Creep and Relaxation
When initially loaded, a wood member deforms elastically.
If the load is maintained, additional time-dependent defor-
mation occurs. This is called creep. Creep occurs at even
very low stresses, and it will continue over a period of years.
For sufficiently high stresses, failure eventually occurs. This
failure phenomenon, called duration of load (or creep rup-
ture), is discussed in the next section.
At typical design levels and use environments, after several
years the additional deformation caused by creep may ap-
proximately equal the initial, instantaneous elastic defor-
mation. For illustration, a creep curve based on creep as a
function of initial deflection (relative creep) at several stress
levels is shown in Figure 5–22; creep is greater under higher
stresses than under lower ones.
Ordinary climatic variations in temperature and humidity
will cause creep to increase. An increase of about 28 °C
(50 °F) in temperature can cause a two- to threefold increase
in creep. Green wood may creep four to six times the initial
deformation as it dries under load.
Unloading a member results in immediate and complete
recovery of the original elastic deformation and after time, a
recovery of approximately one-half the creep at deformation
as well. Fluctuations in temperature and humidity increase
the magnitude of the recovered deformation.
Relative creep at low stress levels is similar in bending, ten-
sion, or compression parallel to grain, although it may be
somewhat less in tension than in bending or compression
under varying moisture conditions. Relative creep across the
grain is qualitatively similar to, but likely to be greater than,
creep parallel to the grain. The creep behavior of all species
studied is approximately the same.
If instead of controlling load or stress, a constant deforma-
tion is imposed and maintained on a wood member, the ini-
tial stress relaxes at a decreasing rate to about 60% to 70%
of its original value within a few months. This reduction
of stress with time is commonly called relaxation. In
limited bending tests carried out between approximately
18 °C (64 °F) and 49 °C (120 °F) over 2 to 3 months, the
curve of stress as a function of time that expresses relaxation
is approximately the mirror image of the creep curve (defor-
mation as a function of time). These tests were carried out
at initial stresses up to about 50% of the bending strength of
the wood. As with creep, relaxation is markedly affected by
fluctuations in temperature and humidity.
Duration of Load
The duration of load, or the time during which a load acts
on a wood member either continuously or intermittently, is
an important factor in determining the load that the member
can safely carry. The duration of load may be affected by
changes in temperature and relative humidity. The constant
stress that a wood member can sustain is approximately an
Figure 5–20. Residual MOR for solid-sawn lumber at 82 °C
(180 °F) and 80% relative humidity (RH); SPF at 66 °C (150
°F) and 75% RH shown for comparison. SPF is Spruce–
Pine–Fir; MSR, machine stress rated; DF, Douglas-fir; and
So. pine, Southern Pine (Green and others 2005).Figure 5–21. Relationship of ultimate stress at short-time
loading to that at 5-min loading, based on composite of
results from rate-of-load studies on bending, compres-
sion, and shear parallel to grain. Variability in reported
trends is indicated by width of band.Figure 5–22. Influence of four levels of stress on creep
(Kingston 1962).Chapter 5 Mechanical Properties of Wood