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Wednesday, 12 August 2015

HARDNESS TESTING METHODS

Hardness – It is a mechanical property of material which shows the Resistance of material against indentation, penetration, scratching & wear.

Hardness Test

Let us see various methods used in industries to determine of hardness of materials as mentioned below 

1. Brinell hardness test 
2. Vickers hardness test  
3. Rockwell hardness test
4. Rockwell superficial hardness test   
5. Micro hardness test  
6. Mohrs hardness test

Let us understand Brinell hardness test

Brinell hardness test method

We use a carbide ball as indenter in this test. Indenter is placed over the surface of work piece & force is applied on indenter for 10 to 15 sec, this time duration is called as dwell time. 

Now indenter is removed from the work piece, indenter leaves a round indent over the surface of work piece.

We measure the size of round indent with the help of portable microscope and by using  following formula to calculate brinell hardness number ,we may aware that how much material is hard. More brinell hardness number shows that material is harder.

BHN = 2F/ [Π D {D-√ (D2-d2)}]       Where F is kg load, diameter in mm.

Tensile strength

Tensile strength is a mechanical property of material which is defined as the maximum amount of tensile stress that material can bear before fracture or rupture.

For example a bar with CA 20mm2 & a force is applied 2000 KN ,material does not fail but as the force is increased beyond 2000 KN ,material rupture take place that means tensile strength of material will be 100 KN/mm2.

There are three types of strength in stress –strain diagram for mild steel  

Yield Point/ strength
Ultimate Point/ strength
Braking Point/ strength
Elastic Region
Uniform Plastic Region
Necking

Let us see 

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