Page 310 - Materials Science and Engineering An Introduction

282 • Chapter 8 / Failure

tion of an engineering nature; constant-stress tests are employed to provide a better
understanding of the mechanisms of creep.
Figure 8.29 is a schematic representation of the typical constant-load creep behavior
of metals. Upon application of the load, there is an instantaneous deformation, as in-
Tutorial Video:
Creep Failure dicated in the figure, that is totally elastic. The resulting creep curve consists of three
What is the regions, each of which has its own distinctive strain–time feature. Primary or transient
Mechanism of Creep? creep occurs first, typified by a continuously decreasing creep rate—that is, the slope of
the curve decreases with time. This suggests that the material is experiencing an increase
in creep resistance or strain hardening (Section 7.10)—deformation becomes more dif-
ficult as the material is strained. For secondary creep, sometimes termed steady-state
creep, the rate is constant—that is, the plot becomes linear. This is often the stage of
creep that is of the longest duration. The constancy of creep rate is explained on the
basis of a balance between the competing processes of strain hardening and recovery,
recovery (Section 7.11) being the process by which a material becomes softer and retains
its ability to experience deformation. Finally, for tertiary creep, there is an acceleration
of the rate and ultimate failure. This failure is frequently termed rupture and results
from microstructural and/or metallurgical changes—for example, grain boundary sepa-
ration, and the formation of internal cracks, cavities, and voids. Also, for tensile loads, a
neck may form at some point within the deformation region. These all lead to a decrease
in the effective cross-sectional area and an increase in strain rate.
For metallic materials, most creep tests are conducted in uniaxial tension using a
specimen having the same geometry as for tensile tests (Figure 6.2). However, uniaxial
compression tests are more appropriate for brittle materials; these provide a better
measure of the intrinsic creep properties because there is no stress amplification and
crack propagation, as with tensile loads. Compressive test specimens are usually right
cylinders or parallelepipeds having length-to-diameter ratios ranging from about 2 to
4. For most materials, creep properties are virtually independent of loading direction.
Possibly the most important parameter from a creep test is the slope of the second-
ary portion of the creep curve ( P/ t in Figure 8.29); this is often called the minimum
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or steady-state creep rate P s . It is the engineering design parameter that is considered
for long-life applications, such as a nuclear power plant component that is scheduled
to operate for several decades, and when failure or too much strain is not an option.
However, for many relatively short-life creep situations (e.g., turbine blades in mili-
tary aircraft and rocket motor nozzles), time to rupture, or the rupture lifetime t r , is the
dominant design consideration; it is also indicated in Figure 8.29. Of course, for its
determination, creep tests must be conducted to the point of failure; these are termed
creep rupture tests. Thus, knowledge of these creep characteristics of a material allows
the design engineer to ascertain its suitability for a specific application.

Concept Check 8.5 Superimpose on the same strain-versus-time plot schematic creep curves
for both constant tensile stress and constant tensile load, and explain the differences in behavior.
[The answer may be found at www.wiley.com/college/callister (Student Companion Site).]

8.13 STRESS AND TEMPERATURE EFFECTS
Both temperature and the level of the applied stress influence the creep characteristics
(Figure 8.30). At a temperature substantially below 0.4T m , and after the initial deformation,
the strain is virtually independent of time. With either increasing stress or temperature,
the following will be noted: (1) the instantaneous strain at the time of stress application
increases, (2) the steady-state creep rate increases, and (3) the rupture lifetime decreases.