Mechanical properties of Engineering materials

Mechanical properties of Engineering 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.