Charpy Impact Test: Understanding the Process and Results

[Femme_physics]

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?

 

[Q_Goest]

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.

 

[Astronuc]

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.

 

[Femme_physics]

Thanks for clearing it up, Q Goest, Astronuc, I fully get it now :)