FIGURE 34.12 Charpy transition curve for A588 Grade 50 (350-MPa yield strength) structural steel.

depends on temperature, as reflected by the variation of CVN with temperature as shown in Figure 34.12. The transition is a result of changes in the underlying microstructural fracture mode.

Brittle fracture on the so-called lower shelf in Figure 34.12 is associated with cleavage of individual grains on select crystallographic planes. Brittle fracture may be analyzed with linear-elastic fracture mechanics theory because the plastic zone at the crack tip is very small.

At the high end of the temperature range, the so-called upper shelf, ductile fracture is associated with the initiation, growth, and coalescence of microstructural voids, a process requiring much energy. The net section of plates or shapes fully yields and then ruptures with large slanted shear lips on the fracture surface.

Transition-range fracture occurs at temperatures between the lower and upper shelves and is associated with a mixture of cleavage and shear fracture. Large variability in toughness at constant temperature and large changes with temperature are typical of transition-range fractures.

Brittle fracture can be thought of as an interruption of what would otherwise be ductile plastic deformation in tension, much like buckling can interrupt ductile plastic deformation in compression. For the purposes of structural engineering, it is usually only necessary to make sure that brittle fracture does not occur, so that is the emphasis in this chapter. As long as the behavior is in the transition range or upper shelf, the resulting ductility is generally sufficient. For example, as discussed in the previous section, moment-frame connections will tolerate a certain minimum number of cycles of large rotations governed by the low-cycle fatigue limit state, as long as specific details are used and minimum CVN requirements are assured. If low-toughness weld metal that does not meet minimum requirements is used, brittle fracture may occur before the expected number of cycles [65].

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