Notched-Bar Impact Tets
Various tests, in recent past, have been enhanced to determine the material tendency's to fracture in brittle manner. Most tests are presented on bar specimens that carry notches in most critically stressed zone beneath impact [strain rate ≥ 103 mm/mm (sec)]. Carrying out the impact tests upon notched bar samples at low temperatures renders most severe situation that might exist in service.
One significant factor that separates impact test from static tests is the great complexity or still impossibility of stress computation at the tip of the notch beneath impact load. The 24 triaxial state of stress such exists around notch tip beneath static load is modified considerably beneath impact load. Because of this difficulty the conclusion of impact tests are not represented in form of stress like is customary in static test. As the relative magnitudes of the stress components on notch tip depend on the dimension of notch and test piece, the impact test specimens are standardized so that results from various sources may be compared.
The property such is measured in impact test is the energy absorbed into fracturing the test piece of standard notch and standard dimensions. The property, measured in joules, or kgf-m is often referred also to like impact toughness.
Because impact toughness is not a stress and it cannot be generalized for a material except remains property of test piece of a material, it is impossible to incorporate that property in design directly. Conversely, this property is a superior qualitative index of behaviour of material in occurrence of notch and at low temperature, and that an index is impossible to acquire from any static test, consequently, this property of material is employed in selection of material as particularly steel and for enhancement of material for exact purposes of inhibiting the tendency of brittle fracture.
Though the impact test can be employed for any material, still now it is steel mostly the structural steel for that this is universally employs. This is due to the fact that steel is the predominant material employed for structures and because of the fact also that steel behaviour is sharply influenced via variation in strain rate and temperature.