Hardness is a must-have property to demonstrate resistance to deformation, wear, or scratching under the load of a material. Measurement of hardness offers useful information about metals, strengths, durability and performance of real-world applications.
Multiple techniques have been developed for measuring hardness with each tailored for application with particular materials and conditions to provide accurate and consistent results. It is vital for the various industries such as automobile to construction, whose material performance has a direct bearing on safety and efficiency, to comprehend the various hardness testing types, their methods, and the standards that regulate them.
What is Hardness Test
A hardness test is a method for evaluating the resistance of a material to give permanent surface deformation, for example, through scratching or indenting. The test allows for a straightforward and uniform assessment of the material's, whether it is a metal, alloy, plastic, or other substances, resistance to deformation due to an applied force. Due to its relative simplicity and lack of destructiveness, hardness testing is a common technique for quality assurance and material analysis in manufacturing, engineering, and research.
Principle of Hardness Testing
The basic concept underlying hardness testing is measuring how much resistance a material exhibits against deformation when an applied force is applied. All of the various methods of hardness testing utilize distinct techniques to measure this resistance and thus yield different scales and values of hardness.
Formula and Unit of Hardness Testing
Here are some common formulas for Hardness Testing:
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Vickers Hardness Number (HV) = 1.854* (F/d²)
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Brinell Hardness Number (BHN) = (2F/πD) × √(D² - d²)
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Rockwell Hardness Number (R)= a-b
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Knoop Hardness Test = F/A = P/CL²
Common units for hardness testing are Rockwell (HRA, HRB, HRC), Brinell (HBW), Vickers (HV) and Leeb (HLD) which are used to quantify resistance to indentation.
Hardness Test Diagram?
On a typical hardness testing diagram, there is a sample of a chosen material secured to a flat base and an indenter above it. The indenter, which can be a steel ball, diamond cone, or diamond pyramid depending on the method used, is pressed into the surface of the material with a specified load, applied vertically downwards. The diagram will typically show the material's surface indentation and may show readings of either diameter or depth of the indentation that will be used to calculate the hardness value. Components that may be present would be the load application system like a lever or raising load system, and scales or dial measuring systems for the hardness reading.
Typically, the diagram will show a contact point of the indenter and also a contact point of the applied load and typically shows how the material resists deformation.
Hardness Test Procedure
A typical hardness test consists of pressing a sharply defined indenter into the surface of a material with a known force for a known time. The following are the steps taken in the process of the Hardness Test:
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Material Preparation: The surface of the material should be flat, clean, and smooth enough for proper indentation measurement.
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Indenter Selection: Appropriate indenter type and geometry (diamond pyramid, steel ball, etc.) should be selected according to the test method (Vickers, Brinell, Rockwell, etc.) and what is being tested.
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Apply Load: Indent the material surface with the indenter with a standard load.
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Dwell Time: Maintain the indenter under load for a specific time (i.e. dwell time).
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Measure the Indentation: Depth of indentation or permanent impression size will be measured after the removal of the load.
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Calculate Hardness: From the correct formula for the test method chosen, calculate the hardness of the material from the indentation measurement and applied force.
Types of Hardness Test
The primary hardness tests are Rockwell, Brinell, Vickers, and Knoop, which are all indentation tests of resistance to a specified indenter.
There are multiple types of Hardness Test:
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Rockwell Hardness Test
The Rockwell hardness test is a quick and differential-depth method to ascertain a material's resistance to indentation, with both a minor shared load and a major load using an indenter of a specific type. The test is commonly applied to metals and plastics and provides a ready measurement of material quality and performance.
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Brinell Hardness Test
The Brinell hardness test is an indentation test that can be explained as the test that is used to establish the resistance of permanent indentation of a material by indenting it with a dense and round metal ball. The resulting indentation area is used to determine the Brinell Hardness Number, and offers a measure of bulk hardness, suitable to cast iron, aluminum alloys and steel, etc.
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Vickers Hardness Test
Vickers hardness test measures how hard a material is by pressing a pyramid-shaped diamond into its surface while keeping the load constant. The diagonals of this indentation are then measured and turned into a Vickers Hardness Number (HV).
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Knoop Hardness Test
The Knoop Hardness Test is another indent test, but with a pyramid diamond indenter with a rhombic base shape. This hardness test also gauges the ability of a material to withstand indentation or scratching on the material surface. Knoop is especially useful for thin or brittle materials and very small materials like coatings and ceramics.
Hardness Testing Standards
Hardness testing standards are available in four main categories: Brinell (ASTM E10, ISO 6506), Rockwell (ASTM E18, ISO 6508), Vickers (ASTM E92/E384, ISO 6507), and Knoop (ASTM E92/E384, ISO 4545).
These standards summarize the method of testing, in addition to providing information on the validity, reliability and reproducibility of the hardness values that could be obtained from different materials and locations.
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Brinell Hardness Test - For coarse-grained or inhomogeneous-grain materials, including castings and forgings.
Standards: ASTM E10, ISO 6506.
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Rockwell Hardness Test - A rapid test that is extensively used for production control, particularly on metallic materials.
Standards: ASTM E18, ISO 6508.
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Vickers Hardness Test - Applicable for all kinds of solid materials, including metals.
Standards: ASTM E92, ASTM E384, ISO 6507.
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Knoop Hardness Test - A microhardness test frequently employed to test brittle materials or to bypass cracking.
Standards: ASTM E384, ISO 4545.
Limitations of Hardness Test
Limitations of the hardness test include surface preparation sensitivity, Material thickness limitation, Limited correlation with material properties and non-uniform material issues.
Here are some of the limitations explained in detail:
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Surface Preparation Sensitivity
The surface condition of the material significantly affects hardness measurements. Looking at rough, roughened, or dirty surfaces will yield erroneous results as the indenter will not make contact equally across the surface. Materials should be cleaned, polished, prepared, etc., but that might not always be practical and can take significant time to accomplish.
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Material Thickness Restriction
Certain hardness test methods, namely Vickers or Knoop, are not suitable for extremely thin material because they are able to indent the material through or affect layers below. Similarly, Brinell testing is not appropriate on a thin sheet which limits the use of hardness testing on small or fragile parts.
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Non-Uniform Material Problems
Hardness testing is done with the assumption of homogeneous materials, but parts having dissimilar microstructure, grain size, and/or finishing can yield inconsistent hardness measurements. A localized condition such as an inclusion, weld, or coating, can significantly affect a hardness measurement (or conversion), leading to inaccuracies that limits the degree to which general material properties can be characterized.
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Limited Correlation with Properties
There is no assurance that hardness will relate in a direct form to other properties, such as tensile strength, ductility or impact resistance. Selecting or making a judgment strictly based on hardness could create the wrong impression of performance in actual loads and conditions.
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Operator and Instrument Dependence
Outcomes may be dependent on operator skill, indenter alignment, application of load, and instrument calibration. Small inaccuracies in applying force and taking impressions can lead to large variations, highlighting the necessity for competent staff and frequent equipment checks to ensure reliability.
Hardness Tester Machine by Testronix Instruments
Testronix Instruments has advanced hardness testing machines that can provide accurate, repeatable, and efficient hardness evaluation. These machines incorporate reliable indenters and strong load systems, and this allows all types of hardness tests (e.g., Rockwell tests, Brinell tests, and Vickers tests) to be conducted.
Hardness tester machine offers consistent results on hard or high-strength metals, as well as testing that is often used in industries such as automotive, aerospace and manufacturing to evaluate materials' hardness. With easy-to-use controls and a digital display, Testronix hardness testers provide all the features you need to make hardness testing easier, while providing a high level of precision.
Testronix hardness testers are robust, flexible, and reliable machines designed with not only accuracy, but with features such as automatic loading, data storage, and compatibility with ASTM, ISO, and BS test standards. The full-featured machines combine refinement with conformance to global standards allowing engineers and quality control professionals to produce better results.
