Page 167 - Mechanical Behavior of Materials

168 Chapter 4 Mechanical Testing: Tension Test and Other Basic Tests

Figure 4.38 Variation in Charpy V-notch impact energy with temperature for normalized
plain carbon steels of various carbon contents. (From [Boyer 85] p. 4.85; used with permission.)

Fig. 4.38. However, even for the same carbon content, and for heat treatment to the same hardness
(ultimate strength), there are still differences in the impact behavior of steels due to the influence of
different percentages of minor alloying elements. This behavior is illustrated by Fig. 4.39.
In Figs. 4.38 and 4.39, there tends to be a region of temperatures over which the impact energy
increases rapidly from a lower level that may be relatively constant to an upper level that may
also be relatively constant. Such a temperature-transition behavior is common in various materials.
The fracture surfaces for low-energy (brittle) impact failures are generally relatively smooth, and in
metals have a crystalline appearance. But those for high-energy (ductile) fractures have regions of
◦
shear where the fracture surface is inclined about 45 to the tensile stress, and they have, in general,
a rougher, more highly deformed appearance, called fibrous fracture. These differences can be seen
in Fig. 4.37.
The temperature-transition behavior is of some engineering significance, as it aids in comparing
materials for use at various temperatures. In general, a material should not be severely loaded at
temperatures where it has a low impact energy. However, some caution is needed in attaching too
much significance to the exact position of the temperature transition. This is because the transition
shifts even for different types of impact tests, as discussed in the book by Barsom (1999). Notch-
impact test results can be quantitatively related to engineering situations of interest only in an
indirect manner through empirical correlations, with this situation applying both to the energies
and to the temperature transition.
By the use of fracture mechanics (as described later in Chapter 8), materials containing cracks
and sharp notches can be analyzed in a more specific way. In particular, the fracture toughness can
be quantitatively related to the behavior of an engineering component, and loading-rate effects can