Page 33 - Materials Science and Engineering An Introduction

1.3 Why Study Materials Science and Engineering? • 5

Finally, probably the overriding consideration is that of economics: What will the
finished product cost? A material may be found that has the ideal set of properties but is
prohibitively expensive. Here again, some compromise is inevitable. The cost of a finished
piece also includes any expense incurred during fabrication to produce the desired shape.
The more familiar an engineer or scientist is with the various characteristics and
structure–property relationships, as well as the processing techniques of materials, the
more proficient and confident he or she will be in making judicious materials choices
based on these criteria.

C A S E S T U D Y

Liberty Ship Failures

he following case study illustrates one role that experienced a ductile-to-brittle transition. Some
Tmaterials scientists and engineers are called of them were deployed to the frigid North Atlan-
upon to assume in the area of materials performance: tic, where the once ductile metal experienced brit-
analyze mechanical failures, determine their causes, tle fracture when temperatures dropped to below
and then propose appropriate measures to guard the transition temperature. 6
against future incidents. • The corner of each hatch (i.e., door) was square;
The failure of many of the World War II Liberty these corners acted as points of stress concentra-
ships 3 is a well-known and dramatic example of the tion where cracks can form.
brittle fracture of steel that was thought to be duc- • German U-boats were sinking cargo ships faster
tile. 4 Some of the early ships experienced structural than they could be replaced using existing con-
damage when cracks developed in their decks and struction techniques. Consequently, it became
hulls. Three of them catastrophically split in half when necessary to revolutionize construction methods
cracks formed, grew to critical lengths, and then rap- to build cargo ships faster and in greater numbers.
idly propagated completely around the ships’ girths. This was accomplished using prefabricated steel
Figure 1.3 shows one of the ships that fractured the sheets that were assembled by welding rather
day after it was launched. than by the traditional time-consuming riveting.
Subsequent investigations concluded one or more
5
of the following factors contributed to each failure : Unfortunately, cracks in welded structures may
propagate unimpeded for large distances, which
• When some normally ductile metal alloys are can lead to catastrophic failure. However, when
cooled to relatively low temperatures, they be- structures are riveted, a crack ceases to propagate
come susceptible to brittle fracture—that is, they once it reaches the edge of a steel sheet.
experience a ductile-to-brittle transition upon • Weld defects and discontinuities (i.e., sites where
cooling through a critical range of temperatures. cracks can form) were introduced by inexperi-
These Liberty ships were constructed of steel that enced operators.

3 During World War II, 2,710 Liberty cargo ships were mass-produced by the United States to supply food and
materials to the combatants in Europe.
4 Ductile metals fail after relatively large degrees of permanent deformation; however, very little if any permanent
deformation accompanies the fracture of brittle materials. Brittle fractures can occur very suddenly as cracks spread
rapidly; crack propagation is normally much slower in ductile materials, and the eventual fracture takes longer.
For these reasons, the ductile mode of fracture is usually preferred. Ductile and brittle fractures are discussed in
Sections 8.3 and 8.4.
5 Sections 8.2 through 8.6 discuss various aspects of failure.
6 This ductile-to-brittle transition phenomenon, as well as techniques that are used to measure and raise the critical
temperature range, are discussed in Section 8.6.
(continued)