Izod impact test | Rockwell hardness test


What are the Izod impact test and Rockwell hardness test?

Answer: Izod impact test | Rockwell hardness test

In the manufacturing of locomotive wheels, coins, and connecting rods, etc., the components are subjected to impact loads or shock loads. These loads are applied suddenly. The stress-induced in components is many times more than the stress produced by gradual loading. Hence materials should be able to sustain such loads. Therefore, impact tests are performed to assess shock absorbing capability of materials subjected to suddenly applied shock loads.

We express these capabilities as:

(i) Rupture energy
(ii) Modulus of rupture

(iii) Notch impact strength

Two tests are performed to measure the impact strength of a specimen.

1. Charpy test

2. Izod test

1. Charpy Test: The Charpy test is carried out on a specimen that is 55 mm  10 mm 10 mm in size and has a 2 mm deep notch at its centre, making an angle of 450 as shown in figure (a). A heavy hammer strike on the specimen from a standard height. The specimen gets fractured due to the applied load. The ductile material becomes brittle due to the presence of a notched specimen. From this test, one can directly measure from the calibrated dial, the energy absorbed for fracturing the specimen in the Newton meter.

2. Izod tests : A specimen of dimension 11 mm X 11 mm X 75 mm is fixed using vice and the other
end is free shown in figure (b). A V‐notch of 2 mm depth and angle 450 is produced at a distance
of 22 mm from the free end. By striking the edge of the free end using a striking hammer, the energy for fracture is determined.

The advantages of impact test are:

1. It can determine the impact strength of the material.

2. After examining the fractured surface, it can identify whether the material is brittle or ductile or a combination of both.

3. The ductile‐brittle transition temperature can be determined by measuring the impact test over a range of temperatures.

Rockwell’s hardness test:

The Rockwell’s hardness test is performed when quick and direct reading is desirable. This test is also performed when the material has a hardness, beyond the range of Brinell’s hardness test. It differs from Brinell’s test in such a way that in this test the loads for making indent are smaller, and thus it makes smaller and shallower indents. It is because of this reason that Rockwell’s hardness test is widely used in the industry.

Process of Rockwell’s hardness test :

● The specimen is placed on the machine.

● The handwheel is providing for rising the specimen up against the steel ball.

● When the minor load is applied on indenter till the needle on the dial shows zero reading.

● The machine is connected to a dial indicator, which shows the reading.

● With the help of a crank mechanism (which is not shown in the figure) major load is applied.

● When a crank mechanism is acting in the reverse direction, the major load is removed and a minor load is applied.

● Finally, the handwheel is rotated, and the specimen is lowered.

● After the above process, the hardness of specimen material can be read from the dial indicator.


Scaling of Rockwell hardness test material :

● It consists of different scales such as the B scale and the C scale.
● The B scale is used to measure the hardness of soft material using a steel ball from RHO to RH 100, where RH stands for Rockwell hardness.

● The C scale is used to measure the hardness greater than RH 100 using a diamond cone indenter

for hard materials.

Mathematically, the hardness of the material is,

Rockwell hardness, RH=h-t/0.002

Where h = Constant of material in mm; t = Depth of material in mm

The merits of the Rockwell hardness test are:

1. This instrument is easy to handle and the readings are noted directly from the reading dial.

2. A small size impression is made before using a larger load and hence, the uneven surface in the specimen is avoided.

3. It is very useful when a large number of samples is used.

4. The process of testing is very fast.

The demerits of the Rockwell hardness test are :

1. Error occurs when over one scale is used to find the hardness of the material in the measurement.

2. The conversion of the Rockwell hardness number into Brinell hardness number and Vickerʹs hardness number is not possible.

3. The accuracy of the Rockwell hardness test is less than the Vickerʹs hardness test, especially when measuring the hardness of materials having slight differences.

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