What is Hardness Testing?

Mechanical Engineering is an extensively diverse field that includes detailed processes for almost every aspect of the job. One such process is hardness testing, used primarily to study materials and how they will deform when subjected to high pressure loading over a small surface area. To understand the benefits and drawbacks of hardness testing, it is important to understand what hardness means to a mechanical of materials engineer.

Unlike different properties of metals, such as elastic modulus, hardness is not an intrinsic material property; methods such as mechanical or chemical strengthening can have a large effect on the hardness of samples of the exact same metal. Even the location of the testing site affects the hardness, meaning that the same sample can have different hardness values if measured at two different locations. As such, material hardness is measured in scales relative to other materials, with no associated units. This differs from material strength, which can be quantitatively measured with units of pressure (Force/Area). While strength and hardness of a material are related, it is important to not confuse the two.

To measure hardness, engineers have developed many methods and scales with the two primary methods being Brinell Hardness and Rockwell Hardness testing. While both methods give a value of hardness with an associated scale, they are completely different tests and the values are not interchangeable. Brinell Hardness indents the metal with a sphere made of tungsten-carbide that is 10 mm in diameter. Once a set load is applied, the hardness number is generated by comparing the applied load to the diameter of the indentation created on the surface.

Rockwell Hardness is a different process, which uses a cone-shaped indenter made synthetic diamond, and different scales depending on the metal being tested. For a Rockwell test, a minor load is first applied to the metal’s surface, followed by a larger major load; Rockwell hardness numbers are generated from a defined scale based in the change in indentation depth between the two loads. The scale used depends on the minor major loads, which are lower for weaker metals such as aluminum and higher for metals such as steel. There are also scales, called Superficial testing scales, which are used for thin samples and are analogous to the scales used for normal samples. These scales use a reduced loading such that the test will not damage the opposite side of the sample. The Brinell scale, as well as the four main Rockwell scales, can only be qualitatively compared to samples tested on the same scale.

By now, the different names, scales, and methods of testing are probably getting confusing; there is a lot of jargon associated with hardness testing, and this is barely scratching the surface (pun intended). Hardness testing is a useful way to compare the behavior of different metals, but it seems to me that the process is more complex than it needs to be. Being a first year student, I have only started to understand the different processes used by mechanical engineers. However, I believe that hardness testing could be improved.

To make hardness testing more universal, engineers should define a single testing method that is standardized for all testing equipment and laboratories. As of now, material hardness can be listed in any one of the different scales. While there is most likely merit to this system, I think that a single method would make comparing materials easier.

When studying materials, a measure of hardness gives more in depth information on how the material behaves when put to use. While currents testing methods and equipment are well understood and give quality information to the engineers who use them, there is always room to grow and improve.

 

-Matthew Santullo

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