Mechanical Properties of Metals, Stress and Strain

Stress

When a material is subjected to an external force, a resisting force induced. The internal resistance force per unit area is called stress.

Unit :    $N/m^2$

Type of stress:

• Normal Stress

• Shear Stress

          

• Bulk Stress

      

Strain

Deformation per unit length in the direction of deformation is known as strain. It is dimensionless quantity.

Types of Strain

• Normal Strain

    

• Shear Strain

• Bulk Strain

    

True Stress and True Strain

True stress is defined as the ratio of load to the instantaneous area. 

Where σ and ε is the engineering stress and engineering strain respectively.

The true strain is defined as

Stress-Strain Curve

NOTE:

• Limit of Proportionality: It is the stress at which the stress-strain curve is straight line(OA).
• Hook's law is valid upto proportionality limit.
• Elastic limit: point in the stress-strain curve upto which the material remains elastic. There is no permanent deformation upto this point.
• Plastic Range: Region between elastic limit and point of fracture.
• Yeild Point: The point just beyond the elastic limit, at which material goes increase in length without further increase in the load.

Properties of Materials

Elasticity

Elasticity is the property by virtue of which a material is deformed under the load and is enabled to return to it original dimension when the load is removed.

Plasticity

The characteristic of the material by which it undergoes inelastic strains beyond those at elastic limit is known as plasticity.

Ductility

Ducitility is the characteristic which permits a material to be drawn out longitudinally to a reduced section, under the action of a tensile force. A ductile material possess a high degree of plasticity strength.

Brittleness

A material is said to be brittle when it cannot be drawn out by tension to smaller section. e.g. glass, ceramic etc

In brittle material failure takes place under load without significant deformation.

Malleability

It is a property of a material which permits the material to be extended in all directions without rapture. A malleable material possess a high degree of plasticity, but not necessarily great strength.

Toughness

It is the property of a material which enable it to absorb energy without fracture. It is desirable in materials which is subject to cyclic or shock loading.

Bend test used for common comparative test for toughness.

Hardness

Hardness is the ability of a material to resist indentation or surface abrasion. Brinell hardness test is used to check hardness. 

Brinell Hardness Number (BHN)   

where, P = Standard load,

                         D = Diameter of steel ball 

                                 d = Diameter of the indent (mm)

Strength

The property enables material to resist fracture under load.

Load required to cause fracture, divided by area of specimen is known as ultimate strength.

Behavior of Various Material