Charpy Impact Test: Understanding the Process and Results

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Discussion Overview

The discussion revolves around the Charpy impact test, focusing on the reasons for using liquid nitrogen in testing and the behavior of metal specimens during impact. It covers both theoretical and practical aspects of the test, including temperature effects and energy measurement.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions the necessity of cooling metals with liquid nitrogen before testing, suggesting that standard testing could be done at 25 degrees instead.
  • Another participant clarifies that liquid nitrogen is not used for all tests and is typically reserved for materials that will be used in cryogenic applications.
  • It is noted that the specimen in a Charpy test usually breaks upon impact, and the energy absorbed by the specimen is measured by the difference in height of the swinging arm before and after the test.
  • A participant mentions that Charpy tests are conducted over a range of temperatures and that testing at temperatures above the ductile-to-brittle transition may result in specimens that do not break.
  • There is a discussion about how energy is absorbed during impact, with some energy going into plastic deformation in ductile materials.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of using liquid nitrogen and the behavior of specimens during impact. There is no consensus on whether testing should always be done at room temperature or if cooling is beneficial, and the discussion remains unresolved regarding the implications of temperature on test results.

Contextual Notes

The discussion highlights the variability in testing conditions and the specific applications of materials, which may influence the choice of testing temperature and the interpretation of results.

Femme_physics
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I was watching this charpy impact test video clip -

I fail to understand 2 things.

A) Why do they cool the metal with liquid nitrogen before they test it? It seems wasteful. Set the standard testing temperature to 25 degrees instead.

B) What happens to the metal after it gets hit? Does it always break apart? If so, how do we measure its impact strength if it always breaks apart?
 
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My area of expertise isn't in impact testing, but I've had a bit of experience. The liquid nitrogen isn't used for all testing. In fact, it's pretty rare that it would be used. I work for a company that designs cryogenic machinery so we often specify that the material should be impact tested at low temperature. But that's an option that is provided by the testing lab. We test at low temperature because we use the material at low temperature, but if the material is used at a different temperature, that other temperature is commonly used.

As far as I know, the specimen is always broken. The amount of energy the specimen absorbs when hit is equal to the amount of energy that comes out of that weighted arm that swings around. So they measure the energy by measuring the difference in height that the arm starts and ends the swing at. The arm always comes up to a height that is lower than what it started the swing at. The difference is the energy absorbed by the specimen.
 
Charpy tests are done over a range of temperatures.

Liquid nitrogen would be used to test a metal alloy at 'cryogenic' temperatures. One would expect the metals in contact with liquid nitrogen to be ductile so as to avoid brittle fracture if struck or impacted.

A Charpy specimen doesn't always break. If one was exploring the ductile-to-brittle transition temperature, then testing at a temperature above that level might produce a result in which the specimen didn't break. If that happens, then one would have to increase the energy to fracture the specimen. In the case of ductility, some energy goes into the plastic strain (over some volume) of the sample, in addition to propagating the fracture.
 
Thanks for clearing it up, Q Goest, Astronuc, I fully get it now :)
 

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