Page 309 - Materials Science and Engineering An Introduction

8.12 Generalized Creep Behavior • 281

thermal fatigue Thermal fatigue is normally induced at elevated temperatures by fluctuating ther-
mal stresses; mechanical stresses from an external source need not be present. The
origin of these thermal stresses is the restraint to the dimensional expansion and/or con-
traction that would normally occur in a structural member with variations in tempera-
ture. The magnitude of a thermal stress developed by a temperature change T depends
Thermal stress— on the coefficient of thermal expansion a l and the modulus of elasticity E according to
dependence on
coefficient of thermal s = a l E T (8.23)
expansion, modulus
of elasticity, and (The topics of thermal expansion and thermal stresses are discussed in Sections 19.3
temperature change
and 19.5.) Thermal stresses do not arise if this mechanical restraint is absent. Therefore,
one obvious way to prevent this type of fatigue is to eliminate, or at least reduce, the
restraint source, thus allowing unhindered dimensional changes with temperature vari-
ations, or to choose materials with appropriate physical properties.
Failure that occurs by the simultaneous action of a cyclic stress and chemical attack is
corrosion fatigue termed corrosion fatigue. Corrosive environments have a deleterious influence and pro-
duce shorter fatigue lives. Even normal ambient atmosphere affects the fatigue behavior
of some materials. Small pits may form as a result of chemical reactions between the envi-
ronment and the material, which may serve as points of stress concentration and therefore
as crack nucleation sites. In addition, the crack propagation rate is enhanced as a result of
the corrosive environment. The nature of the stress cycles influences the fatigue behavior;
for example, lowering the load application frequency leads to longer periods during which
the opened crack is in contact with the environment and to a reduction in the fatigue life.
Several approaches to corrosion fatigue prevention exist. On one hand, we can take
measures to reduce the rate of corrosion by some of the techniques discussed in Chapter
17—for example, apply protective surface coatings, select a more corrosion-resistant
material, and reduce the corrosiveness of the environment. On the other hand, it might
be advisable to take actions to minimize the probability of normal fatigue failure, as
outlined previously—for example, reduce the applied tensile stress level and impose
residual compressive stresses on the surface of the member.
Creep

Materials are often placed in service at elevated temperatures and exposed to static
mechanical stresses (e.g., turbine rotors in jet engines and steam generators that experi-
ence centrifugal stresses; high-pressure steam lines). Deformation under such circum-
creep stances is termed creep. Defined as the time-dependent and permanent deformation of
materials when subjected to a constant load or stress, creep is normally an undesirable
phenomenon and is often the limiting factor in the lifetime of a part. It is observed in
all materials types; for metals, it becomes important only for temperatures greater than
about 0.4T m , where T m is the absolute melting temperature. Amorphous polymers,
which include plastics and rubbers, are especially sensitive to creep deformation, as
discussed in Section 15.4.

8.12 GENERALIZED CREEP BEHAVIOR
A typical creep test consists of subjecting a specimen to a constant load or stress while
11
maintaining the temperature constant; deformation or strain is measured and plotted as
a function of elapsed time. Most tests are the constant-load type, which yield informa-

11 ASTM Standard E139, “Standard Test Methods for Conducting Creep, Creep-Rupture, and Stress-Rupture Tests
of Metallic Materials.”