Hardness represents the resistance of a material to indentation, penetration and scratching. In hardness testing, a loaded ball or diamond is pressed against the surface of a material which causes the plastic deformation of the same. This deformation is measured by one of the following methods:
(i) Brinell Hardness test-
In this method, a steel hardened ball is pressed into the surface of the material under a specified load. The load is held in position for a fixed period and then released. This leaves a permanent impression in the surface of the material. Then either measure the diameter or the depth of the impression.
The Brinell specimen Hardness Number (BHN) is defined as the ratio of the applied load to the spherical area of the impression.
Where, P is in Newton.
Conversion tables are also available to determine the hardness number.
(ii) Vicker Pyramid Diamond Method
This method is also similar to the Brinell method except that the indenter is a 136° pyramid diamond on a square base. As hardness of diamond is excessively high. It can be used for the whole range of materials.
The Vicker Pyramid Number (VPN) is defined as the ratio of applied load to the impressed area. The area is calculated by measuring the length of the diagonal of the square impression on the surface of the material.
(iii) Rockwell Hardness Method –
The scale ranges between 0‐100. It uses either a diamond 120° cone indenter or ball indenter made of hardened steel.
Depending on the combination of indenter and load there are several Rockwell hardness scales. Three most commonly used Rockwell hardness scales are given in table.
The applied load depends on the hardness of material. As a thumb rule the load used for measuring the hardness of steel = 30D2 kg; where D is the diameter of the ball. If D = 10mm the load to be used = 3000kg.
Static tests are useful only when the loads are static in nature. These tests do not indicate the resistance of a material against shock or impact loads to which usually the automobile parts are subjected to. In such cases, an impact test has to be undertaken. An impact test indicates the toughness of a material which is defined as the energy absorbed by the specimen without fracture.
The following are the main types of impact tests undertaken:
(i) Izod Impact Test
Figure shows an Izod impact testing machine. It consists of an anvil in which a notched specimen can be fixed. The specimen is taken of some standard dimensions. While fixing, care is to be taken to have the notch on the side of the falling hammer and the level with the level of top face of the hammer.
(ii) Charpy Impact Test
This test is similar to the Izod impact test except that instead of fixing the notched specimen in the anvil, it is supported at each end as a beam as shown in Figure. The hammer strikes at notch in the centre. Impact tests are important as they can reveal the temper brittleness in heat treated materials such as nickel chrome steels.
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