Civil Engineering Reference
In-Depth Information
Strain
ε
Fracture
ε
creep rate
Slope
t
ε
Transition
point
t
Initial or
'instantaneous'
strain
Time
t
1st stage
Primary creep
2nd stage
Constant
creep rate
Secondary creep
3rd stage
Tertiary creep
F
IGURE
8.18
Typical creep curve
Closely related to creep is
relaxation.
Whereas creep involves an increase in strain
under constant stress, relaxation is the decrease in stress experienced over a period of
time by a material subjected to a constant strain.
8.6 F
ATIGUE
Structural members are frequently subjected to repetitive loading over a long period
of time. For example, the members of a bridge structure suffer variations in loading
possibly thousands of times a day as traffic moves over the bridge. In these circum-
stances a structural member may fracture at a level of stress substantially below the
ultimate stress for non-repetitive static loads; this phenomenon is known as
fatigue.
Fatigue cracks are most frequently initiated at sections in a structural member where
changes in geometry, e.g. holes, notches or sudden changes in section, cause
stress
concentrations.
Designers seek to eliminate such areas by ensuring that rapid changes
in section are as smooth as possible. Thus at re-entrant corners, fillets are provided as
shown in Fig. 8.19.
Other factors which affect the failure of amaterial under repetitive loading are the type
of loading (fatigue is primarily a problem with repeated tensile stresses due, probably,
to the fact that microscopic cracks can propagate more easily under tension), temper-
ature, the material, surface finish (machine marks are potential crack propagators),
corrosion and residual stresses produced by welding.
Frequently in structural members an alternating stress,
σ
alt
, is superimposed on a static
or mean stress,
σ
mean
, as illustrated in Fig. 8.20. The value of
σ
alt
is the most important
factor in determining the number of cycles of load that produce failure. The stress,
σ
alt
, that can be withstood for a specified number of cycles is called the
fatigue strength
of the material. Some materials, such as mild steel, possess a stress level that can be
