Physical Properties of Engineering Materials

Physical properties of Engineering materials:

          These properties concerned with such properties as melting,

temperature, electrical conductivity, thermal conductivity, density,

corrosion resistance, magnetic properties, etc. and the more important

of these properties will be considered as follows :

    1.   Density

    2.   Electrical conductivity :

    3.   Melting temperature of a material

    4.   Semiconductor

    5.   Thermal conductivity

    6.   Fusibility

     7.    Reluctance (as magnetic properties)  

     8.    Temperature stability

1. Density :

          Density is defined as mass per unit volume for a material. The

derived unit usually used by engineers is the kg/m3 . Relative

density is the density of the material compared with the density of

the water at 4˚C. The formulae of density and relative density are:

2. Electrical conductivity :

          Figure 1 shows a piece of electrical cable. In this example copper

wire has been chosen for the conductor or core of the cable because

copper has the property of very good electrical conductivity. That is,

it offers very little resistance to the flow of electrons (electric

current) through the wire. A plastic materials such as polymerized

has been chosen for the insulating sheathing surrounding the wire

conductor. This material has been chosen because it is such a bad

conductor, where very few electrons can bass through it. Because

they are very bad conductors they are called as insulators. There is

no such thing as a perfect insulator, only very bad conductors.

          For example, metallic conductors of electricity all increase in

resistance as their temperatures rise. Pure metal shows this effect

more strongly than alloys. However, pure metals generally have a

better conductivity than alloys at room temperature. The

conductivity of metals and metal alloys improves as the temperature

falls.

Conversely, non-metallic materials used for insulators tend to

offer a lower resistance to the passage of electrons, and so become

poorer insulators, as their temperatures rise. Glass, for example, is an

excellent insulator at room temperature, but becomes a conductor if

raised to red heat.

3. Melting temperature of material :

          The melting temperatures and the recrystallisation temperatures

have a grate effect on the materials and the alloys of the materials

properties and as a result on its applications.

4. Semiconductors :

          So far we have examined the conductivity of the metals and the

insulating properties of the non-metals (exception : carbon). In

between conductors and isolators lies a range of materials known as

semiconductors. These can be good or bad conductors depending

upon their temperatures. The conductivity of semiconductor

materials increases rapidly for relatively small temperature increases.

This enable them to be used as temperature sensors in electronic

thermometers.

          Semiconductor materials are capable of having their conductors

properties changed during manufacture. Examples of semiconductor

materials are silicon and germanium. They are used extensively in

the electronics industry in the manufacture of solid-state devices

such as diodes, thermistors, transistors and integrated circuits.

5. Thermal conductivity :

          This is the ability of the material to transmit heat energy by

conduction. Figure 2 shows a soldering iron. The bit is made from

copper which is a good conductor of heat and so will allow the heat

energy stored in it to travel easily down to the tip and into the work

being soldered. The wooden handle remains cool as it has a low

thermal conductivity and resists the flow of heat energy.

6. Fusibility :

          This is the ease with which materials will melt. It can be seen

from figure 3 that solder melts easily and so has the property of high

fusibility. On the other hand, fire bricks used for furnace linings only

melt at very high temperatures and so have the properties of low

fusibility. Such materials which only melt a very high temperatures

are called refractory materials. These must not be confused with

materials which have a low thermal conductivity and used as thermal

insulators. Although expanded polystyrene is an excellent thermal

insulator, it has a very low melting point ( high fusibility ) and in no

way can it be considered a refractory material.

7. Reluctance (as magnetic properties) :

          Just as some materials are good or bad conductors of electricity,

some materials can be good or bad conductors of magnetism. The

resistance of magnetic circuit is referred to as reluctance. The good

magnetic conductors have low reluctance and examples are the

ferromagnetic materials which get their name from the fact that they

are made from iron, steel and associated alloying elements such as

cobalt and nickel. All other materials are non-magnetic and offer a

high reluctance to the magnetic flux field.

8. Temperature stability :

          Any changes in temperature can have very significant effects on

the structure and properties of materials. However, there are several

effects can appear with changes in temperature such as creep.

Creep is defined as the gradual extension of a material over a

long period of time whilst the applied load is kept constant. It is also

an important factor when considering plastic materials, and it must

be considered when metals work continuously at high temperatures.

For example gas-turbine blades. The creep rate increases if the

temperature is raised, but becomes less if the temperature is lowered.