ASTM D256 and ISO 180 , It belongs to Common Mechanical Tests
ASTM impact energy is expressed in J/m or ft-lb/in. Impact strength is calculated by dividing impact energy in J (or ft-lb) by the thickness of the specimen. The test result is typically the average of 5 specimens.
ISO impact strength is expressed in kJ/m2. Impact strength is calculated by dividing impact energy in J by the area under the notch. The test result is typically the average of 10 specimens.
SO Charpy specification has two expressed . One is J .the others is kJ/m2.