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Thanks for your question. Impact testing was first carried out on metals and those tests were adapted to plastics. You are correct in saying that charpy is the test most often used when testing metals but it is not unheard of to do a metals izod or even a metals tensile impact test. Also, both charpy and izod are used for testing plastics. Izod historically has been the test of choice here in the US but the rest of the world tends to favor charpy testing according to ISO 179. In the past decade, more US users have started to use charpy testing, particularly those that supply the automotive industry. The auto industry is global in nature so 10-15 years ago they didn’t want data from 2 tests so they selected the ISO charpy test since it was in use globally. The point of the notch in the specimen is to try to get it to break in the same spot. For izod, the notch faces the striker so that you get a hinge type breaking action. For charpy you are getting a three point bend type action on the specimen. Hope this helps. Regards, Harry