Australian Wood Review – September 2019

http://www.woodreview.com.au 97

TIMBER

requirement to timber reconditioning of collapse prone
ash eucalypts.)

Considering the stress-strain behaviour of wood during
softening, the response in compression within the plastic
range is magnified and becomes large, up to 10x that in the
dry range, while not affecting the tension properties.

Initially it may be thought that green wood might be
easily softened with heat or steam, however as in drying,
prolonged heating of green wood increases the drying rate,
but reduces its strength markedly. Steaming time should be
kept to a necessary minimum, even for dry wood.

Experience has shown in Australia¹ and elsewhere that while
air-dry timber (notionally 12%) can be used, the ‘goldilocks’
MC for softening timber for bending is approximately 20–25%.
Consequently, moisture plays a key role in the ability of wood
to be bent. It was noted that while some pines (Pinus) also
bend well when softened green, this is however unsatisfactory
for hardwoods, probably because the timber becomes more
collapse and check prone on subsequent drying.

Kiln dried timber, unless dried aggressively, should also be no
different from air-dried. Still, if the intent is to dry for bending,
then kiln drying is an unnecessary step as the timber would need
to be steamed for longer to raise its moisture content for optimal
softening. Furthermore, the timber’s MC in the core should also
be below fibre saturation point (FSP) otherwise severe internal
and surface checking is likely to develop upon final drying.

A further point to consider is that air-dried species are prone
to surface checking (as with for example messmate, karri
and others of high density). Here the steaming used for
softening tends to reopen otherwise closed surface checks
which do not close after the bent timber is set and dried.

Stretching and compression

Considering the actual amount that wood surfaces can stretch
and shorten, normally flat pieces can absorb a difference
between outside and inside radiuses of 2–3% (without using
end pressure or steaming). Where a more severe radius is
needed, up to 20 to 30 times the wood’s thickness, steaming
and end pressure and retraining straps can be used.

Softened timber can compress considerably without fracture
however the outer (convex) face tends to stretch only a little
more than non-softened before it tears apart (fracture). Because
of this, softened timber can only be bent to a large radius if a
means of preventing the stretch on the outside face is applied.

A practical way of reducing tension during bending is to
bend slowly and to prevent the ends from expanding by using
bending straps with tightly fitting ends – the idea being to

AUSTRALIAN TIMBER SPECIES

AND THEIR BENDING PROPERTIES

Excellent

Red tulip oak Tarrietia peralata N.Qld

Myrtle beech Nothofagus cunninghamii Ta s

Southern sassafras Atherosperma moschatum Ta s

Celery top pine Phyllocladus rhomboidalis Ta s

Very good

Northern silky oak Cardwellia sublimis N.Qld

Silver quandong Elaeocarpus grandis NSW, N.Qld

Huon pine Lagarostrobus franklinii Ta s

King William pine Athrotaxis selaginoides Ta s

Southern blue gum Eucalyptus globulus Vic, Tas

Radiata pine Pinus radiata plantation

Good

Silver ash Flindersia pubescens,

Flindersia bourjotiana

N.Qld

Yellow siris Albizzia xanthoxylon N.Qld

Rose mahogany Dysoxylum fraseranum N.Qld

Red siris Albizzia toona Qld

Yellowwood Flindersia oxleyana NSW, Qld

Spotted gum Corymbia maculata NSW, Qld

Silver wattle Acacia dealbata Vic, NSW, Tas

Blackwood Acacia melanoxylon Tas, Vic

Mountain ash Eucalyptus regnans Vic, Tas

Silvertop ash Eucalyptus sieberiana Vic, Tas

Messmate

stringybark

Eucalyptus obliqua Vic, Tas

Red stringybark Eucalyptus macrorrhyncha Vic, Tas

Alpine ash Eucalyptus gigantea Vic, NSW, Tas

Mountain grey gum Eucalyptus goniocalyx Vic

Manna gum Eucalyptus viminalis Vic, Tas

Brown stringybark Eucalyptus capitellata Vic

Karri Eucalyptus diversicolor WA

Fair

Spotted gum Corymbia maculata* NSW, northern

NSW, Qld

Red silkwood Lucuma galactoxylon

Blackbutt Eucalyptus pilularis

Poor

Qld maple Flindersia brayleyana N.Qld

Hoop pine Araucaria cunninghamii NSW, Qld

Bad

Rose gum Eucalyptus grandis NSW, Qld

* Spotted gum from the south coast of NSW is said to be superior to

the more highly interlocked material from northern NSW and Qld

Although an extensive classification of Australian timbers produced in

1948 is given in the Trade Circular¹ unfortunately only a few species

listed are available today.

Oposite: Ross Annels (centre) and students work together to bend
steamed timber around a form which it will then be secured in order
to constrain the outside-face stretch. Photo: Linda Nathan