Notched-Bar Impact Tests

Several tests, in recent past, have been developed to determine the tendency of material to fracture in brittle manner. Most tests are performed on bar specimens which carry notches in most critically stressed zone under impact [strain rate ≥ 103 mm/mm (sec)].Carrying out the impact tests on notched bar samples at low temperatures renders most severe conditions which might exist in service.

One important factor which separates impact test from static tests is the great difficulty or even impossibility of stress calculation at the tip of the notch under impact load. The triaxial state of stress that exists around notch tip under static load is modified considerably under impact load. Due to this difficulty the result of impact tests are not presented in form of stress as is customary in static test. Since the relative magnitudes of the stress components at notch tip depend upon the dimension of notch and specimen, the impact test specimens are standarised so that results from different sources may be compared.

The property that is measured in impact test is the energy absorbed in fracturing the specimen of standard dimensions and standard notch. The property, measured in kgf-m, or joules is also often referred to as impact toughness.

Since impact toughness is not a stress and it cannot be generalised for a material but remains property of specimen of a material, it is not possible to incorporate this property in design directly. on the other hand, this property is a good qualitative index of behaviour of material in presence of notch and at low temperature, and such an index is not possible to obtain from any static test, therefore, this property of material is used in selection of material (particularly steel) and for development of material for specific purposes of inhibiting the tendency of brittle fracture.

Although the impact test can be used for any material, yet it is steel (particularly the structural steel) for which this is universally used. This is because of the fact that steel is the predominant material used for structures as well as due to the fact that steel behaviour is sharply influenced by variation in temperature and strain rate.