials

Ferrous metallic	Non-ferrous metallic	Organic	Inorganic
Steel, gray cast iron, wrought iron, malleable iron	Aluminum, copper, magnesium, nickel, lead, zinc, titanium	Leather, wood, rubber, natural fibers, resin	Ceramic, glass, graphite

Ferrous materials contain iron as the base metal and range from plain carbon steel containing more than 98 percent iron to high alloy steel containing up to 50 percent of a variety of alloying elements. Non-ferrous metallic materials can be further sub-divided into light metals, such as aluminum, magnesium and titanium, low-melting point metals (e.g. lead and tin), refractory metals (molybdenum, tantalum and tungsten), and precious metals (e.g. gold, silver and platinum).

Within the group of alloyed metallic materials, we can further classify them according to (a) chemical composition, e.g. carbon content and alloy content in steels, (b) finishing, e.g. hot rolled or cold rolled, and (c) product form, e.g. bar, plate, sheet, tubing, structural shape.

Metallic materials can also be divided into cast alloys and wrought alloys according to the method of production. Cast alloys make up about 20 percent of all industrial metallic materials and are directly cast into the final shape of the product. Wrought alloys are usually shaped by hot or cold working into semi-finished materials such as plates, sheets, rods, wires or tubes. These are used as the input material for further processing into the final product.

Besides cast and wrought alloys, another type of metallic material that is gaining attention in the industries is powder metals and alloys. The powders are compacted and sintered (that is, heated without melting) to produce components that are ready to use and need very little further processing.

The most common non-metallic materials are the polymeric materials. They have a wide range of mechanical, physical and chemical properties. These materials are characterized by their low density, good thermal and electrical insulation, high resistance to most chemicals and ability to be process using different colors and opacities. Compared to metals and alloys, these materials are mechanically weaker and less stiff. These drawbacks, however, can be overcome by reinforcing the materials with various types of fibers.

One of the main reasons why polymers, such as plastics, are increasing used in making products is because they can be easily processed into complicated shapes in one step with little need for further processing or surface treatment. Polymers can also be easily machined and joined. They are also light and need less processing power compared to metals. Products made from plastics can have high dimensional accuracy and surface finish. They can be made in attractive colors at high speed and low cost.

Plastics are classified into thermoplastics and thermosets. Thermoplastics soften when heated and harden when cooled no matter how often the heating and cooling processes are repeated. Thermosets, however, does not soften upon heating once they are fully cured. To a product designer, the difference between thermoplastics and thermosets is important because they need different manufacturing methods.

Rubbers are similar in structure to plastics. The difference is based on the degree of stretching. Rubbers can be stretched to at least twice their original length. Once the stress is released, they return to their original length.

Ceramics are inorganic compounds of one or more metals with a non-metallic element. Some examples of ceramics and their non-metallic elements are shown in Table 2.2. Ceramics have high hardness, stiffness and stability. They are also generally good insulators to heat and electricity. They are, however, brittle.

Table 2.2        Ceramics and Their Non-metallic Elements

Ceramic compounds	Non-metallic elements
Aluminum oxide	Oxygen
Silicon oxide	Carbon
Silicon nitride	Nitrogen

DID YOU KNOW:ENGINEERING MATERIALS

DID YOU KNOW:

ENGINEERING MATERIALS

Materials are one of the main input elements in a production process. The process converts the raw materials into useful products. You will notice that nowadays many products ranging from automobile parts and electrical appliances to medical application are made from plastics, composites and other polymers. The commonly used metals, however, will still be used for load-bearing and heat-resistant structures.

Materials that can be used to manufacture a product have such diverse properties that even when we consider the performance and cost, it is still difficult to decide what material to use for a particular application. One material may have higher strength but poor corrosion resistance. Another material may have high strength and good corrosion resistance but difficult or expensive to be processed into the finished product. It is important for us to understand their properties, reliability and manufacturability before we

can effectively and efficiently use them in making new products

DID YOU KNOW: wrought iron furniture

DID YOU KNOW:WROUGHT IRON

-Wrought iron is pure iron.

-It is ductile. This iron is processed using a pudding furnace. This iron produced has gone through processes such as knocking, squeezing and rolling to get the different shaped of round bar, plates and sheet.

-It is used to manufacture pulleys, anvils and crane hook.

DID YOU KNOW: carbon in steel

DID YOU KNOW:Carbon Steel

·Low Carbon Steel

-Carbon content is between 0.15%  to 0.30%.

-Products of this steel are rods, steel sheet, steel structure, rivet, wire and others.

-This type of carbon steel are ductile, malleable and machinable. It can be formed and welded but its hardenability is low.

·Medium Carbon Steel

-Carbon content is between 0.30% and 0.80%.

-Products of this steel are moulds blocks, wire, screw driver, chisel, wrench, heavy duty wrought hammer, gear and spindle.

-This type of carbon steel has a higher strength compared to low carbon steel and its mechanical properties such as hardness and durability can be improved by heat treatment.

·High Carbon Steel

-Carbon content in this type o9f steel is between 0.80% and 1.5%.

-Products of this steel are cutting tools.

-This type carbon of steel has a combination of mechanical properties such as hard and durable.

DID YOU KNOW:METALLIC

DID YOU KNOW:METALLIC

Most important engineering material in manufacturing.

chosen in the manufacturing industry due to the factors such as:

üHigh durability

üAble to resist abrasion and corrosion

üMetals are economical and can be shaped