General Properties of Engineering Materials

General Properties of Engineering Materials :

          The principle properties of materials which are of importance to the engineer in selecting materials. These can be broadly divided into:

Physical properties of 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.

Mechanical properties of materials :

          These properties are concerned with the following properties :

         1.   Tensile strength

         2.   Toughness  

         3.   Malleability

         4.   Hardness

         5.   Ductility

         6.   stiffness

         7.   brittleness

         8.   elasticity

         9.   plasticity

1. Tensile strength (TS) :

          It is the ability of a material to withstand tensile ( stretching )

loads without breaking. For example, figure 4 shows a heavy load

being held up by a rod fastened to beam. As the force of gravity

acting on the load is trying to stretch the rod, the rod is said to be

in tension. Therefore, the material from which the rod is made

needs to have sufficient tensile strength to resist the pull of the

load.

Strength:

           It is the ability of a material to resist applied forces

without fracturing.

2. Toughness :

          It is the ability of the materials to withstand bending or it is

the application of shear stresses without fracture, so the rubbers

and most plastic materials do not shatter, therefore they are tough.

For example, if a rod is made of high-carbon steel then it will be

bend without breaking under the impact of the hammer, while if a

rod is made of glass then it will broken by impact loading as

shown in figure 5.

3. Malleability :

          It is the capacity of substance to withstand deformation under

compression without rupture or the malleable material allows a

useful amount of plastic deformation to occur under compressive

loading before fracture occurs. Such a material is required for

manipulation by such processes as forging, rolling and rivet

heading as shown in figure 6.

4. Hardness :

          It is the ability of a material to withstand scratching

(abrasion) or indentation by another hard body , it is an indication

of the wear resistance of the material. For example, figure 7 shows

a hardened steel ball being pressed first into a hard material and

then into a soft material by the same load. As seen that the ball

only makes a small indentation in the hard material but it makes a

very much deeper impression in the softer material.

5. Ductility :

          It refer to the capacity of substance to undergo deformation

under tension without rupture as in wire drawing (as shown in

figure 8 ), tube drawing operation.

6. Stiffness :

          It is the measure of a material's ability not to deflect under an

applied load. For example, although steel is very much stronger

than cast iron, then the cast iron is preferred for machine beds and

frames because it is more rigid and less likely to deflect with

consequent loss of alignment and accuracy.

Consider figure 9 (a): for a given load the cast iron beam deflect

less than the steel beam because cast iron is more rigid material.

          However, when the load increased as shown in figure 9 (b), the

cast iron beam will break, whilst the steel beam deflects little

further but not break. Thus a material which is rigid is not

necessarily strong.

7. Brittleness :

          It is the property of a material that shows little or no plastic

deformation before fracture when a force is applied. Also it is

usually said as the opposite of ductility and malleability.

8. Elasticity :

          It is the ability of a material to deform under load and return to

its original size and shape when the load is removed. If it is made

from an elastic material it will be the same length before and after

the load is applied, despite the fact that it will be longer whilst the

load is being applied. All materials posses elasticity to some

degree and each has its own elastic limits. As in figure 10.

9. Plasticity :

          This property is the exact opposite to elasticity, while the

ductility and malleability are particular cases of the property of the

plasticity . It is the state of a material which has been loaded

beyond its elastic limit so as to cause the material to deform

permanently. Under such conditions the material takes a

permanent set and will not return to its original size and shape

when the load is removed. When a piece of mild steel is bent at

right angles into the shape of a bracket, it shows the property of

plasticity since it dose not spring back strength again, this is

shown in figure 11.

          Some metals such as lead have a good plastic range at room

temperature and can be extensively worked (where working of

metal means squeezing, stretching or beating it to shape). This is

advantage for plumber when beating lead flashings to shape on building sites.