science

Knoop hardness test

Knoop hardness test is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Knoop hardness test rather than just read about it. In short: The Knoop hardness test is a microhardness test – a test for mechanical hardness used particularly for very brittle materials or thin sheets, where only a small indentation may be made for testing purposes. A pyramidal diamond point is pressed into the polished surface of the test material with a known (often 100 g) load, for a specified dwell time, and the resulting indentation is measured using a microscope.

Key takeaways

Knoop hardness test belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Knoop hardness test to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Knoop hardness test from memory before moving on to harder problems.

Reference excerpt

The Knoop hardness test is a microhardness test – a test for mechanical hardness used particularly for very brittle materials or thin sheets, where only a small indentation may be made for testing purposes. A pyramidal diamond point is pressed into the polished surface of the test material with a known (often 100 g) load, for a specified dwell time, and the resulting indentation is measured using a microscope. The geometry of this indenter is an extended pyramid with the length to width ratio being 7:1 and respective face angles are 172 degrees for the long edge and 130 degrees for the short edge. The depth of the indentation can be approximated as 1/30 of the long dimension. The Knoop hardness HK or KHN is then given by the formula:

H K = load ( k g f ) impression area ( m m 2 ) = P C p L 2 {\displaystyle HK={{{\text{load}}(\mathrm {kgf} )} \over {{\text{impression area}}(\mathrm {mm} ^{2})}}={P \over {C_{\text{p}}L^{2}}}}

where:

L is the length of indentation along its long axis Cp is the correction factor related to the shape of the indenter, ideally 0.070279 P is the load HK values are typically in the range from 100 to 1000, when specified in the conventional units of kgf⋅mm−2. The SI unit, pascal, is sometimes used instead: 1 kgf⋅mm−2 = 9.80665 MPa. The impression area represents a projected area based on the surface projection of the elongated diagonal measurement. This is different from the area term used for other types of indentation hardness testing, such as the Vickers hardness test, which utilize a contact surface area term.

The test was developed by Frederick Knoop and colleagues at the National Bureau of Standards (now NIST) of the United States in 1939, and is defined by the ASTM E384 standard. The advantages of the test are that only a very small sample of material is required, and that it is valid for a wide range of test forces. The main disadvantages are the difficulty of using a microscope to measure the indentation (with an accuracy of 0.5 micrometre), and the time needed to prepare the sample and apply the indenter. Variables such as load, temperature, and environment, may affect this procedure, which have been examined in detail.

Illustrations

Worked examples

Example 1 — a first encounter with Knoop hardness test

Start with the simplest possible case. Write down what Knoop hardness test claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Knoop hardness test before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Knoop hardness test ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Knoop hardness test

In research: Knoop hardness test appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Knoop hardness test in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Knoop hardness test is common in secondary-school and first-year university syllabi. It links to neighbouring topics Hardness tests, so understanding it makes those chapters shorter.
In everyday life: Look for Knoop hardness test outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Knoop hardness test in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Knoop hardness test means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Knoop hardness test out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Knoop hardness test in simple terms?

Why does Knoop hardness test matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Knoop hardness test?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Knoop hardness test.