Hardness | Definition | Measuring Tests [Brief Explanation]

By | January 1, 2019

What is hardness?

Hardness is the property of a material to resist surface indentation (and scratches).

Some materials are harder than other materials. For example, metals are harder than plastics, wood etc.

Difference between hardness and toughness

Hardness is a property which is related to the surface of a material while toughness is a property which is related to whole material.

Toughness is the ability of a material to absorb energy until ultimate failure happens. It means more tough material will be fractured after less tough material.

Hardness test

There are several tests to measure hardness of a material, some of them are.

  1. Brinell Hardness Test
  2. Rockwell Hardness Test
  3. Knoop Hardness Test
  4. Vickers Hardness Test

Brinell Hardness Test

Brinell Hardness Test is most commonly used to test materials that have a structure that is too coarse or that have a surface that is too rough to be tested using another test method (for example parts made by castings and forging). Brinell testing often use a very high-test load (3000 kgf) and a 10mm diameter indenter so that the resulting indentation averages out most surface and sub-surface inconsistencies.

Rockwell Hardness Test

Rockwell hardness test

Rockwell hardness test

Image attributions: By Three-quarter-ten – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=11461977

Rockwell method measures the permanent depth of indentation produced by a force/load on an indenter. First, a preliminary test force (commonly referred to as preload or minor load) is applied to a sample using a diamond or ball indenter. This preload breaks through the surface to reduce the effects of surface finish. After holding the preliminary test force for a specified dwell time, the baseline depth of indentation is measured.

Knoop Hardness Test

In Knoop hardness test a pyramidal diamond point is pressed into the polished surface of the test material with a known (often 100g) load, for a specified dwell time, and the resulting indentation is measured using a microscope.

Vickers Hardness Test

Vickers Hardness Test

Vickers Hardness Test

Image attributions: By Freundchen – Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=19350656

In Vickers Hardness Test a diamond pyramid is pressed against the solid with a certain normal load and the hardness is calculated based on the imprint left on the surface.

Also read:

What is residual stress?

What is cold working process?

What are different types of heat treatments?

References:

Brinell Hardness Test: https://www.hardnesstesters.com/test-types/brinell-hardness-testing

Rockwell Hardness Test: https://www.hardnesstesters.com/test-types/rockwell-hardness-testing

Knoop Hardness Test: https://en.wikipedia.org/wiki/Knoop_hardness_test

Vickers Hardness Test: http://www.tribonet.org/hardness-calculation-vickers-indenter/

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