The resistance of a material to deformation or indentation is known as its hardness. This property indicates the material’s ability to resist scratching, cutting, and abrasion. Hardness tests are commonly used to measure the amount of permanent deformation caused by a concentrated load. A material with higher hardness can withstand external loads without losing its shape. Understanding the level of hardness of a material is important for determining its machinability and behavior during application. In this article, we will discuss five major tests used to determine the hardness of a material.
5 Tests to Measure Hardness of Construction Materials
There are several methods available for testing the hardness of a material, and each technique has its own way of defining hardness on its particular scale. The five most commonly used tests to measure material hardness are the Brinell Hardness Test, the Rockwell Hardness Test, the Vickers Hardness Test, the Knoop Hardness Test, and the Mohs Hardness Test.
The Brinell Hardness Test involves applying a load to a spherical indenter and measuring the resulting indentation’s diameter. The Rockwell Hardness Test also uses an indenter, but in this case, the load is applied by a diamond cone or a steel ball. The hardness value is then calculated based on the depth of the indentation.
The Vickers Hardness Test is similar to the Brinell and Rockwell tests but uses a diamond-shaped indenter to create a square-shaped indentation. The hardness is calculated based on the size of the indentation. The Knoop Hardness Test also uses a diamond-shaped indenter but creates a much smaller indentation, making it useful for measuring the hardness of very thin materials.
Finally, the Mohs Hardness Test is used specifically for minerals and involves determining a mineral’s hardness on a scale of 1 to 10 based on its scratch resistance. Overall, each of these hardness tests has its unique advantages and limitations and is used for specific applications, depending on the material being tested and the type of data required.
1. Brinell Hardness Test
The Brinell hardness test is a method used to measure the hardness of a material by creating an indentation through the application of a concentrated force. The force is applied using an indenter, which is typically a steel or carbide sphere. After applying the force for a period of 10 to 15 seconds, the indenter is removed, leaving behind a spherical indentation on the material. The diameter of this indentation is then measured to determine the Brinell hardness (HB) of the material.
The advantage of using a steel ball indenter is that it can be applied to a larger surface area, allowing for the consideration of any irregularities in the metal’s surface and grain. This is in contrast to the other four hardness testing methods which use different indenters and apply forces with different values and durations, resulting in smaller indentations.
However, the Brinell hardness test is slower compared to the other methods and creates a larger permanent impression on the specimen. This can be a disadvantage when working with small or delicate parts where minimizing damage is important. Nonetheless, the Brinell hardness test remains a valuable technique for assessing the hardness of a material, especially in applications where larger surface areas are involved.
2. Rockwell Hardness Test
The Rockwell hardness test is a method used to measure the hardness of a material by determining the permanent depth of indentation caused by the application of a concentrated load. The load is applied using a ball indenter, which can be made of diamond or steel. Prior to the actual measurement, a preliminary pressure is applied to the indenter, and the depth of indentation caused by this preload is recorded as a reference point.
Figure-1. Rockwell Hardness Test
Image Courtesy: Matsuzawa
After applying the preload, a significant load is applied to the indenter for a designated dwell time period, causing further indentation of the material. The permanent depth of indentation is determined by taking the difference between the reference position and the depth of indentation caused by the major load. This test is less destructive and more cost-effective compared to the Brinell test. By measuring hardness at different depths, errors resulting from surface imperfections can be minimized. Additionally, no additional equipment is required to read the hardness value.
3. Vickers Hardness Test
The Vickers hardness test is a method for measuring the hardness of a material based on the surface area of the indentation formed by a four-sided square pyramid indenter. The test begins by adjusting the instrument to lift the specimen until it makes contact with the sample. The indenter then slowly applies a test force onto the specimen until it reaches the desired load value, which is maintained for a specified dwell time. Finally, the area of the indentation, which may be diamond or square-shaped, is calculated to determine the material’s hardness.
Figure-2. Vickers Hardness Testing
Image Courtesy: Manufacturing Guide
The Vickers hardness (HV) is calculated using the formula HV = Applied force in kilogram-force (kgF) divided by the surface area of the indentation. Unlike the Brinell and Rockwell tests, which use different indenters, the Vickers test employs a square pyramid indenter, resulting in lower force but higher accuracy. However, the Vickers test can be expensive as it requires optical and measuring equipment, as well as preparation. Additionally, it can be time-consuming to perform due to the complex process involved in obtaining accurate results.
4. Knoop Hardness Test
The Knoop hardness test is a method that shares similarities with the Vickers hardness test. It involves the use of a diamond or pyramid-shaped indenter to create an indentation on the material being tested. The indenter is applied with a specific dwell time, or the amount of time it is kept in contact with the material. What sets the Knoop test apart is its use of an elongated diamond-shaped indenter, which makes it suitable for testing the hardness of brittle and thin materials without causing any cracks or damage.
Figure-3. Knoop Hardness Testing
Image Courtesy- Manufacture Guide
The Knoop indenter, with a diagonal that is three times longer than the Vickers diagonal, is ideal for testing brittle materials such as ceramics as it allows for penetration to about half the depth of the Vickers test. The indentation created by a Knoop hardness test is rhombohedral in shape, with one diagonal being seven times larger than the other diagonals. The Knoop hardness, denoted as HK, can be calculated using the formula: HK = Applied Test force in kgF divided by the surface area of the indentation in mm2.
5. Mohs Hardness Test
The Mohs hardness test is a method used to determine the hardness of a material by assessing its resistance to scratches. In this test, a material with a known hardness is used to scratch the surface of the specimen being tested. The resulting scratches on the specimen are then observed to determine its hardness.
Figure-4. Mohs Hardness Test
Image Courtesy: Geology.com
In this method, unlike other techniques, an indenter is not used to measure hardness. Instead, materials are ranked based on a hardness scale known as the Mohs scale, which consists of 10 minerals. The Mohs scale is used to determine the relative hardness of minerals, and it is not suitable for measuring the hardness of metals. It’s important to understand the advantages and disadvantages of each hardness testing method in order to determine the most appropriate approach for a given material, as there is no universal measurement scale or unit for defining hardness.
FAQs
How to determine Brinell hardness of a material?
The Brinell hardness test is a method used to measure the hardness of a material by determining the indentation caused by a constant force applied to a specimen. This test uses an indenter, which is a steel or carbide sphere, to apply the force to the material.
To begin the test, the steel ball indenter is brought into contact with the material before a constant force is applied. The force remains on the material for a duration of 10 to 15 seconds, which is known as the dwell time. Once the dwell time is over, the indenter is removed, leaving behind a spherical indentation on the material.
The Brinell hardness of the material is then calculated using a formula that involves dividing the applied force in kilograms force (kgF) by the surface area of the indentation in square millimeters (mm2). This formula provides a numerical value for the material’s hardness, which can be used for various applications.
How is the hardness test performed on metals?
The measure of a material’s hardness is typically determined by examining the indentation caused by an indentor. This indentor applies a specific load to the material for a fixed period known as the dwell time. This method is widely used to gauge the hardness of a given material. By analyzing the depth and size of the indentation, one can determine how hard or soft the material is. It is an essential aspect of materials science, as the hardness of a material can impact its ability to withstand pressure, deformation, and wear and tear. Therefore, accurately measuring a material’s hardness is crucial for its various applications in different industries.
What are the different ways to determine the hardness of materials?
There are five primary tests utilized to determine the hardness of a material, including the Brinell Hardness Test, Rockwell Hardness Test, Vickers Hardness Test, Knoop Hardness Test, and Mohs Hardness Test.
The Vickers Hardness Test is one of the methods used to measure the hardness of a material. This test involves using a diamond indenter to create a square impression on the surface of the material, and the size of the indentation is measured to determine the material’s hardness.
The Brinell Hardness Test is another method used to measure material hardness. This test involves using a hard ball indenter to create an indentation on the material’s surface. The size of the indentation is measured to determine the material’s hardness.
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