Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

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2nd Revised Pages

9.8 Impact Fracture Testing • 313

ductile-to-brittle transition, features of both types will exist (in Figure 9.20, displayed

by specimens tested at –12◦C, 4◦C, 16◦C, and 24◦C). Frequently, the percent shear

fracture is plotted as a function of temperature—curveBin Figure 9.19.

For many alloys there is a range of temperatures over which the ductile-to-brittle

transition occurs (Figure 9.19); this presents some difficulty in specifying a single

ductile-to-brittle transition temperature. No explicit criterion has been established,

and so this temperature is often defined as that temperature at which the CVN energy

assumes some value (e.g., 20 J or 15 ft-lbf), or corresponding to some given fracture

appearance (e.g., 50% fibrous fracture). Matters are further complicated inasmuch as

a different transition temperature may be realized for each of these criteria. Perhaps

the most conservative transition temperature is that at which the fracture surface

becomes 100% fibrous; on this basis, the transition temperature is approximately

110 ◦C (230◦F) for the steel alloy that is shown in Figure 9.19.

Structures constructed from alloys that exhibit this ductile-to-brittle behavior

should be used only at temperatures above the transition temperature, to avoid

brittle and catastrophic failure. Classic examples of this type of failure occurred, with

disastrous consequences, during World War II when a number of welded transport

ships, away from combat, suddenly and precipitously split in half. The vessels were

constructed of a steel alloy that possessed adequate ductility according to room-

temperature tensile tests. The brittle fractures occurred at relatively low ambient

temperatures, at about 4◦C (40◦F), in the vicinity of the transition temperature of the

alloy. Each fracture crack originated at some point of stress concentration, probably

a sharp corner or fabrication defect, and then propagated around the entire girth of

the ship.

In addition to the ductile-to-brittle transition represented in Figure 9.19, two

other general types of impact energy-versus-temperature behavior have been ob-

served; these are represented schematically by the upper and lower curves of Figure

9.21. Here it may be noted that low-strength FCC metals (some aluminum and cop-

per alloys) and most HCP metals do not experience a ductile-to-brittle transition

(corresponding to the upper curve of Figure 9.21), and retain high impact energies

(i.e., remain ductile) with decreasing temperature. For high-strength materials (e.g.,

high-strength steels and titanium alloys), the impact energy is also relatively insen-

sitive to temperature (the lower curve of Figure 9.21); however, these materials are

also very brittle, as reflected by their low impact energy values. And, of course, the

characteristic ductile-to-brittle transition is represented by the middle curve of Fig-

ure 9.21. As noted, this behavior is typically found in low-strength steels that have

the BCC crystal structure.

Impact energy

Low-strength (FCC and HCP) metals

Low-strength steels (BCC)

High-strength materials

Temperature

Figure 9.21 Schematic curves for

the three general types of impact

energy-versus-temperature behavior.