MECHANICAL PROPERTIES OF MATERIALS

MECHANICAL PROPERTIES OF ENGINEERING MATERIALS

The mechanical properties of materials describes the behavior of the material under the action of external forces. 

Mechanical properties play important role in material selection. 

(1) Strength: 

Definition- Ability of material to resist external forces or loads, without fracture.

On the basis of type of stresses induced by loads, strengths are categorized as tensile strength, compressive strength, shear strength, Torsional strength, Yield strength and Ultimate tensile strength. 

Tensile strength- Ability of material to withstand (without fracture) tensile forces which causes Tensile Stress.

Compressive strength- Ability of material to withstand (without fracture) compressive forces which causes Compressive Stress.

Shear Strength- Ability of material to withstand (without fracture) Forces forces which causes Shear Stress.

Yield Strength- The maximum stress at which noticeable elongation is seen without increase in load.

Ultimate Tensile strength- The maximum stress value that can be reached in tension test.

(2) Elasticity: 

Definition- Ability of the material to return to its original shape and size after the deformation, when the loads are removed. 

Every engineering metals are elastic in nature, but the degree of elasticity varies from metal to metal. 

Steel is elastic in nature but only within elastic limit. The elastic deformation which a steel can undergo is very small.

For elastic materials, stress-strain relationship is linear within elastic limit

(3) Plasticity: 

Definition- Ability of the material to, continue deformation, under load on permanent basis. 

Plasticity is exactly opposite to elasticity

material with plasticity property can not regain its normal shape and size.

Plasticity is considerable property for press working processes.

(4) Stiffness: 

Definition- Ability of the material to resist deformation under the action of loads.

Deformation and stiffness are inversely proportional, that means lesser the deformation stiffer the material

Modulus of elasticity is related to stiffness, it can give idea about stiffness. As the values of modulus of elasticity for aluminum alloy is 71,000 N/mm²  and carbon steel is 2,07,000 N/mm², Therefore, carbon steel is more stiffer than aluminum alloy.

Stiffness is important consideration in design of transmission shaft.

(5) Resilience: 

Definition- Ability of the material to absorb energy when undergo elastic deformation under the load and to release this energy when unloaded. 

A resilient material absorbs energy within elastic range without any permanent deformation. 

This property is essential criteria for spring materials. 

Modulus of resilience- is the strain energy per unit volume that is required to stress the specimen in tension test up to elastic limit point.

Modulus of resilience is the area under the stress strain curve from origin to the elastic limit point.

(6) Toughness: 

Definition- Ability of the material to absorb energy before fracture.

This property tells about the capacity of taking impact loads.

Modulus of toughness- The whole area (from origin to break point) under stress-strain curve in tension test,

It is also the work done to fracture the specimen.

In practical applications, toughness is measured by Izod and Charpy impact testing machines.

Toughness increases as the temperature decreases.

(7) Malleability: 

Definition- Ability of the material to deform (into sheets) under the action of pressure or compressive force.

Malleable metals use extrusion, forging and rolling because these processes involve shaping under compressive force.

Gold and silver are good example of malleable metals.

(8) Ductility: 

Definition- Ability of the material to deform prior to crack, when it is subjected to tensile force.

Mild steel (MS), copper and aluminium are ductile materials.

Shaping process like forming, drawing or bending are used for ductile materials.

Ductility is desirable property in machine components in which unwanted overloads or impact loads can occur.

Ductility is inversely proportional to temperature. So ductility decreases as the temperature increases; as metals become weak with increasing temperature.

All ductile materials are also malleable, but reverse is not always true.

Presence of impurities in the metal reduces the both malleability as well as ductility.

Ductile materials fails due to yielding.

(9) Brittleness: 

Definition- If the material shows negligible plastic deformation in tension test then the material is brittle.

Brittleness property is opposite to ductility.

Cast iron is a best example of brittle material.

A brittle material component fails by sudden fracture.

Differentiating factor between ductile and brittle materials is a tensile strain of 5% at fracture in tension test.

(10) Hardness: 

Definition- Ability of material to resist the penetration or permanent deformation.

Hardness signifies the resistance to abrasion, scratching, cutting or shaping. That’s why it becomes important property in material selection for parts which rub to one another.

Ex- pinion and gear, cam and follower, rail and wheel and parts of ball bearing.

Wear resistance can be improved by increasing surface hardness by case hardening.

Following are the primary methods of measuring hardness –

Brinell hardness test,

Rockwell hardness test,

Vicker hardness test

Shore scleroscope.

The hardness increases, the strength also increases. 

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