Engineering Materials: Physical & Mechanical Properties

In this article we will discuss about the physical and mechanical 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.

The more important of these properties will be considered as follows:

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:

Density (p) = Mass (m)/volume (V)

Relative density (d) = Density of the material/Density of pure water at 4°C

2. Electrical Conductivity:

Figure 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 pass 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. 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 great effect on the materials and the alloys of the materials properties and as a result on its applications.

4. Semiconductors:

In between conductors and insulators 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 enables 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 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 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 felid.

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.

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 Engineering Materials:

These properties are concerned with the following properties:

1. Tensile Strength:

It is the ability of a material to withstand tensile (stretching) loads without breaking. 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 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 broke by impact loading.

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.

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.

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), tube drawing operation. For more ductile material ε_p > 15%, for less ductile material ε_p > 5.1 ε_p < 15%.

6. Stiffness:

It is the measure of a material’s ability not to deflect under an applied load.

For example, 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.

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.

For brittle material ε_D < 5%.

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 possess elasticity to some degree and each has its own elastic limits.

9. Plasticity:

This property is 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 it 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 does not spring back strength again.

10. Creep:

The permanent deformation (strain) of a material under steady load as a function of time is called creep.

Length of our waist belt increases after some duration, is due to creep effect.

Thermally actuated process, and hence is influenced by temperature. Appreciable at temperature above 0.4. T_m where T_m is melting point of material in degree kelvin.

Creep occurs at room temperature in many materials such as lead, zinc, solder wire (an alloy of Pb and Sn), white metals, rubber, plastics and leather etc. e.g. consider zinc where melting point is 420°C (693 K). Its creep rate is considerable above a temperature of (0.4 x 693 K = 277 K) is at about 4°C only.

11. Fatigue:

The behavior of materials under fluctuating and reversing loads (or stresses) is termed as fatigue. This behaviour is different from that under the steady load. Fatigue is, however, not a dynamic effect. The rate of loading is usually not a factor is fatigue behavior. Fatigue behavior is experienced by all materials whether metals, plastics, concretes, or composites.

Main Effects of Fatigue:

i. Loss of ductility,

ii. Loss of strength, and

iii. Enhanced uncertainty in strength and the service life of materials.

Salient Features:

Fatigue occurs at stresses well within the elastic range. Various types of fluctuating and reversing stresses are shown in figure (a-d) in a simplified manner.

In practical situations the stress variations may be drastic, unpredictable and complex. One such example, Figure (d), is that of an aeroplane during its take-off, flight and landing.

Fatigue Limit (or Endurance Stress) and S-N Diagram:

Total number of cycles N required to bring about fracture in a material (or its specimen) under an applied stress defines its basic fatigue life. Fatigue life of a material is expressed in percent survival out of large number of specimens. Generally median fatigue life is estimated at 50% survival.

It is evaluated from the data of stress (S or σ) and number of cycles of failure N by conducting fatigue test. The plot is referred to as S-N diagram. The curve of mild steel is asymptotic beyond B. It implies that the material can withstand a stress equal to σ_c for any number of cycles without fatigue fracture. This stress is called as endurance stress. It is also known as fatigue limit. This stress is generally 0.3 to 0.5 times the ultimate strength of materials.

Ferrous metals usually have a fatigue limit but non-ferrous metals and non-metals often do not. Fatigue response of these materials is specified for a number of stress cycle normally 10⁷ and known as fatigue strength.

Critical Applications of Fatigue Bahaviour:

i. Wings of aeroplanes and other aircrafts.

ii. Leaf springs used on automobiles and railways rolling stocks.

iii. Connecting rods of internal combustion engines.

iv. Rotating shafts and machine parts.