Notched impact strength

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The notched impact strength is a measure of the resistance of a material against an abrupt (dynamic) strain. The unit is the impact work performed based on the fracture area in joules per unit area [J / cm²]. In the case of metallic materials, the notched impact strength is an important guideline value that records the essential properties of the material. In addition to tensile tests , bending tests , creep tests and fatigue tests , the test is one of the classic strength tests . The test or specimen by means of a notched bar impact test is a destructive test method .

Test procedure

Tester for the notched bar impact test

The notched impact strength is determined in the notched impact test. The dynamic bending caused by the sudden loading causes a break , often without the flow of the material observed during slow loading .

The notched bar impact test is carried out on a calibrated hammer . First, a notched impact test is clamped at the bottom dead center on the radius of the notch hammer at the end of the pendulum. For the experiment, the pendulum is turned up to a fixed higher position and locked. The actual test then takes place when the pendulum swings down freely and thus notches or pierces the sample with exactly the same kinetic energy . The notch is then measured or, if it breaks through, the height to which the pendulum swings back is recorded (using a tracking pointer). The impact energy consumed can be calculated from the weight of the pendulum and the difference between the initial and final positions, whereby the impact energy is the product of the specimen cross-section and the notched impact strength.

Test conditions

The toughness depends on the temperature . Materials are generally tougher at higher temperatures. At low temperatures, however, they generally become inelastic and are more likely to break. A typical value for structural steel is an impact energy of 27 J at +20 ° C. In addition, despite the same initial conditions, there are greater variations between the tests. For this reason, notched impact tests with the same test objects, conditions and settings are repeated in a series and an average value is determined.

Sample - test piece

The impact test is carried out on a notched specimen. A square bar with a cross section of 10 by 10 mm and a length of 55 mm made from the material to be tested has a 2 mm deep “notch” in the middle. The notch angle is 45 °. The radius of the notch base is 0.25 mm (see DIN EN ISO 148-1). The test rod is freely inserted into the pendulum impact tester without being clamped at its ends. The free falling pendulum punctures the specimen with its cutting edge exactly behind the notch. Other designs of the sample are possible. They are described in DIN 50115.

The stress state that occurs during the experiment is multiaxial due to the notch. In body-centered cubic materials, the higher the temperature, the higher the notched impact strength. You are ductile .

Impact work as a function of temperature

The movements of dislocations in the crystal lattice, which are necessary for deformation, cannot take place equally easily on all levels. In general, only the planes that are most densely packed with atoms slide, since lower stresses are necessary here. These planes are called slip planes. Slip planes are the diagonal surfaces in the face-centered cubic grid and the top and base surfaces in the hexagonal grid. With the body-centered cubic lattice, there are no such preferred planes, here the diagonal surfaces take over the sliding process. 

There are different numbers of (preferred) slip planes in different crystal lattice systems. The more slip planes there are, the easier it is to deform the material. Of course, with an easily deformable material with many slip planes, less notched impact work will have to be performed than with a less easily deformable material with correspondingly few slip planes.


  • Hermann Schenck: Mechanical and physical test methods to determine the processes involved in quenching and deformation aging. West German publishing house, Cologne and Opladen 1961.
  • Erich Siebel (Hrsg.): Handbook of materials testing. Second edition, second volume, Springer Verlag Berlin Heidelberg GmbH, Berlin Heidelberg 1955.

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