Page 279 - Materials Science and Engineering An Introduction

C h a p t e r 8 Failure

© William D. Callister, Jr.

(a) Neal Boenzi/New York Times Pictures/Redux Pictures

Have you ever experienced
the aggravation of having to
expend considerable effort to tear
open a small plastic package that (b)
contains nuts, candy, or some other
confection? You probably have also noticed that when a small incision (or cut) has been made into an edge, as appears in
photograph (a), 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.
Photograph (b) 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.
Photograph (c) 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 that the cause was
metal fatigue 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.

Courtesy of Star Bulletin/Dennis Oda/© AP/ Wide World Photos.

(c)

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