Hardness tests

• Definition of hardness:Hardness is a measure of a material’s resistance to

localized plastic deformation(e.g., a small dent or a scratch).

• The hardness test is performed on a material to know its resistance against indentation and abrasion.

Basic principle:

• In which a small indenter is forced in to the surface of a material to be tested, under controlled condition of load and rate of application.

• Then the depth or size of the resulting indentation is measured.

• A large and deeper impression indicate a soft material, and a small impression indicates a hard material

Types of hardness test

• Brinell hardness test• Vickers hardness test• Rockwell hardness test

Brinell hardness test• In this test, a standard hardened steel ball is

pressed into the surface of the specimen by a gradually applied load .

• The obtained impression is measured by a microscope and the brinell hardness number(B.H.N) is found out by the BHN equation.

• If the BHN value is higher, then the material is said to be harder. BHN value is less, it indicate the metal is soft.

Brinell hardness testing machine

Measurement of diameter of indentation by a microscope

Testing procedure• The test is carried with a hardened steel or

carbide ball of 10 mm diameter.• The standard loads range between 500 and

3000 kg in 500 kg increments.• The load is maintained constant for a specific

time (between 10 and 30 sec).• Harder materials require greater applied load.• The Brinell hardness number (BHN) is function

of both the magnitude of the load and the diameter of the resulting indentation.

• This diameter is measured with a special low power microscope, utilizing a scale that is etched on the eyepiece.

• The measured diameter is then converted to the appropriate HB number using the following equation.

BHN =

=

Where,P = Load applied on diameter in kgD= Diameter of steel ball indenter in mmd= Diameter of ball impression

Load on the ball

Area of indentation of steel ball

P

π D [ D-√(D2-d2) ]

2

Hardness testing technique

Table 1

Vickers hardness test• In vickers hardness test, a polished square

based pyramid diamond tool with an angle of 136° between the faces is used.

• As in the brinell test, the indenter is forced into the surface of the materials under the action of a static load for 10 to 15 seconds.

• An advantages of vickers over Brinell is greater precision in measurement of the diagonal of the square as compared to the dia. of the circular Brinell impression.

• Refer table 1 (Hardness testing technique)

• The diamond pyramid hardness(DPH) number or vickers hardness number(VHN or VPH) is defined as the load divided by surface area of indentation.

VHN = =Where,

P = Applied load in Kg

θ = Angle between the opposite faces

d = Mean length of the two diagonals in mm

Applied load

Surface area of impression

2P sin(θ/2)

d2

Rockwell hardness test

• The Rockwell hardness test is probably the most widely used method of hardness testing.

• Rockwell tester use much smaller penetrator and loads than does the brinell tester.

• Four sizes of hard ball from 1/16 in. to ½ in. in diameter are available as well as a cone shape diamond.

• For testing metallic materials the 1/16 in. ball and the diamond penetrator are most commonly used

• The penetrator chuck is mechanically connected to a dial indicator which responds to vertical motion of the penetrator

• Refer Table 1 (Hardness testing technique)

Rockwell hardness testing machine

Testing procedure

• The specimen is placed on the anvil of the machine and the penetrator seated by means of 10kg minor load.

• The dial indicator is zeroed and then a major load of 60, 100 or 500 Kg is applied, forcing the penetrator into the specimen.

• After the indicating pointer has come to rest, the major load is removed

• With the major load removed, the pointer now indicate the Rockwell hardness number on the appropriate scale of the dial.

• Each scale is represented by a letter of the alphabet.

• The two commonest scales are the HRB scales and HRC scales, respectively standing for the 1/16 in. ball with 100Kg, and the diamond penetrator with the 150 Kg major load

Rockwell Hardness Scales

Impact test

• An impact test signifies toughness of material, that is ability of material to absorb energy during plastic deformation.

• The impact test is performed to study the behavior of materials under dynamic loadi.e., suddenly applied load.

• The capacity of a metal to withstand blows without fracture is known as impact strength.

• The impact test also indicates the notch sensitivity of a material.

• The notch sensitivity refers to the tendency of some normal ductile materials to behave like a brittle materials in the presence of notches.

• In an impact test, a notch is cut in a standard test piece which is struck by a single blow in a impact testing machine.

• Then the energy absorbed in breaking the specimen can be measured from the scale provided on the impact testing machine.

Standard specimen for Impact testing

Schematic arrangement of Impact testing machine

• Based on the types of specimen used on impact testing machine, the impact tests can be classified in to:1. Izod test2. Charpy test

Izod test

• The Izod test uses a cantilever test piece of 10mm X10mm section specimen having standard 45° notch 2mm deep.

• This is broken by means of a swinging pendulum which is allowed to fall a certain height to cause an impact load on the specimen.

Izod Impact test

• The angle rise of the pendulum after rupture of the specimen or energy to rupture the specimen is indicated on the graduated scale by a pointer.

• The energy required to rupture the specimen is the function of the angle of rise.

Charpy test

• This test is more common than Izod test and it uses simply supported test piece of 10mm X10mm section.

• The specimen is placed on supports or anvil so that the blow of striker is opposite to the notch

Charpy Impact test

The energy used in rupturing the specimen in both Izod and Charpy test is calculated as follows:

Impact test- Calculation of Energy relation

• From the previous figure, W = Weight of the pendulum / striker,H = Height of fall of centre of gravity of pendulum

/ striker,H’ = Height of rise of centre of gravity of

pendulum / striker,α = Angle through which the pendulum falls,β = Angle through which the pendulum rises, R = Distance from centre of gravity of pendulum to

axis of rotation O.

• Initial energy = W H= W (R-R cos α)= WR (1- cos α)

• Energy after rupture = W H’ = W (R-R cos β) = WR (1- cos β)

• Energy used to rupture specimen= WH – WH’= WR[(1- cos α)- (1- cos β)]= WR(cos β - cos α)