Hardness Testing Methods Hardness testing of materials can be summed up as a repeatable method which produces a comparable value of a material's ability to resist plastic deformation.
When a uniform force is applied to two materials, one hard like tool steel and one other soft like aluminium or copper then it’s logical to assume that the softer materials will experience a greater amount of plastic deformation than the harder one. This is the practical basis for almost all hardness testing.
There are several types of hardness testing commonly in use and many of these can operate over several different scales for the different ranges of hardness found in various materials. The basic principle is to apply a known force to a special shaped tool called an indentor which results in a small area of plastic deformation to the test sample, the measurement of either the width or depth of the deformation can then be used to determine the hardness of the material on a given scale.
Another thing to consider when choosing a hardness testing method is the size of deformation created, which is commonly known as a spot. Generally the greater the area of the spot the more accurate your test result will be due to an averaging effect over the area, however the drawback to this is that it could result in unacceptable damage to the part due to a) the large size of the spot and 2) the high forces that need to be used in order to create the large spot. Often, such tests (whilst offering better accuracy) are not used due to unacceptable levels of damage that are caused to the part and the trade off between smaller sized spots and less mechanical damage but somewhat less accuracy and repeatability of results is deemed acceptable.
It has been found that the hardness values of metals correlates to the tensile strength of the sample, and so if the hardness of a metal can be determined it may be possible to make an estimated guess of the materials tensile strength.
The three main types of test that are commonly used for metallic hardness testing are.
The Rockwell hardness testing method was invented in the USA by Hugh Rockwell and Stanley Rockwell around the time of World War One. Hugh and Stanley were not thought to be related and it was purely coincidental that they both shared the same surname. Over the course of the next decade Stanley Rockwell made several improvements to the method and later teamed up with Charles Wilson to commercialise and standardise the Rockwell method.
The Rockwell hardness testing method is unique in that it uses two loads, an initial minor load followed by a major load. The minor load is first applied to the test piece, and the depth display zeroed, then the major load is applied for a short period of time, then removed. After the major load is removed the depth of the indentation will be greater than before, the Rockwell hardness is calculated from the difference between the depth of indentation from before and after the application of the major load.
The indentation is created via a special conical diamond indenter often called a brale or a 1/8” or 1/16” tungsten carbide ball bearing mounted in a housing. The type of indentation used and the values of the major and minor forced give rise to the different Rockwell Hardness scales. For example the commonly used HRC scale uses the brale diamond indentor, a minor load of 10kgf and a major load of 150 kgf.The advantages of the Rockwell Hardness method are as follows:
The Brinell hardness test was invented in 1900 by Johan August Brinell and was the first standardised hardness test to be used in the fields of engineering and metallurgy.
The method works by clamping the test piece between an anvil base and a ball bearing of a known diameter (typically 10mm) a known force is then applied (typically equivalent to 3000kg) and the ball bearing is pushed into the test piece. After at least 10 seconds the force is removed and the test piece taken out of the machine.
A special microscope is then used to read the diameter of the resulting indentation. This microscope will typically be able to read indentations up to 6mm with an accuracy of 0.05mm. Once the diameter of the indentation has been measured a simple converstion chart is used to give the correct Brinell Hardness valve for the test piece.The advantages of the Brinell Hardness method are as follows:
The Vickers hardness test was developed in 1921 by Robert L. Smith and George E. Sandland who both worked for the engineering conglomerate Vickers. The test was designed to be a viable alternative to the Brinell method.
The method works by applying a known force to a pyramid shaped diamond indentor for a period of around 10 to 15 seconds. Once the load has been removed the distance from corner to corner (across corners) of the square shaped indentation is measured using a high magnification microscope to an accuracy of 0.01mm. With a typical indentation having a size of around 0.5mm
The Vickers hardness test is one of the most versatile methods for testing hardness and the same indentor is used regardless of scale. With the applied force being the only variable. Applied forces are usually in the range of 5kg to 100kg, with 10kg and 30kg being most common.
A special application of the Vickers test is that of a micro-Vickers hardness test. Applied forces can range from 10 grams to 1kg and these low force tests are often used on ceramics, foils and other situations where a very small indentation is desirable.The advantages of the Vickers Hardness method are as follows: