Notched Bar Impact Test (Charpy)

The notched bar impact test (Charpy impact test) is a destructive testing method that determines, in terms of energy, the resistance of a material to fracture under dynamic (sudden) loading, in other words its fracture toughness. The physical principle of the method is based on releasing a pendulum (hammer) of known mass from a specified height so that it strikes, at high velocity, a standard test specimen with a V- or U-shaped notch machined at its mid-point. The difference between the potential energy of the pendulum at the moment of impact and the energy remaining when it rises on the opposite side after breaking the specimen gives the energy absorbed by the material in fracturing (impact energy, KV or KU). This value is measured in joules (J) and quantitatively reveals whether the material behaves in a ductile or a brittle manner. The notch concentrates stress in a single cross-section and creates a multiaxial stress state, which makes it possible to measure the fracture tendency of the material under the most unfavourable conditions.

The test is performed by placing the prepared specimen on the supports of the machine so that the notch faces exactly opposite the surface to be struck. A standard Charpy specimen is generally 55 mm long with a 10 x 10 mm square cross-section; the notch depth and angle (for a V-notch, 2 mm depth, 45° angle, 0.25 mm root radius) are precisely defined by the standards. For thin materials in which a full-size cross-section cannot be prepared, subsize specimens of 7.5 mm or 5 mm are used and the results are reported for that cross-section. Because even small deviations in notch geometry change the measured energy significantly, specimen preparation (machining, notch broaching) requires high precision.

The capacity of the pendulum impact machine used is typically of the order of 150 J or 300 J, and the impact velocity is kept within the 5–5.5 m/s range specified by the standards. The energy measurement system of the machine may be dial (analogue) or electronic encoder based; the geometry of the striking edge and the position of the supports must be verified regularly. Since the test result is highly sensitive to temperature, specimens are brought to the target temperature in a controlled environment (liquid bath or climatic chamber); in low-temperature tests the specimen must be broken within 5 seconds of removal from the bath, because temperature drift directly affects the result.

At an application-specific level of detail, the Charpy test is of critical importance particularly in determining the ductile-to-brittle transition temperature (DBTT) of ferritic-pearlitic steels. A series of tests carried out at different temperatures allows an energy-temperature transition curve to be plotted; this curve shows below which temperature a material carries the risk of brittle fracture and forms the basis of design safety for pressure vessels, pipelines and structural steels operating at low temperature. In welded joints, the notch is positioned in the weld metal, on the fusion line or in the heat-affected zone (HAZ) so that the toughness of these critical regions is evaluated separately.

In reporting, the absorbed impact energy (J), test temperature, notch type and specimen size are stated for each specimen; the average of three specimens is generally given together with the individual values. To complete the result, the fracture surface must also be evaluated: lateral expansion is measured and the percentage of ductile (fibrous/matt) and brittle (crystalline/shiny) fracture is determined visually. Acceptance criteria are evaluated on the basis of the minimum energy values in the material specification, product standard or customer specification. TÜV AUSTRIA SILA KALİTE performs the notched bar impact test as a service accredited to TS EN ISO/IEC 17025; the results obtained therefore carry national and international recognition and provide independent, traceable evidence of the reliability of the material under dynamic loading.