Charpy impact toughness criterion

[AnnyB]

Materials like 17-4PH H1150 might be used at -196°C but exhibit Charpy-V values less than 27J which corresponds to the impact energy at the DBTT.
I know people accepting value as low as 5J. Other say 5J is taking too big of a risk and require 10J. Other accept only above 15J.
Is really 15J safer than 10J and safer than 5J whereas we are below the DBTT anyway or is it the same? Would really a material with 15J sustain a tougher chock than the material at 5J?
How can we fix such a value?

 

[metengr]

First, you need to understand that CVN impact energy is not fracture toughness - tolerance for cracks in material. CVN impact testing is a subjective test to evaluate the material fracture behavior under severe strain rate conditions at a particular service temperature.

So, CVN impact energy values that are considered acceptable may be based in whole or in part on fracture response of the material in a particular service. 15J may have been proven to provide suitable or safe service experience given a particular design stress level.

Minimal acceptable CVN impact energy values are either based on design or experience in service. In some cases you cannot change the value easily because they are either part of a material specification, code or standard.

 

[AnnyB]

Dear metengr,
Thank you very much for your answer