products may be reduced in strength to a greater extent than
is lumber. This additional effect on fiber-based composites
may be more a function of internal bond damage caused by
waterborne-treatment-induced swelling rather than actual
chemical hydrolysis.Initial kiln-drying temperature—Although initial kiln
drying of some lumber species at 100 to 116 °C (212 to
240 °F) for short durations has little effect on structural
properties, such drying results in more hydrolytic degrada-
tion of the cell wall than does drying at lower temperature
kiln schedules. Subsequent preservative treatment and re-
drying of material initially dried at high temperatures cause
additional hydrolytic degradation. When the material is sub-
sequently treated, initial kiln drying at 113 °C (235 °F) has
been shown to result in greater reductions over the entire
bending and tensile strength distributions than does initial
kiln drying at 91 °C (196 °F). Because Southern Pine
lumber, the most widely treated product, is most often ini-
tially kiln dried at dry-bulb temperatures near or above
113 °C (235 °F), treatment standards have imposed a maxi-
mum redrying temperature limit of 74 °C (165 °F) to pre-
clude the cumulative effect of thermal processing.
Incising—Incising, a pretreatment mechanical process in
which small slits (incisions) are punched in the surface of
the wood product, is used to improve preservative penetra-
tion and distribution in difficult-to-treat species. Incising
may reduce strength; however, because the increase in
treatability provides a substantial increase in biological
performance, this strength loss must be balanced against the
progressive loss in strength of untreated wood from the inci-
dence of decay. Most incising patterns induce some strength
loss, and the magnitude of this effect is related to the size
of material being incised and the incision depth and density
(that is, number of incisions per unit area). In <50-mm-
(<2-in.-) thick, dry lumber, incising and preservative treat-
ment induces losses in MOE of 5% to 15% and in static
strength properties of 20% to 30%. Incising and treating
timbers or tie stock at an incision density of ≤1,500 inci-
sions m–2 (≤140 incisions ft–2) and to a depth of 19 mm
(0.75 in.) reduces strength by 5% to 10%.
In-service temperature—Both fire-retardant and preserva-
tive treatments accelerate the thermal degradation of bend-
ing strength of lumber when exposed to temperatures above
54 °C (130 °F).
In-service moisture content—Current design values apply
to material dried to ≤19% maximum (15% average) mois-
ture content or to green material. No differences in strength
have been found between treated and untreated material
when tested green or at moisture contents above 12%.
When very dry treated lumber of high grade was tested at
10% moisture content, its bending strength was reduced
compared with that of matched dry untreated lumber.
Duration of load—When subjected to impact loads, wood
treated with chromated copper arsenate (CCA) does not
exhibit the same increase in strength as that exhibited by
untreated wood. However, when loaded over a long period,
treated and untreated wood behave similarly.
Polymerization
Wood is also sometimes impregnated with monomers, such
as methyl methacrylate, which are subsequently polymer-
ized. Many of the mechanical properties of the resultant
wood–plastic composite are greater than those of the origi-
nal wood, generally as a result of filling the void spaces in
the wood structure with plastic. The polymerization process
and both the chemical nature and quantity of monomers in-
fluence composite properties.Nuclear Radiation
Wood is occasionally subjected to nuclear radiation. Exam-
ples are wooden structures closely associated with nuclear
reactors, the polymerization of wood with plastic using
nuclear radiation, and nondestructive estimation of wood
density and moisture content. Very large doses of gamma
rays or neutrons can cause substantial degradation of wood.
In general, irradiation with gamma rays in doses up to about
10 kGy has little effect on the strength properties of wood.
As dosage exceeds 10 kGy, tensile strength parallel to grain
and toughness decrease. At a dosage of 3 MGy, tensile
strength is reduced about 90%. Gamma rays also affect
compressive strength parallel to grain at a dosage above
10 kGy, but higher dosage has a greater effect on tensile
strength than on compressive strength; only approximately
one-third of compressive strength is lost when the total
dose is 3 MGy. Effects of gamma rays on bending and shear
strength are intermediate between the effects on tensile and
compressive strength.Mold and Stain Fungi
Mold and stain fungi do not seriously affect most mechani-
cal properties of wood because such fungi feed on substanc-
es within the cell cavity or attached to the cell wall rather
than on the structural wall itself. The duration of infection
and the species of fungi involved are important factors in
determining the extent of degradation.
Although low levels of biological stain cause little loss in
strength, heavy staining may reduce specific gravity by 1%
to 2%, surface hardness by 2% to 10%, bending and crush-
ing strength by 1% to 5%, and toughness or shock resistance
by 15% to 30%. Although molds and stains usually do not
have a major effect on strength, conditions that favor these
organisms also promote the development of wood-destroy-
ing (decay) fungi and soft-rot fungi (Chap. 14). Pieces with
mold and stain should be examined closely for decay if they
are used for structural purposes.Decay
Unlike mold and stain fungi, wood-destroying (decay) fungi
seriously reduce strength by metabolizing the cellulose frac-
tion of wood that gives wood its strength.Chapter 5 Mechanical Properties of Wood