DETERMINATION OF IMPACT STRENGTH OF METALS

Purpose of the work: to study the design and operation of a pendulum-type impact testing machine; to master the impact strength determination technique; to investigate the influence of carbon content on strain (reduction in area) ofimpact specimens and impact strength of carbon steel.

Theory. Many parts and structures in normal operational conditions undergo impact loading that promotes brittle fracture. Susceptibility to brittle fracture is enhanced by increase in loading rate and grain size, decrease in temperature, and presence of notches. Chemical content also influences the impact strength: it is reduced with the increase in content of carbon and detrimental impurities, such as sulphur, phosphorus, hydrogen, nitrogen and oxygen. Static tests do not reveal the resistance of materials to brittle fracture. Therefore the dynamic tests are used.

The dynamic impact bending tests (GOST 9454-78) are the most wide spread. They are of the most severe type and promote brittle fracture. Notched-bar test specimens are commonly used in these tests. The impact strength KC is determined as a ratio of the work required to fracture the specimen to its cross-sectional area and, hence, represents the specific work required to fracture. The area is gauged (at the notched section before fracture (specimens of brittle materials do not require notches). The impact strength is normally denoted (signed) in accordance with the form of the notch (U-notched specimen with the notch radius of 1 mm; V-notched, with the radius of 0.25 mm (Charpy) and crack-notched): KCU, KCV and KCT correspondently. The work required to fracture decreases with the decrease in notch radius.

The initial angle through which the pendulum of mass P (Fig. 1.9) is raised corresponds to the potential energy level of PH. After the test (impact) and fracturing of the specimen the pendulum raises up to the height of h and possesses the energy level Ph. The work required to fracture is equal to the loss of potential energy:

where t is length of the pendulum, m;

a₁ is angle to which the pendulum is raised before the impact;

a₂ is angle to which the pendulum raises after fracturing the specimen.

Fig. 1.9. Pendulum-type impact testing machine (a); notched-bar test specimen (b); principle of the impact test (c)

The initial angle a₁ is a constant value and the angle a₂ is measured on a dial gage of the machine. Special tables are available for determining the work K, J. These tables list the work of impact for each value of the angle a₂. Next, the impact strength may be found by the formulas

KC = K/F,

where F is the cross-sectional area of the specimen at the notch before fracture (normally 80 mm²).

The impact strength mainly depends on work consumed in deforming the specimen before rupture, i.e. on plasticity of a material. Reduction in width of the specimen after fracture may be regarded as one of the plasticity characteristics. According to this index the reduction in area of the specimen may be determined as follows:

where b_o and b₁, is width of the specimen before and after the test correspondently, mm.