International Academy of Wood Science

ACADEMY LECTURE

WOOD AS NATURAL SMART MATERIAL

Boris UGOLEVMoscow State Forest University, Russian Federation

Saint-Petersburg – Moscow, Russia2009

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«It will not be too pessimistic to state that we shall not learn to know the wood ultrastructuretruths before 2050 or perhaps 3000. Anyhow it is most fascinating science to deal with .»

A. Bjorkman “Wood Science1920-2003-?”

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• Nature that created wood as ultramicroscopic miracle, prompted to mankind to produce diverse modern artificial materials ranging from reinforced concrete to nanocomposites. Further studies of wood nanostructure discover new possibilities of biomimetic approach to creation of effective materials.

• Material scientists predict a prominent role of artificial smart materials in future. This term designates material which usefully reacts to the changes in environmental parameters.

• One of the main features of smart materials is «shape-memory effect». It means that these materials after forced change of the form are able to restore their initial form when original physical condition is recovered.

• Metallic shape memory alloys were synthesized the first. Later were created ceramics and polymers with the same property.

• We find a very broad spectrum of smart materials applications from deployable space structures to self-repairing auto bodies, switches, sensors, kitchen utensils, tools up to minimally invasive surgery and implants in biomedicine.

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• fiber-reinforced elastic memory composites (Abrahamson et al. 2002)

• problems of behaviour of the shape memory polymers (Lendlein and Kelch 2002)

(Примеры актуальных проблем для искусственных умных материалов)

Examples of Topical Problems for Artificial Smart Materials

• internal stress induced by constrained expansion of memory polymer nanocomposites (Gall et al. 2004)

• model development for shape memory polymers(Siskind et al. 2008)

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• Hygroscopicity of wood gives possibility to use it as a sensor of surrounding air humidity.

• Wood constructions of buildings regulate, in some degree, the air humidity in living premises, by drying or humidifying of the air at changing weather.

• Swelling of wood tightens joints in articles and structures.

• Pit props of pine and spruce wood emit crackling which warns of coming mine destruction.

• Acoustic emission is used for monitoring of lumber stress state at drying.

• Sonorous ability of wood reveals itself in musical instruments.

• Piezoelectric properties of wood permit to create nondestructive methods of strength testing.

• Et cetera.

(Примеры полезных свойств древесины, проявляющихся

при внешних воздействиях)

Examples of Wood Useful Properties under External Influence

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The notion «memory of wood» was introduced by us at the beginning of the 1980s. Previously some researchers (Т. Takemura (1973) et al.) had used the analogous term for indication of purely temporal phenomena. In our case this is a metaphor which reflects the ability of wood to react to the restoration of initial physical state determined by its moisturecontent and temperature.

«Wood memory effect» is based on quazi-residual «frozen»strains

They were experimentally discovered by us at constrained shrinkage of wood in the early 1960s.

(FS)

(Древесина обладает доминантным признаком умных

материалов – «эффектом памяти» )

Wood Possesses «Memory Effect» –The Dominant Feature of Smart Materials

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ε = –βW + αW+ ∫ +τ στσ0

21 ),(),( TWEHd

TWEH

&

Integral Law of Wood Straining at Drying and Wetting

Here: ε – strainβ – coefficient of shrinkageW=∆w – moisture content decrease from limit saturation of cell wall (FSP)α– coefficient of swellingσ – stressE – stiffness modulusТ – temperature decrease from 100˚CW(τ),T(τ) and are functions of time τH1 and H2 are Havisade`s functions accordingly for drying and wetting

)(τσ&

(Общий закон деформирования древесины при сушке и увлажнении)

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Here׃ εe – elastic strain; εv – viscous strain; εev – elastic- viscous strain; εc – creep; σ – stress

εεεεe

εεεεc

εεεεv

εεεεe

εεεεc

εεεεev

σσσσ====const σσσσ====0

ττττ

ε

τ

Strain (ε) – Time (τ) Relation of Wood at Constant Moisture Content

(Зависимость деформаций(ε) древесины от времени (τ) при постоянной влажности)

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Here: σ – stress; ε – strain; ∆w – drop of moisture content.1– wet wood ( w > 30%)2 – dry wood

σσσσ

1

2

4 1 1 9 7

0 1 8 6 5

10 ′

1 2

∆W

0 3 1' 2' εεεε

Changes of Wood Hygromechanical Strains(Изменения гигромеханических деформаций древесины)

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Changes of Strains ε, Stresses σ and Moisture Content w in Time τ of Experiment

1 – wet wood2 – dry wood

,h

(Изменения деформаций ε, напряжений σ и влажности w во времени τ)

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Scheme of Forming of Frozen Elastic-ViscousStrain at Wood Drying

εf = εev1– εev2

Frozen strainsare the result of temporary reconstruction of wood nanostructure under the control load influence at increasing wood stiffness processes of drying or cooling

(Схема образования замороженных упруго-эластическихдеформаций древесины при сушке)

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Contribution of Frozen Strains to Stress-Strain State of Wood at Drying

The FS as a part of set-strain are the main reason for the forming of dried lumber casehardening

εs = εf + εr

(Вклад замороженных деформаций в напряженно-деформированное

состояние древесины при сушке )

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Frozen Strains Were Taken into Account at the Drying Stress Calculation

( )15,0 −+= jj EEEw∆

σ – stressβ – coefficient of shrinkageE – stiffness modulus

– average stiffness modulus– moisture contents drop from limit of cell wall saturation (FSP)

j – step numberi – slice numbern – quantity of slices

Here:

∆−∆+= ∑ ∑= =

−n

i

n

i

ji

ji

ji

ji

ji

ji

ji EwEwE

1 1

1 /βσσ

Step-by-step method for multy-sliced model of drying lumber

(Замороженые деформации были учтены при разработке метода

расчета сушильных напряжений )

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Lumber drying schedules parameters

Product quality parameters

• temperature

• humidity

• time

• moisture content

• moisture content gradient

• stress

Lumber Drying Schedules and Product Quality(Режимы сушки пиломатериалов и качество продукции)

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Detection of Lumber Drying Stresses

(Определение сушильных напряжений в пиломатериалах)

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Method of Lumber Drying Stresses Monitoring by Differential Shrinkage (DS)

(Метод контроля сушильных напряжений в пиломатериалах

по дифференциальной усадке)

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Scheme of Wood Deformative Conversions at Heating

0

5

1

2

6

3

8

4

7

θθθθ

σσσσ

εεεε εεεε′′′′=const

θθθθΙΙΙΙΙΙΙΙ

θθθθΙΙΙΙ

εεεε0=const

εεεεf

t1 σσσσr

σσσσR

Here: Ө = 100° -t – drop of temperatureσR = Εt(ε0 – εf ) at ε0 = const (case a)σr = Εt(ε′– εf ) at ε′ = const (case b)Εt – modulus of elasticity

(Схема деформационных превращений древесины при нагревании)

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Wood «Stress Memory Effect» at Heating

loaded wood and constanttotal strain

unloaded wood and constantfrozen strain

0

0,2

0,4

0,6

20 40 600

0,4

0,8

1,2

20 40 60

1

3 2

6

4

32

7

1

8

σσσσ,МPа σσσσ,МPа

σσσσR

σσσσr

t, °Ct, °Ca b

Pinus sibiricaDu Tour, compression, tangential direction across the grain,

ε0=0,009

Quercus roburL., tension ,radial direction across the grain,

ε′=0,007

(«Эффект силовой памяти» древесины при нагревании)

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Change of the Form of Ash Veneer and «Strain Memory Effect»

a) original state (heated wood)

b) under load c) after cooling

d) after unloading( «frozen strains»)

e) after heating

(Изменение формы ясеневого шпона и «эффект

деформационной памяти древесины)

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The Scheme of the Wood Specimen Loading

b= 10А

140

а

t

r I Р

h=10А

l =100

II Р

(Схема нагружения образца древесины)

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«Strain Memory Effect» at Heating of Previously Loaded (Tension – Compression) and then Unloaded Wood

-0,002

-0,001

0

0,001

0,002

0,003

0,004

0,005

0,006

0,007

εεεε

20 40 60 t , °°°° C 1

6

2

3

2

7 5

4 4

а)

b) After loading – cooling – unloading

After heating

(«Эффект деформационной памяти при нагревании предварительно

нагруженной (растяжение – сжатие) и затем разгруженной древесины)

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Contribution of Hygrofrozen Strains (HFS) and ThermofrozenStrains (TFS) in Complex Rrozen Strains (CFS)

Complex frozen strain (CFS)

Wood drying and cooling under loadconsecutive simultaneous

HFS

TFSTFS

CFS+SESCFS

HFS

(Вклад гигрозамороженных деформаций (HFS) и термозамороженных

деформаций (TFS) в комплексные замороженные деформации (CFS))

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Using Hygro-Thermofrozen Strains to RemoveVeneer Waviness

02468

1012

0 30 60 90 120 150 180 210 240

Width of specimen, mm

Hei

gh

t of

wav

es,

m

m

1

2

1 – original state, 2 – after cooling and drying of loaded specimen.

(Использование гигро-термозамороженных деформаций для

устранения волнистости шпона)

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Wood Stress-Strain Behavior of the Surface Zone of a Board at Drying and Conditioning Moisture-Heat Treatment

0

1

3

2

4

-σσσσ

+εεεε

+σσσσ

-εεεε

(Напряженно-деформированное состояние поверхностной

зоны доски при сушке и кондиционирующей

влаго-теплообработке)

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IR- Absorbtion Spectrum of Birch Wood

- - - - after free drying—— after constraind shrinrage

(ИК- спектры поглощения древесины березы)

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Experimental Device

(Экспериментальная установка)

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Scheme of Wood Deforming at Constant Load, Drying and Unloading

βf

βf

εv +εεεε εe εc

εs β*

β

εf εv εc εf εe

εs

-εεεε

σσσσ

β*

7′′′′ 2

2′′′′ 2′′′′

(2) 1 0′′′′

5 6 7 3 4 0 (4)

εevc

Ee2 Eev1 Eev2

Ee2> Eev2 >Eev1

Here׃ 0-1– tension of wet wood1-2– drying of loaded wood2-3-4–unloading and time exposure of dry woodsections0-4׃ – «reduced shrinkage»β*

4-5 – frozen elastic-viscous strain εf 5-6 – creep εs4-6 – quasi-residual set-strain εs

6-7 – « frozen shrinkage»βf0-7 – free shrinkage β

(Схема деформирования древесины при постоянной нагрузке, сушке и разгрузке )

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Effect of Tension Load on Reducing Shrinkage Degree

1–Fraxinus excelsior L., t = 80 °C (our date)2–Fagus orientalis, t = 20°C (by N.Chulitsky)

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

K=ββββ*/ββββ

0 0,2 0,4 0,6 0,8 1 1,2 1,4 σσσσ, MPa

1

2

(Влияние растягивающей нагрузки на степень

уменьшения усушки )

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Detection of «Frozen Strain» and «Frozen Shrinkage»at Drying Fastened Specimen

0

0,01

0,02

0,03

0,04

0,05

2 4 6 8 10 ττττ , h

0

30

60

90

0 2 4 6 8 10

0

1

2

0 2 4 6 8 10

- εεεε ,,,, ββββ ,,,, - αααα

σσσσ , МPа

W, %

W fsp

ττττ , h

ττττ , h

ββββ f

2 **

εεεε f

εεεε c

2

3 4

5

1

ββββ αααα

6 *

6

2 *

6 **

7

7 ′′′′

6 ′′′′ (5 ′′′′ ) 4 ′′′′

1 ′′′′

2 ′′′′

3 ′′′′

1 ′′′′ ′′′′ 2 ′′′′ ′′′′ 3 ′′′′ ′′′′

4 ′′′′ ′′′′

5 ′′′′ ′′′′ 6 ′′′′ ′′′′ 7 ′′′′ ′′′′

ββββ *

Fraxinus excelsior L., tangential direction across the grain, t = 80 °C

(Определение «замороженной деформации» и «замороженной

усушки» при сушке закрепленного образца)

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Free Shrinkage and Stress-Strain State at Wood Drying

1

2

3

4

5

-0,05 -0,04 -0,03 -0,02 -0,01 0 0,01 0,02 0,03 0,04 0,05

σσσσ, МПа

εεεε -εεεε

1 2 3

4

5

∆∆∆∆σσσσ

6 6** 6*

ββββf

ββββ' ββββ

4′′′′

2*

εεεεev=ββββ* εεεεc εεεεf

Fraxinus excelsior L., tangential direction across the grain, t=80 °C

(Свободная усушка и напряженно-деформированное состояние

древесины при сушке)

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∑=

∆⋅=n

iii twEwK

1

),()(σσ β

Drying Stress Calculation Equation with Reducing Shrinkage Coefficient

Here׃ Кβ – coefficient of shrinkage∆w – moisture content drop from limit saturation of cell wall (FSP)σ – stressE – stiffness modulust – temperature decrease from 100 °Cw – moisture content

(Уравнение для расчета сушильных напряжений с

уменьшающимся коэффициентом усушки)

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Decreasing of Wood Stiffness at Hygrofatigue

n– number of sorption–desorption cycles: MC from 12 to 20 %Picea abies; tangential direction across the grain

(Уменьшение жесткости древесины при гигроусталости)

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Wood Deformative Conversions at Change of Moisture and/or Content or Temperature

(Деформационные превращения древесины при изменении

влажности и/или температуры)

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Wood Strains at Drying or Wetting(Деформации древесины при сушке или увлажнении)

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Classification gives an opportunity to consider it as a peculiar Mendeleev's table and detect “blank spots”, lay out

research programs. Further research of deformativeconversions will permit to improve wood technology and

create new smart wood composites.

Thank you for your attention