Page 280 - Materials Science and Engineering An Introduction

WHY STUDY Failure?

The design of a component or structure often calls design principles that may be employed to prevent in-
upon the engineer to minimize the possibility of failure. service failures. For example, in Sections M.14 through
Thus, it is important to understand the mechanics of M.16 of the Mechanical Engineering Online Support
the various failure modes—fracture, fatigue, and Module, we discuss material selection and processing is-
creep—and, in addition, be familiar with appropriate sues relating to the fatigue of an automobile valve spring.

Learning Objectives
After studying this chapter, you should be able to do the following:
1. Describe the mechanism of crack propagation 6. Define fatigue and specify the conditions under
for both ductile and brittle modes of fracture. which it occurs.
2. Explain why the strengths of brittle materials 7. From a fatigue plot for some material, determine
are much lower than predicted by theoretical (a) the fatigue lifetime (at a specified stress
calculations. level) and (b) the fatigue strength (at a
3. Define fracture toughness in terms of (a) a specified number of cycles).
brief statement and (b) an equation; define all 8. Define creep and specify the conditions under
parameters in this equation. which it occurs.
4. Make a distinction between fracture toughness 9. Given a creep plot for some material, determine
and plane strain fracture toughness. (a) the steady-state creep rate and (b) the
5. Name and describe the two impact fracture rupture lifetime.
testing techniques.

8.1 INTRODUCTION
The failure of engineering materials is almost always an undesirable event for several
reasons; these include putting human lives in jeopardy, causing economic losses, and
interfering with the availability of products and services. Even though the causes of
Tutorial Video: failure and the behavior of materials may be known, prevention of failures is difficult
Cyclical to guarantee. The usual causes are improper materials selection and processing and
Fatigue Failure inadequate design of the component or its misuse. Also, damage can occur to structural
What are Some parts during service, and regular inspection and repair or replacement are critical to safe
Real-World Examples design. It is the responsibility of the engineer to anticipate and plan for possible failure
of Failure? and, in the event that failure does occur, to assess its cause and then take appropriate
preventive measures against future incidents.
The following topics are addressed in this chapter: simple fracture (both ductile
and brittle modes), fundamentals of fracture mechanics, fracture toughness testing,
the ductile-to-brittle transition, fatigue, and creep. These discussions include failure
mechanisms, testing techniques, and methods by which failure may be prevented or
controlled.

Concept Check 8.1 Cite two situations in which the possibility of failure is part of the
design of a component or product.
[The answer may be found at www.wiley.com/college/callister (Student Companion Site).]

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