MATERIALS ENGINEERING CONCEPTS - Materials for Civil and Construction Engineers - page 19

degrees. Other materials, such as metals or concrete, are less affected by tem-
peratures, especially when they are near ambient temperature.
Ferrous metals, including steel, demonstrate a change from ductile to
brittle behavior as the temperature drops below the transition temperature .
This change from ductile to brittle behavior greatly reduces the toughness of
the material. While this could be determined by evaluating the stress-strain
diagram at different temperatures, it is more common to evaluate the tough-
ness of a material with an impact test that measures the energy required to
fracture a specimen. Figure 1.13 shows how the energy required to fracture
a mild steel changes with temperature (Flinn and Trojan 1986). The test re-
sults seen in Figure 1.13 were achieved by applying impact forces on bar
specimens with a “defect” (a simple V notch) machined into the specimens
(ASTM E23). During World War II, many Liberty ships sank because the
steel used in the ships met specifications at ambient temperature, but be-
came brittle in the cold waters of the North Atlantic.
In addition to temperature, some materials, such as viscoelastic materi-
als, are affected by the load duration. The longer the load is applied, the larg-
er is the amount of deformation or creep. In fact, increasing the load
duration and increasing the temperature cause similar material responses.
Therefore, temperature and time can be interchanged. This concept is very
useful in running some tests. For example, a creep test on an asphalt con-
crete specimen can be performed with short load durations by increasing the
temperature of the material. A time-temperature shift factor is then used to
adjust the results for lower temperatures.
Viscoelastic materials are not only affected by the duration of the load,
but also by the rate of load application. If the load is applied at a fast rate,
the material is stiffer than if the load is applied at a slow rate. For exam-
ple, if a heavy truck moves at a high speed on an asphalt pavement, no per-
manent deformation may be observed. However, if the same truck is parked
on an asphalt pavement on a hot day, some permanent deformations on the
pavement surface may be observed.

1.2.9 Failure and Safety
Failure occurs when a member or structure ceases to perform the function
for which it was designed. Failure of a structure can take several modes,

90

60

30

0

50 30 10 10 30 50

FIGURE 1.13 Fracture toughness
of steel under impact testing.

Temperature, C