Charpy Testing

History

The Charpy test or Charpy v notch impact test was developed by and named after the French scientist Georges Charpy in 1905. It was central to understanding fractures that can occur in steel used at low temperatures.

During the second world war, twelve allied ships broke in half without warning due to fractures. It was thought. initially, that this was caused by the welding of inexperienced shipbuilders. Constance Tipper, an engineer at Cambridge university showed that fractures that occurred at temperatures below a critical point could spread across welded joints. Shipbuilders and manufacturers of offshore and nuclear equipment use the test. Bridge builders, construction companies and pressure vessel manufacturers also use material that has been Charpy tested.

Toughness vs strength

It is useful to compare strength against toughness. The yield and tensile strength measure material under strain. It is measured in units of force such as Newtons. Toughness is how that material reacts to sudden impact loading and this is measured in units of energy, typically joules.

The Charpy test measures the toughness of steel at differing temperatures. This is done because at lower temperatures a transition occurs in the steel from ductile to brittle and potential catastrophic failure.

Method

A notched sample is prepared. The location of the notch and shape of the sample are standard.

Prepared test piece with V notch

The sample is placed in a Charpy testing machine (see diagram). The points of support of the sample, as well as the impact of the hammer, must bear a constant relationship to the notch.

Charpy testing machine

Results

It is usual for a set of three tests to be performed and the results recorded. The average result of the three tests is used. Of course it is good to see a regular set of results indicating that the sample behaved consistently. Because of this only one result out of the three is usually permitted to be below the minimum stated figure eg. 27 joules.

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