# Mechanical Properties of Solids

## What are mechanical properties of solids?

A solid material which is either a metal or a non-metal, may possess any one or more of the mechanical properties. These properties are classified as –

### Mechanical properties of solids resisting deformation

Material properties, that resist the deformation of solids are –

1. Strength.
2. Rigidity.
3. Fatigue resistance.
4. Elasticity.
5. Plasticity etc.

### Mechanical properties of solids resisting wear & tear

Material properties, that resist the wear and tear of solids are –

1. Hardness.
2. Hardenability.
3. Toughness.

### Mechanical properties facilitating extrusion

Material properties, that facilitate deformation under extrusion process are –

1. Ductility.
2. Malleability.
3. Brittleness.

### Mechanical properties facilitating fabrication

Material properties, that facilitate in fabrication are –

1. Machinability.
2. Weldability.

## Strength

Among all mechanical properties, strength is of prime importance.

It is the property of a material by virtue of which it can withstand the forces applied to it.

If a material can bear much force, it is said of high strength. But if it fails ( i.e. breaks, bends or torn etc. ) under the action of a small amount of force, it is said of low strength.

Example

Steel is stronger than Wood.

### Types of Strength

There are three kinds of strengths –

1. Tensile strength.
2. Compressive strength.
3. Shearing strength.

#### 1. Tensile strength –

The mechanical properties of a material by virtue of which it can bear tensile forces ( i.e. pulling forces ) applied to it are called tensile strength.

#### 2. Compressive strength –

The mechanical properties of a material by virtue of which it can bear compressive forces ( i.e. squeezing, shortening or hammering forces ) applied to it are called compressive strength.

#### 3. Shearing strength –

The mechanical properties of a material by virtue of which it can bear shearing forces ( i.e. a force which is applied to a body at right angles to its length ) applied to it are called shearing strength.

TO BE NOTED –

A material which is strong in one kind of strength may be weak in another kind of strength.

Example

1. Concrete possesses much higher compressive strength but its tensile strength is very low.
2. Mild-steel possesses much high tensile strength. Its compressive strength is also high. But its shearing strength is low.
3. Cast iron has high compressive strength but low tensile strength.

## Hardness

It is that mechanical property of a material by virtue of which it resists scratching, abrasion and penetration in it.

With a sharp knife or any other sharp tool, if we can easily put scratching marks on the surface of a material, or can make holes at ease in it or rub its surface easily by means of some abrasive material then the material is said to be of low hardness or soft.

But for a hard material we can not do the above functions easily. Greater the hardness of a material, greater will be its resistance to scratching, abrasion and penetration.

A material which is possessing high strength may also possess high hardness simultaneously.

TO BE NOTED –

1. “Scratching” means “putting indentation marks on a surface”.
2. “Abrasion” means “rubbing away” of material particles from surface.
3. “Penetration” means “making a hole” or dig on to surface of a material body.

Example –

1. Cast iron, glass, diamond etc. are very hard materials.
2. High carbon steel possesses both high strength and high hardness.
3. Aluminium has high strength but low hardness.
4. Glass possess high hardness but less strength.

### Units of Hardness

Various units used to specify hardness of materials are –

1. Brinell hardness number ( BHN ) .
2. Rockwell hardness number ( RHN ) .
3. Vickers hardness number ( VHN ) .

### Brinell Hardness Number –

Brinell hardness number is determined by means of a test called Brinell hardness test. In this test, a hardened steel ball of ( 10 \ mm ) diameter is pressed against a horizontal flat polished surface of the material by a load of ( 3000 \ kgf ) . The depth of penetration of hardened steel ball determines the BHN .

## Ductility

Mechanical properties of a material by which it go on increase in length to a great extent under the action of tensile forces without failure are called ductility.

Example

Aluminium, copper etc. are very good ductile materials and thin wires can be easily made from these materials.

## Malleability

Mechanical properties of a material by which it can be hammered into thin sheets without breaking are called malleability.

Example –

1. Gold is the most malleable and ductile material of all metals.
2. Lead is a good malleable material but not a good ductile material.

Malleability is measured by the minimum thickness of the leaf or foil which is possible to be produced with it.

## Brittleness

It is that property of a material for which the material easily break under a little force.

Example

1. Glass, Cast iron etc. are brittle materials.

## Toughness

Toughness of a material is defined as the amount of energy absorbed by the material before it fracture.

Example

High carbon steel is a tough material because it requires much energy before fracture.

## Fatigue resistance

A materials can withstand much higher value of steady load. But, the same material fail easily at low loads if the load is variable, repeating or cyclic in nature. This is called fatigue failure.

Fatigue resistance is the property of a material by virtue of which it can withstand fluctuating, repeating, variable or cyclic loads which are applied to it.

Example

• A hard wire can be broken easily by bending it repeatedly in opposite directions, as it loses strength due to elastic fatigue.
• Fatigue resistance of “High carbon steel” is very high.

## Hardenability

The ability of a metal of being hardened by a heat treatment process is known as hardenability.

(“Heat treatment” means “a process of heating and cooling of metals in controlled environment to bring a change in its micro structure resulting in a desired property”.)

Example

All ferrous metals can be hardened by heat treatment process but non-ferrous metals can not be hardened.

## Machinability

Ability of a metal of being machined easily is known as its machinability.

Example

1. Mild steel and non-ferrous metals like Copper, Aluminium, Brass, Bronze etc. has high machinability and can be machined easily.
2. Cast iron has low machinability and can not be machined so much easily.

## Weldability

The term weldability indicates the capacity of a metal of being fabricated into a required shape and structure by a welding process.

The important factors on which the weldability of a metal depends are –

1. Chemical composition of the metals ( i.e. the kind and percentage of elements present in the metals to be joined by the welding process )
2. Effect of temperature changes on the various elements present in the metals to be welded.
3. Expansion and contraction characteristics of the metals to be welded.
4. Filler metal used.
5. Flux used in welding process.
6. Welding procedure including the skill of the operator who does the welding operation.

## Creep

When a metal is raised in temperature and is subjected to a load it shows signs of flow like a viscous liquid. This flow of metal which takes place very slowly is called creep. Creep continues so long as the load is applied and ultimately the material breaks.

Example

Soft metals like lead, tin, zinc, etc have low melting point temperature and possess creep even at low temperatures. Hence these materials are not used at the regions of high temperatures.

## Rigidity

It is the property of any material by virtue of which it offers resistance to deformation due to applied force.

A body having good rigidity will resist any type of deformation under the influence of a force.

### Rigid body

A rigid body is defined as a hard solid object having a definite shape and size.

Solid bodies are said to be rigid but, in reality they are not perfectly rigid. They can be stretched, compressed and bent.

## Deformation

When an external force is applied on a rigid body, the body may get deformed. Deformation means a change in shape, size or dimensions of a rigid body under the influence of applied external force.

When a force is applied on a rigid body which is firmly held and restricted to move or rotate, the molecules of the body are forced to undergo a change in their relative positions. As a result, the body may undergo a change in shape, size or dimensions.

Deformation may be temporary or permanent. A temporary deformation is one in which the body completely regains its shape and size after removal of the applied force. But in case of permanent deformation, the changes persist even after removal of the applied force.

### Deforming force

A force which changes the size or shape of a body is called a deforming force.

Deformations produced in rigid bodies depends upon following two properties of matters –

1. Elasticity.
2. Plasticity.

## Elasticity

If a body regains its original size and shape after the removal of deforming force, it is said to be elastic body and this property is called elasticity.

Example

If we stretch a rubber band and release, it snaps back to its original length. It is said an elastic material.

### Perfectly Elastic body –

If a body regains its original size and shape completely and immediately after the removal of the deforming force, it is said to be a perfectly elastic body.

No any material body is perfectly elastic. The nearest approach to a perfectly elastic body is quartz fiber.

## Plasticity

If a body does not regain its original size and shape even after the removal of deforming force, it is said to be a plastic body and this property is called plasticity.

Example

If we stretch a piece of chewing-gum and release it, it will not regain its original size and shape.

### Perfectly Plastic body –

If a body does not show any tendency to regain its original size and shape even after the removal of deforming force, it is said to be a perfectly plastic body.

No any material body is perfectly plastic. Putty and paraffin wax are nearly perfectly plastic bodies.

TO BE NOTED –

1. No any material is perfectly elastic or perfectly plastic in nature. The nature of all bodies lie in between these two limits.
2. When the elastic behavior of a body decreases its plastic behavior increases and vice versa.