Hardness testing of materials is one of the most common tests used to determine a material’s strength. The test measures the material’s resistance to a permanent deformation, usually by a non-destructive penetration of the material. Hardness testing is easy to perform and can be used to test nearly any material from hard materials such as metals, plastics and concrete to softer materials like leather and rubbers.
Hardness testing plays an important role in quality control and material selection in many manufacturing industries such as metallurgy, aerospace, automotive, oil and gas, machine shops and many more. Hardness testing can help determine the types of treatments and coatings to be used for specific metals or can indicate the strength, machinability and durability of materials.
Hardness Testing Equipment
Hardness Testers can be larger, stand-alone machines or they can be portable, hand-held devices that can perform hardness testing in less than a minute. They can indicate the hardness values with either a graduated dial or a digital LCD screen. Recent advances in technology have led to testing equipment that is highly accurate and can even offer capabilities to download and transfer data to computers to be used with sophisticated software for advanced structural or failure analysis calculations.
Hardness Measuring Principles
The measuring principles of the most common Hardness Testers are based on the penetration of a ball or diamond-shaped indenter into the material being tested. Other forms of hardness testing such as the calculation of a rebound force or ultrasonic testing are also used in a wide range of materials and applications.
However, indentation hardness testing offers the easiest and most reliable test. A specific indenter, based on the geometry and properties of the material, is placed on the material at a specific force for a defined length of time. A measurement is then made from either the depth of the penetration or the overall dimensions of the resulting indentation. Typically, softer materials will have larger indentations.
Indention Test Methods
There are four main hardness tests that are used through the penetration method. The type of test and the choice of indenter is based on the size and characteristics of the test material.
The Rockwell hardness test is the most frequently used testing method and is usually used on metals and alloys. The test involves applying a small load to the material to establish a zero-reference position. Then a larger load is applied for a specific amount of time, and the hardness number will be determined by the difference in depth between the zero-reference position and the indent due to the higher load.
The Brinell hardness test is normally used for testing larger steel, aluminum and cast-iron material applications. For the Brinell Hardness Test, a carbide ball indenter is pressed into the material with an accurate force for a specific amount of time. A formula is used to calculate its hardness based on the dimensions of the round indentation left from the test.
The Vickers hardness test is used mostly for thin materials and applied coatings. The Vickers test uses a diamond-shaped indenter applied to the material for a specific amount of time with a controlled force. The test load and impression measurements are used to calculate the material’s hardness.
The Knoop hardness test is used on small or delicate parts that cannot be tested by the other methods. The Knoop test uses a very light testing force, typically less than 1000 grams. The resulting impression is usually measured with a microscope or magnifying lens and measurement reticle.
Hardness Tester Calibration
Through repeated use, Hardness Testers can begin to show inaccuracies in their measurement results over time. It is important that they are regularly calibrated to ensure that they maintain their manufacturer’s specifications.
The most common way of calibrating Hardness Testers is with the use of certified hardness test blocks. The blocks are of a known hardness value and are used to check various readings on the Hardness Tester’s scale. This type of calibration is quick and easy. The only requirement is to have a range of harness block values to check various readings over the full range of the tester being calibrated.
can calibrate many types of Hardness Testers. For more information on Hardness Testers or for a listing of our full calibration capabilities, contact e2b calibration.
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