Callister - An introduction - 8th edition

Chapter 8
Failure
(a)
Have you ever experienced
the aggravation of having to
expend considerable effort to
tear open a small plastic package
that contains nuts, candy, or
some other confection? You
probably have also noticed when
a small incision (or cut) has been
(b)
made into an edge, as appears in photograph (a), that a minimal force is required to tear the package open. This phenomenon
is related to one of the basic tenets of fracture mechanics: an applied tensile stress is amplified at the tip of a small incision
or notch.
The (b) photograph is of an oil tanker that fractured in a brittle manner as a result of the propagation of a crack completely
around its girth. This crack started as some type of small notch or sharp flaw. As the tanker was buffeted about while at sea,
resulting stresses became amplified at the tip of this notch or flaw to the degree that a crack formed and rapidly elongated,
which ultimately led to complete fracture of the tanker.
The (c) photograph is of a Boeing 737-200 commercial aircraft (Aloha Airlines flight 243) that experienced an
explosive decompression and structural failure on April 28, 1988. An investigation of the accident concluded the cause was
metal fatigue that was aggravated by crevice corrosion (Section 17.7) inasmuch as the plane operated in a coastal (humid and
salty) environment. Stress cycling of the
fuselage resulted from compression and
decompression of the cabin chamber during
short hop flights. A properly executed maintenance
program by the airline would have
detected the fatigue damage and prevented
this accident. (Photograph of the oil tanker
by Neal Boenzi. Reprinted with permission
(c)
from The New York Times. Photograph of
the Boeing 737-200 courtesy of Star
Bulletin/Dennis Oda/© AP/Wide World
Photos.)
(c)
234 •