Types of Hardness Testing Methods and Impact Testing

Hello friends, Today we are going to talk about types of hardness testing methods and impact testing.

Hardness tests are employed to ascertain the hardness of materials, indicating their resistance to indentation or abrasion. On the other hand, impact testing is utilized to determine the toughness of materials, providing insights into their ability to absorb energy under sudden forces.

To comprehend the various hardness testing methods and impact testing, it is essential to first understand the concepts of hardness and toughness.

So, without wasting time let’s know the types of hardness testing methods.

What is Hardness?

Hardness is a measure of a material’s resistance to deformation, indentation, or scratching. It is an essential property that indicates the material’s ability to withstand localized surface abrasion or penetration. Hardness is often associated with the strength and durability of a material and is determined through various testing methods that assess how the material responds to applied force or pressure. The measurement of hardness is valuable in selecting materials for specific applications, such as in engineering, metallurgy, and material science.

Types of Hardness Testing Methods

There are the following types of hardness testing methods are used to determine the hardness of materials.

• Rockwell Hardness Test
• Brinell Hardness Test
• Vickers Hardness Test
• Shore Scleroscope Hardness Test
• Knoop Hardness Test
•  Mohs Hardness Test
• Barcol Hardness Test

Rockwell Hardness Test

The Rockwell Hardness Test is a widely used method to determine the hardness of a material. It measures the depth of penetration of an indenter into the material under a specific load, and the hardness value is indicated directly on a scale. Here are the key features of the Rockwell Hardness Test:

Indenter

The indenter typically used is a diamond cone (for hard materials) or a hardened steel ball (for softer materials). The indenter is pressed into the material’s surface under a predefined load.

Load Application

Two loads are commonly used in Rockwell hardness testing: the minor load (preliminary load) and the major load (additional load). The minor load is first applied to eliminate surface irregularities, followed by the major load.

Depth Measurement

The depth of penetration of the indenter into the material is measured, and it is used to calculate the Rockwell hardness number (HR).

Scale Selection

There are different Rockwell scales (e.g., Rockwell B and Rockwell C), each designed for specific materials or hardness ranges. The selection depends on the type of material being tested.

Reading the Hardness Value

The hardness value is read directly from the Rockwell hardness scale. A higher hardness number indicates a harder material.

Advantages

Quick and easy to perform.

 Minimal sample preparation.

Can be used on a variety of materials.

Limitations

Surface finish and homogeneity can affect results.

Not suitable for very thin or small samples.

Care must be taken with an appropriate scale for different materials.

The Rockwell Hardness Test is widely used in quality control and material testing across various industries due to its simplicity and versatility. It provides a quantitative measure of hardness, making it valuable for comparing and assessing the hardness of different materials.

Read also: Try Square: Parts, Types, Grades, Checking Accuracy, Uses

Show in the figure some different Rockwell hardness scales for different materials.

 

Brinell Hardness Test

The Brinell Hardness Test is a method for determining the hardness of a material by measuring the diameter of an impression made by a hardened steel ball. Here are the key features of the Brinell Hardness Test:

Indenter

The indenter used in the Brinell test is a spherical ball made of hardened steel or tungsten carbide. The typical ball diameters used range from 1 to 10 mm, depending on the material being tested.

Load Application

A specified load is applied to the indenter, creating an indentation on the material’s surface. The load is typically in the range of 500 to 3000 kilograms-force (kgf).

Indentation Measurement

The diameter of the indentation is measured using a microscope or an optical system. The Brinell hardness number (BHN) is calculated based on the applied load and the diameter of the impression.

Calculation of Hardness

Brinell Hardness Number (BHN) is calculated using the formula:

\[ BHN = \frac{{2 \times \text{{Applied Load}}}}{{\pi \times \text{{Indentation Diameter}}}} \]

Advantages

Suitable for a wide range of materials, including both metallic and non-metallic materials.

Provides an average hardness over a larger surface area.

Less sensitive to the effects of surface roughness.

Limitations

Not suitable for very hard or very soft materials.

Limited applicability for thin or small samples due to the size of the indentation.

Surface finish can affect results.

The Brinell Hardness Test is commonly used in engineering and metallurgy to assess the hardness of materials, especially those with coarse or inhomogeneous structures. It is widely employed for quality control and material selection in various industries.

Vickers Hardness Test

The Vickers Hardness Test is a method for assessing the hardness of a material by measuring the size of an indentation created by a diamond-shaped indenter. Here are the key features of the Vickers Hardness Test:

Indenter

The indenter used in the Vickers test is a diamond pyramid with a square base and an angle of 136 degrees between opposite faces.

Load Application

A specified load is applied to the indenter, causing it to penetrate the material and create a square-shaped indentation. The typical loads range from a few grams to several kilograms-force.

Indentation Measurement

The two diagonals of the square-shaped indentation are measured using a microscope or an optical system. The Vickers hardness number (VHN) is calculated based on the applied load and the average of the two diagonals.

Calculation of Hardness

Vickers Hardness Number (VHN) is calculated using the formula:
\[ VHN = \frac{{1.854 \times \text{{Applied Load}}}}{{\text{{Average Diagonal Length}}^2}} \]

Advantages

Applicable to a wide range of materials, including metals, ceramics, and some plastics.

Provides precise and accurate hardness measurements.

Well-suited for thin samples and small indentations.

Limitations

Indentation size can be relatively small, making it challenging to use on very hard or coarse-grained materials.

Requires a flat surface for accurate measurements.

Sensitive to surface roughness.

The Vickers Hardness Test is widely used in quality control and research settings, offering a reliable and versatile method for assessing the hardness of materials. Its ability to provide accurate results on a variety of materials makes it valuable in metallurgy, engineering, and materials science.

Shore Scleroscope Hardness Test

The Shore Scleroscope Hardness Test is a method used to determine the hardness of materials, particularly metals, by measuring the rebound height of a hammer from the surface of the material. Here are the key features of the Shore Scleroscope Hardness Test:

Indenter

The indenter is a small, spherical steel hammer with a hardened tip. It is dropped from a known height onto the material’s surface.

Load Application

The hammer is released, and it impacts the material’s surface, causing plastic deformation.

Rebound Measurement

The rebound height of the hammer is measured, typically with the help of a graduated scale.

Calculation of Hardness

The hardness value is determined based on the ratio of the rebound height to the drop height. The result is expressed as the Shore Scleroscope hardness number.

Advantages

Portable and relatively simple to use.
Applicable to large or immovable objects.
Quick and non-destructive.

Limitations

Limited accuracy compared to other hardness tests.
Sensitive to variations in the material’s surface condition.
Not suitable for very hard or brittle materials.

The Shore Scleroscope Hardness Test is often used in situations where other hardness testing methods may be impractical. While it may not provide the precision of some other hardness tests, its portability and non-destructive nature make it useful in certain field applications, such as in shipbuilding and construction.

Knoop Hardness Test

The Knoop Hardness Test is a microhardness testing method used to assess the hardness of brittle materials or thin specimens. Here are the key features of the Knoop Hardness Test:

Indenter

The indenter used in the Knoop test is a rhombic-based pyramidal diamond with a long, asymmetrical indentation. It is particularly suitable for testing brittle materials.

Load Application

A specified load is applied to the indenter, creating an indentation on the material’s surface. The load is typically in the range of a few grams to several kilograms-force.

Indentation Measurement

The indentation made by the Knoop indenter has a distinct elongated shape. The length of the long diagonal is measured using a microscope or an optical system.

Calculation of Hardness

Knoop Hardness Number (KHN) is calculated using the formula:
\[ KHN = \frac{{\text{{Applied Load}}}}{{\text{{Long Diagonal Length}}^2}} \]

Advantages

Well-suited for microhardness testing of small or delicate specimens.
Can be used on brittle materials without causing excessive damage.
Provides detailed information on localized hardness variations.

Limitations

Limited applicability to coarse-grained or ductile materials.
Time-consuming for large-scale testing.
Surface preparation and cleanliness are crucial for accurate results.

The Knoop Hardness Test is commonly employed in materials science, metallurgy, and semiconductor industries, where detailed information about localized hardness is necessary, especially for small and delicate samples. It is particularly useful in assessing microstructures and the hardness of materials in microscopic regions.

Mohs Hardness Test

The Mohs Hardness Test is a qualitative method for determining the relative hardness of minerals based on their ability to scratch one another. This test was developed by Friedrich Mohs in 1812. Here are the key features of the Mohs Hardness Test:

Indenter

The indenter in the Mohs test is the material itself. Minerals are compared by their ability to scratch one another.

Scale of Hardness

Mohs assigned a hardness value to ten common minerals, ranging from 1 (softest) to 10 (hardest). The scale is as follows:
1. Talc
2. Gypsum
3. Calcite
4. Fluorite
5. Apatite
6. Orthoclase Feldspar
7. Quartz
8. Topaz
9. Corundum
10. Diamond

Testing Procedure

A mineral is tested against known minerals of different hardness levels by attempting to scratch them. If a mineral can scratch another, it is considered harder.

Limitations

The Mohs scale is qualitative and does not provide a precise measurement of hardness.
It is not suitable for assessing the hardness of materials outside the mineral context.

Application

Commonly used in geology and mineralogy for field identification of minerals based on their relative hardness.

The Mohs Hardness Test is primarily used in mineralogy and geology for a quick and practical assessment of mineral hardness in the field. While not suitable for precise hardness measurements, it remains a valuable tool for identifying minerals and understanding their relative hardness levels.

Barcol Hardness Test