In circumstances where safety is extremely critical, the full-scale engineering com­ponents may be tested in their worst possible service condition. An example of a full-scale engineering test could be crash of a train carrying the spent fuel casks to see the effect of crash on the integrity of the casks. However, such full-scale tests are extremely expensive and very rarely conducted. Before the advent of fracture mechanics as a discipline, impact-testing techniques were used to determine the

fracture characteristics of materials. Impact tests are designed to measure the resistance to failure of a material under a sudden applied force in such a way so as to represent most severe conditions relative to the potential of fracture — (i) defor­mation at low temperatures, (ii) high deformation rate, and (iii) a triaxial state of stress (by introducing a notch in the specimen). The impact tests measure the impact energy or the energy absorbed prior to failure. The most common methods of measuring impact energy are Charpy and Izod (Figure 5.19a). The techniques differ in the manner of specimen support and specimen design. Charpy V-notch test is most commonly used in the United States (Figure 5.19b). In this test, the load is applied as an impact blow from a weighted pendulum hammer that is

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Volume of second phase, %

Figure 5.18 Effect of second-phase particles on tensile ductility. From Ref. [2].

released from a fixed position at a fixed height (h1). Upon release, a knife-edge mounted on the pendulum strikes and fractures the specimen at the notch that acts as a stress raiser site for the high-velocity impact blow. After fracturing the specimen, the pendulum continues in its trajectory to reach a height h2, depending on the absorbed energy during impact. The energy absorption is calculated from the difference between the pendulum’s static energies at h1 and h2, that is, the impact energy is Mg(h1 — h2), where M is the pendulum’s mass and g is the acceler­ation due to gravity. In reality, the machine is equipped with a scale and a pointer that shows the impact energy after the test is done. Nowadays, more sophisticated, instrumented impact testers are used that can follow the load versus time or dis­placement during the impact event.

Variables including specimen size and shape as well as notch configuration and depth influence the test results. One of the main purposes of the Charpy test is to determine whether or not a material experiences a ductile-brittle transition with decreasing temperature. Figure 5.20 shows a description of ductile-brittle

transition behavior. Plane strain fracture toughness is quantitative in nature in that a specific property of the material is determined (i. e., KIC). The results of impact tests are more qualitative in nature and are of little use for design purposes. Impact energies are of interest mainly in a relative sense and for making comparisons — absolute values are of little significance. Attempts have been made to correlate frac­ture toughness and Charpy impact energy, but with limited success.