Wood Handbook, Wood as an Engineering Material

have been used to adjust some lumber properties for the re-

versible effects of temperature.

Irreversible Effects

In addition to the reversible effect of temperature on wood,

there is an irreversible effect at elevated temperature. This

permanent effect is one of degradation of wood substance,

which results in loss of weight and strength. The loss de-

pends on factors that include moisture content, heating

medium, temperature, exposure period, and to some extent,

species and size of piece involved.

The permanent decrease of modulus of rupture caused by

heating in steam and water is shown as a function of tem-

perature and heating time in Figure 5–15, based on tests of

clear pieces of Douglas-fir and Sitka spruce. In the same

studies, heating in water affected work to maximum load

more than modulus of rupture (Fig. 5–16). The effect of

heating dry wood (0% moisture content) on modulus of rup-

ture and modulus of elasticity is shown in Figures 5–17 and

5–18, respectively, as derived from tests on four softwoods

and two hardwoods.

Figure 5–19 illustrates the permanent loss in bending

strength of Spruce–Pine–Fir, Southern Pine, and Douglas-

fir standard 38- by 89-mm (nominal 2- by 4-in.) lumber

heated at 66 °C (150 °F) and about 12% moisture content.

Figure 5–20 illustrates the permanent loss in bending

strength of Spruce–Pine–Fir, Southern Pine, Douglas-fir,

and yellow-poplar standard 38- by 89-mm (nominal 2- by

4-in.) lumber heated at 82 °C (180 °F) and about 12% mois-

ture content. The curves for Spruce–Pine–Fir heated at

66 °C (150 °F) and about 12% moisture content are included

for comparison. The trends in Figure 5–20 can be compared

with the trends in 5–19. In general, there is a greater reduc-

tion in MOR with time at the higher temperature. During

the same time periods shown in Figures 5–19 and 5–20,

modulus of elasticity barely changed. Acid hydrolysis of

hemicellulose, especially of arabinose, appears to be the

fundamental cause of strength loss resulting from thermal

Table 5–16. Percentage change in bending properties of lumber with

change in temperaturea

Property

Lumber

gradeb

Moisture

content

((P–P 70 )/P 70 )100 = A + BT + CT^2

Temperature

range

A B C Tmin Tmax

MOE All Green 22.0350 0.4578 0 0 32

Green 13.1215 0.1793 0 32 150

12% 7.8553 0.1108 (^0)  15 150
MOR SS Green 34.13 0.937 0.0043  20 46
Green 0 0 0 46 100
12% 0 0 0  20 100
No. 2 Green 56.89 1.562 0.0072  20 46
or less Green 0 0 0 46 100
Dry (^0 0 0)  20 100
aFor equation, P is property at temperature T in °F; P 70 , property at 21 °C (70 °F).
bSS is Select Structural.
Figure 5–15. Permanent effect of heating in water (solid
line) and steam (dashed line) on modulus of rupture of
clear, defect-free wood. All data based on tests of Doug-
las-fir and Sitka spruce at room temperature.
Figure 5–16. Permanent effect of heating in water on work
to maximum load and modulus of rupture of clear, defect-
free wood. All data based on tests of Douglas-fir and Sitka
spruce at room temperature.
Chapter 5 Mechanical Properties of Wood