Rockwell Hardness Test

    The Rockwell test determines the hardness by measuring the depth of penetration of an indenter under a large load (60Kgf - 200Kgf) compared to the penetration made by a preload (10Kgf). There are different scales, which are denoted by a single letter, that use different loads or indenters. The result, which is a dimensionless number, is noted by HRX where X is the scale letter.

      The determination of the Rockwell hardness of a material involves the application of a minor load followed by a major load, and then noting the depth of penetration, vis a vis, hardness value directly from a dial, in which a harder material gives a higher number. The chief advantage of Rockwell hardness is its ability to display hardness values directly, thus obviating tedious calculations involved in other hardness measurement techniques. 

This method is widely used in Industry as the reading is available easily & quickly. 

The above info is taken from Wikipedia. Please do refer to them for more info. 

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Knoop Hardness Test

      The Knoop hardness test  is a microhardness test - a test for mechanical hardness used particularly for very brittle materials or thin sheets, where only a small indentation may be made for testing purposes

    A pyramidal diamond point is pressed into the polished surface of the test material with a known force, for a specified dwell time, and the resulting indentation is measured using a microscope. The geometry of this indenter is an extended pyramid with the length to width ratio being 7:1 and respective face angles are 172 degrees for the long edge and 130 degrees for the short edge. The depth of the indentation can be approximated as 1/30 of the long dimension. 

The Knoop hardness HK or KHN is then given by the formula:

where:

L = length of indentation along its long axis

C_p = correction factor related to the shape of the indenter, ideally 0.070279

P = load

        The advantages of the test are that only a very small sample of material is required, and that it is valid for a wide range of test forces. The main disadvantages are the difficulty of using a microscope to measure the indentation (with an accuracy of 0.5 micrometre), and the time needed to prepare the sample and apply the indenter.

       The above information is taken from Wikipedia. Please do visit eh same for more information.

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Brinell Hardness Test

       The Brinell hardness test method consists of indenting the test material with a 10 mm diameter hardened steel or carbide ball subjected to a load of 3000 kg. 

     The objective of harness test is define the hardness number which represents an arbitrary quantity used to provide a relative idea of material properties. The hardness number derived in this test is called Brinell harness number and is designated as BHN

      For softer materials the load can be reduced to 1500 kg or 500 kg to avoid excessive indentation. The full load is normally applied for 10 to 15 seconds in the case of iron and steel and for at least 30 seconds in the case of other metals. The diameter of the indentation left in the test material is measured with a low powered microscope. 

      The Brinell harness number is calculated by dividing the load applied by the surface area of the indentation. The formula is shown in the picture shown below.

Where F = Force applied in kgF
          D = diameter of indenter
       

     This method is not used in industry since it is quite slow, deforms the specimen excessively and requires setup to calculate the depth of the indentaion..

    The above information has been taken from www.gordonengland.co.uk. Please do refer to them for more info.

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Hardness tests of materials

Hardness of a material refers to the resitance the material offers to permanent plastic deformation when an external force is applied. Wikipedia defines it as the measure of how resistant solid matter is to various kinds of permanent shape change when a force is applied. 

In view of syllabus of material science, Impact hardness tests are important and necessary. Impact hardness refers to resitance offered by material when the force applied is impact in nature i.e. for short period with high magnitude. 

The four important tests covered in the syllabus are

• Rockwell Hardness test 
• Knoop Hardness test
•  Vickets hardness test
• Brinell's Hardness test

The hardness tests are performed since 

• They are easy, simple 
• The set up is in-expensive
• The test doesn't damage the entire specimen. Usually small specimen is sufficient
• Other physical properties can be told from this tests

with warm regards

Abhishek Boinapalli