503 335w4 01 wood structure 1 v2

Wood s structure and variability 1 Part A The macroscopic structure of wood Characteristics we can readily see Characteristics we can see www.csaw.utas.edu.au KDA503/335 Timber, its

Trang 1

Wood s structure and variability 1

Part A

The macroscopic structure of wood

Characteristics we can readily see

Characteristics we can see

www.csaw.utas.edu.au

KDA503/335 Timber, its origin and characteristics

Content

•  Structure from the cycle of

growth (Part A)

•  Variation in wood

properties (Part B)

–  Species, climate and age

effects

–  Knots and other impacts of

natural action on structure

and character

Constraints

•  There are numerous species of trees, growing

in a vast array of locations and climate

•  Given this genetic and material diversity, only broad issues can be covered here

•  There will always be exceptions to the items discussed

Major botanical grouping of species

•  Gymnosperms, softwoods,

species of trees bearing

seeds in cones,

–  needle-like leaves

–  generally lower densities

–  often light in colour

•  Common Australian

softwoods include

–  radiata pine

–  cypress pine

–  hoop pine

Major botanical grouping of species

•  Angiosperms, hardwoods,

flowering trees bearing covered seeds

–  broad leaf –  generally higher densities –  often dark in colour

•  Common Australian hardwoods include:

–  All eucalypts

•  Jarrah, messmate, mountain ash, yellow gum, blue gum

–  brown box –  spotted gum

Trang 2

Tree growth and the character of wood

Wood s structure and properties

•  Wood is the natural

material drawn from the

trunks of trees

•  Its structure and properties

relate directly to the tree s:

–  cycle of growth

–  species

–  surrounding environment,

particularly climate

–  age

Structure from the cycle of growth

Tree growth

•  Seeds germinate

•  Seedling establishes

•  New wood forms on the growing tip and the outside of stem

•  The young tree grows tall quickly to compete for light

•  Further wood layers form on the outside of the stem

•  As the tree ages, parts of the wood mature

Drawings by Bruce Greaves

Tree Growth

•  New wood on outside of tree

–  oldest wood on inside of tree –  youngest wood on outside –  diameter largest at base –  one ring (layer) per year

•  Trees grows towards light

–  trunk straight –  lower branches die –  leaving small knots in wood

•  Bark protects wood from damage

–  tree sheds bark each year

Zones of wood

Pith / heart

•  cells from the original sapling or growing tip

Cambium

•  growth cells that created wood cells on the

inside face and bark cells on the outside

Sapwood

•  newest wood on the outside areas of the stem that

transports nutrients between the root and the leaves

Heartwood

•  older wood cells filled with extractives and other

material to increase durability and support for the tree

Bark

•  A protection layer New bark is made

continually with portions falling off each year

Zones of wood

Heartwood

Pith / heart Bark

Cambium Sapwood

Trang 3

Zones of wood: pith

Pith / heart / juvenile wood

•  The pith or heart is found near

the centre of the stem

•  Made up of soft spongy tissue

from the original sapling or

growing tip, its function is

nutrient storage and transport

•  Generally, its quality is much

lower than mature wood, and

this complicates both production

and design

Pith / heart / juvenile wood - material

Compared to mature wood, pith or heart material often:

•  has shorter length cells and fewer latewood cells

•  has a tendency for spiral grain

•  in hardwoods, shrinks and moves excessively during drying

•  is only 85 to 90% as dense

•  is only 50 to 70% as strong or stiff

•  is relatively low durability

Pith / heart / juvenile wood - material

Shrinkage of heart

wood in floor joists

Juvenile wood

Effect of juvenile wood on physical and mechanical properties:

a.  Juvenile core located in the interior of the tree bole

b.  properties that increase from juvenile to mature wood

c.  properties that decrease from juvenile to mature wood

Zones of wood: cambium

Cambium

•  A layer of cells capable of

repeated division that remains

active throughout the tree life

•  Located at the periphery of the

wood, it produces phloem (bark)

on one side of tree and sapwood

(new wood) on the other

•  This increases the diameter of the

tree and pushes the cambium

progressively outward

Zones of wood: bark

Bark

•  an outer protective layer on the

wood consisting of an outer bark and inner layer called phloem

•  The phloem conveys nutrients from the leaves to the various parts of the tree

•  As the tree’s diameter increases, the bark has to stretch, fracture, peel or fall off

Trang 4

Bark patterns

Zones of wood: sapwood

Sapwood

•  The more recently formed wood on the outside areas of the stem

•  Living and open cells and vessels that transports water and nutrients between the root and the leaves

•  Containing starches but lacking protective extractives, sapwood is attractive to insects & fungi

•  It is always classed as low durability

•  Cell walls increasing in thickness as tree grows

•  They are often pale in colour and distinct, especially in hardwoods

Zones of wood: heartwood

Heartwood

•  The heartwood is dead, extractive-filled

cells whose main function is

mechanical support to the tree

•  The tree needs to maintain both

adequate conducting tissue and

adequate support as it grows

•  As new wood is formed each year,

some part of the inner sapwood

becomes redundant for water transport

and is converted into non-conducting

heartwood

•  So, the outer heartwood boundary

continually moving outwards

Heartwood growth

•  Young vigorously growing trees may contain little or no heartwood and have a broad sapwood band

•  In some species, heartwood may not develop for many seasons

•  Once heartwood starts to form, it develops at a greater rate than diameter growth

Sapwood band in 19yo E globulus

Extractives and their effects

•  Extractives are the products of decomposition of

starches, sugars, fats, waxes, oils, gums, tannins and

aromatics, stored in the heartwood

•  Toxic extractives impart decay and insect resistance

This is the case in redwood, Huon pine and most

cedars

•  Aromatic extractives impart a fragrance

•  Other extractives strengthen heartwood while making

it difficult to dry and penetrate with chemical

preservatives

•  Significant amount of extractives can make the

heartwood heavier than sapwood

Heartwood colour

•  Hardwoods have a wider range of heartwood colouration than softwood

•  Generally sapwood is distinctly lighter in colour

–  In some woods, heartwood and sapwood show no colour differences

Blackwood, Acacia melanoxylon Darwin Stringybark, E tetradonta

Trang 5

Comparing sapwood and heartwood

•  Compared to sapwood, heartwood is

generally:

– heavier,

– stronger,

– more highly figured, and

– more resistant to decay

•  Being wood formed in a more mature

tree, sapwood in young trees can be

denser and stronger than adjacent

heartwood

–   more on this later

Part B

The macroscopic structure of wood

Characteristics we can readily see

Growth generates directionality

•  Wood is anisotropic Its properties vary:

–  radially: across the wood’s growth sequence –  tangentially: around the wood’s growth sequence –  longitudinally - along the direction of the wood

radial

tangential

softwood

Impact of directionality

•  Wood directionality is arguably

the largest single consideration

in timber design and

production, influencing:

–  Strength,

–  Appearance,

–  Other primary design

considerations

–  Shrinkage

•  from green to EMC and

movement in service

–  Primary production constraints

Directionality and strength

Along the grain: 10 units

Across the grain: > 1 unit

Trang 6

Directionality and appearance

Back sawn or crown cut

mixed grain direction that can

include cathedrals, crowns or

swirls along the face of the

board

Quarter sawn or quarter cut

relatively straight, even and parallel

grain running along the board

Directionality and shrinkage

Species Radial% Tang.%

Softwood

Hardwood

Timber shrinks and moves at different rates in its principal directions

Shrinkage from 25% to 12% MC

The results of poor installation practice and predictable expansion in a domestic timber floor

Growth generates stress

Growth for height is a key evolutionary trait

Growth and stress

As thin cantilevered columns, trees generate peripheral tensile stresses to resist horizontal forces This is balanced in the standing tree by an inner compressive reaction

Longitudinal strain profile in log originating from growth stress, showing pattern for

different sized logs (Waugh 2000 from Kubler 1959)

Consequences of growth stress

Sawmill waste: 12-year-old E nitens thinned and pruned

Reaction wood

•  Trees produce reaction wood in response to eccentricity in loads

on the stem: wind, branches or slope

•  Reaction wood in:

–  hardwoods is tension wood: Pulling against the load –  softwoods is compression wood: Pushing against the load

From Bamber, 2001

Trang 7

Uneven growth and reactions

•  Softwoods produce compression wood on the

underside of eccentric branches or leans to “push”

them back in line

•  Compression wood is expressed as

–  An eccentric pith on the upper side of the stem

–  exceptionally wide growth rings on the lower

compression side of the stem

•  These rings contain a higher proportion of latewood

•  The cells contains more lignin, and under a

microscope, the cells appear more rounded than

rectangular

•  Compression wood has greater longitudinal

shrinkage compared to normal wood (1-2% vs

0.1-0.2%)

•  Usually denser (up to 40% higher at times), its

strength is about the same as normal wood

•  Opposite wood refers to wood formed on the

opposite side of the stem from compression wood

Drawings by Rob Whiltshire

Eccentric pith location at the base of a harvested pine log

•  Hardwoods produce tension wood on the

upper side to “pull” them back

•  Stems with tension wood are often

elliptical

•  Like compression wood, tension wood:

–  has wider rings compared to rings on the

opposite side of the stem

–  shrinks excessively along the grain

–  has strength properties inferior to normal

wood of similar density

–  is not easy to detect and difficult to

discard during manufacture

•  Under a microscope, tension wood cells

have very thick walls and very small

lumens

•  Thick and loosely attached secondary

walls manifests as fussy and woolly

surfaces when the timber is cut,

especially when processed green Drawings by Rob Whiltshire

Species, climate, and age effects

Species effects

•  Each species has a unique genetic make-up

•  Aspects of material properties vary:

–  between species groups

•  softwood/hardwood, collapse prone/non-collapse prone

–  within species groups

Species effects

rays

cells!

fibres vessels hardwood earlywoo

d rays

latewood

softwood

Softwood

•  Mainly conifers with needle-like leaves

•  The cell structure is relatively open &

all sapwood cells transport nutrients

•  Most softwoods have relatively thin cell walls and are lighter in colour

Hardwood

•  Mainly broad leafed

•  Vessels transport nutrients

•  The cell structure in the heartwood is generally closed

•  Most hardwoods have relatively thick cell walls and are darker in colour

Trang 8

Climate effects: growth rings

•  Climatic fluctuations such as summer/winter or

wet/dry season affect the tree s growth rate

•  The availability of light, nutrients and water influences cell formation in the cambium

•  As days become shorter and colder or moisture levels declines, cell division in the cambium slows and can ceases During

this slowdown, latewood forms

–  higher density wood of smaller cells with thicker

walls and smaller lumens

•  As seasons change and days lengthen and become warmer or moisture become available, cell formation in the cambium

increases and earlywood forms

–  lower density wood of often bigger cells with thinner walls and bigger lumens

Photos: Geoff Boughton

•  In a true growth ring, the latewood to earlywood

transition is abrupt

•  When normal growth is interrupted by drought or late

frost, false rings may occur with two or more rings

being formed in a single year

•  False rings tend to have a gradual transition of the cell

size on both sides

www.swst.org/teach/set2/redwoo~1.jpg

Age effects: property development

zone of wood property change

zone of unchanging wood properties

(age of deposited wood – years)

(age of log – years)

Knots and other features

The marks of growth, insects and

living on the wood

Knots

•  Knots are parts of a branch enclosed within the wood of the stem

•  If the branch is living, the cambium of the stem and branch

are continuous and an inter-grown or tight knot resulting

•  If the branch has died, the cambium is not continuous and an

encased or loose knot result

•  The appearance of the knot depends on the direction of cut through the included branch

•  If a cut is made along the axis of the original branch, the knot appears as a spiked knot

Encased knots

Trang 9

Occluded knots

•  If living branches are pruned,

the branch stub will occlude

(grow over)

•  If a dead branch is pruned,

the branch stub may be

drawn out by the growing

tree leaving a resin-tract

Kino or resin tract Occluded branch

Limb trace

Insects

•  Various insects live in the bark and stem, leaving trails in the wood The appearance of the tracks varies with insect and the direction of cut

Fire, shakes, impacts and other damage

Fire, impacts and wind damage can wound the

stem The tree responds with sap or kino, especially

in eucalypts

Summary

•  There are numerous species of trees, growing in a vast array of locations and climate

•  Wood structure and properties relate directly to the tree s cycle of growth; species; surrounding environment, particularly the climate; and age

•  Wood is anisotropic Its properties vary radially, tangentially and longitudinally

•  Each species has a unique genetic make-up and aspects of material properties vary both within and between species groups

•  These variations can be critical to how we:

–  make wood products –  use them in design

Định dạng
Số trang	9
Dung lượng	1,58 MB