MATERIALS ENGINEERING CONCEPTS - Materials for Civil and Construction Engineers - page 13

High
strength

Fracture

High
toughness

Toughness

Modulus of
resilience

Strain Strain Strain

(a) (b) (c)

FIGURE 1.8 Areas under stress-strain curves: (a) modulus of resilience,
(b) toughness, and (c) high-strength and high-toughness materials.

For instance, as will be discussed in Chapter 3, increasing the carbon con-
tent of steel increases the yield strength, but reduces ductility. Therefore,
the strength is increased, but the toughness may be reduced.

1.2.6 Time-Dependent Response
The previous discussion assumed that the strain was an immediate response
to stress. This is an assumption for elastic materials. However, no material
has this property under all conditions. In some cases, materials have a de-
layed response. The amount of deformation depends on the duration of the
load, the temperature, and the material characteristics. There are several
mechanisms associated with time-dependent deformation, such as creep
and viscous flow . There is no clear distinction between these terms. Creep is
generally associated with long-term deformations and can occur in metals,
ionic and covalent crystals, and amorphous materials. On the other hand,
viscous flow is associated only with amorphous materials and can occur
under short-term load duration. For example, concrete, a material with pre-
dominantly covalent crystals, can creep over a period of decades. Asphalt
concrete pavements, an amorphous-binder material, can have ruts caused by
the accumulated effect of viscous flows resulting from traffic loads with a
load duration of only a fraction of a second.
Creep of metals is not relevant in typical civil engineering applications.
In steel, creep can occur at temperatures greater than 30% of the melting
point on the absolute scale. This may be a concern in the design of boilers
and nuclear reactor containment vessels. Creep of metals occurs in three
phases. The first phase is the result of dislocation movements in the molec-
ular structure of the metal. The second phase is associated with slip at the
grain boundaries, similar to plastic deformation, but accelerated due to the
high temperature. The third phase is associated with an increase in the strain
due to a reduction of the cross section of the specimen. Creep is also consid-
ered in the design of wood and advanced composite structural members.