What is the definition of hardness testing?
Hardness testing applications allow you to evaluate the properties of the material, such as strength, ductility, or wear resistance, and thus let you decide whether a material or material treatment is appropriate for the purpose you need.
This definition of hardness testing is ‘a test to determine the resistance a material exhibits to permanent deformation by penetration of another harder material.’ However, hardness is not a fundamental property of a material. Therefore, while drawing conclusions from the hardness test, you must always evaluate the quantitative value in relation to
The applied load on the indenter
A particular loading time profile and a particular duration of load
Individual indenter geometry
How do hardness tests work?
A hardness test is normally carried out by forcing a special sized and loaded object known as the indenter into the surface of what you are testing. The hardness can either be evaluated by measuring the depth of penetration of the indenter or measured by the size of impression left by the indenter.
Depth of penetration is a measurement used in hardness tests like Rockwell, Instrumented Indentation Testing, and Ball Indentation Hardness.
These tests measure the size of the impression left by the indenter. Hardness tests include: Vickers, Knoop, and Brinell.
SELECTION OF THE OPTIMAL HARDNESS TEST METHOD
How to choose the test method The hardness test you are going to choose depends on the microstructure – e.g. the homogeneity – of the material you like to test as well as on the kind of material, the dimension of the part and its condition.
In all hardness tests, the material under the indent should be representative of the whole microstructure; except when you’re trying to find out how many different constituents there are in the microstructure. Thus, if a microstructure is very coarse and heterogeneous, you require a bigger impression than for a homogeneous material.
There are four major hardness tests, which vary in benefits and requirements. Different standards of these tests detail the procedures and application of the hardness test in detail.
Important factors to consider when a hardness test method should be chosen include:
The type of material to be hardness tested
Whether compliance with a standard is required
Approximate hardness of material
The material’s homogeneity/heterogeneity
Size of the part
Whether mounting is required or not
Numbers of sample to be tested
Amount of accuracy needed by the result.
The four most common indentation hardness tests
A hardness test is normally carried out by forcing a special sized and loaded object known as the indenter into the surface of what you are testing. The hardness can either be evaluated by measuring the depth of penetration of the indenter or measured by the size of impression left by the indenter.
Depth of penetration is a measurement used in hardness tests like Rockwell, Instrumented Indentation Testing, and Ball Indentation Hardness.
These tests measure the size of the impression left by the indenter. Hardness tests include: Vickers, Knoop, and Brinell.
Rockwell Hardness Testing
How to choose the test method The hardness test you are going to choose depends on the microstructure – e.g. the homogeneity – of the material you like to test as well as on the kind of material, the dimension of the part and its condition.
In all hardness tests, the material under the indent should be representative of the whole microstructure; except when you’re trying to find out how many different constituents there are in the microstructure. Thus, if a microstructure is very coarse and heterogeneous, you require a bigger impression than for a homogeneous material.
There are four major hardness tests, which vary in benefits and requirements. Different standards of these tests detail the procedures and application of the hardness test in detail.
Important factors to consider when a hardness test method should be chosen include:
The type of material to be hardness tested
Whether compliance with a standard is required
Approximate hardness of material
The material’s homogeneity/heterogeneity
Size of the part
Whether mounting is required or not
Numbers of sample to be tested
Amount of accuracy needed by the result.
Vickers hardness test
Vickers hardness is a test on any solid material, including metallic materials. Calculation of the Vickers Hardness (HV) is based on the measurement of the diagonal lengths of an indent in the sample material caused by the introduction of a diamond pyramid indenter under a certain load. The diagonals of the indent are measured by the naked eye to find the hardness using a table or formula.
It is used for testing the hardness of all solid materials, metallic material being an example.
Designed for a wide range of applications
Includes a subgroup of hardness testing of welds
The Knoop hardness test
The Knoop hardness test
HK is a substitute for the Vickers test in the micro hardness testing range, and it is mainly used to avoid cracking in brittle materials and to enable the hardness testing of thin layers. The indenter is an asymmetrical pyramidal diamond, and the indent is measured by optically measuring the long diagonal.
For hard and brittle materials like ceramics:
Useful for small elongated areas, such as coatings
The Brinell hardness test
The Brinell hardness test is used for hardness testing larger samples in materials with a coarse or inhomogeneous grain structure. The Brinell hardness test (HBW) indentation leaves a relatively large impression, using a tungsten carbide ball. The size of the indent is read optically.
Used for materials with a coarse or inhomogeneous grain structure
Used for larger samples
Suitable for forgings and castings where the structural elements are large
