Paola MAZZANTI

IUFRO Division 5 Conference

5.02.00 Physiomechanical properties of wood and wood based materials and their

applications

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University of FlorenceDISTAF Wood Technology SectionVia S. Bonaventura, 1350145 FlorenceItaly

Mechanical characteristics of Poplar wood (Populus alba L.) across the grain

Paola MAZZANTI*Luca UZIELLI

Italy

European Union

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Aim of work and methods

The main aim of this research is the knowledge of Poplar wood rheological behaviour in order to apply it to a better conservation of Wooden Cultural Heritage, and specifically to a mathematical modelling of deformations and stresses in painted panels, when subjected to variations of environmental parameters (Temperature and Relative Humidity)

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Aim of work and methods

In fact, since several years Researchers at DISTAF are engaged towards improving knowledge and conservation of wooden artworks.

Several activities have been developed towards such objective, including:

- ongoing research on Leonardo da Vinci’s “Mona Lisa” at Louvre Museum (together with French Colleagues from Montpellier and Nancy)

- proposing and leading the new COST Action IE0601 “Wood Science for Conservation of Cultural Heritage (WoodCultHer)” www.cost.esf.org www.woodculther.com

- monitoring behaviour of mock panels and original artworks, in Laboratory and in Churches and Museums

- national and international cooperations with Wood Scientists, Conservators and Restorers

DISTAF activities

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Aim of work and methods

Painted panels are complex structures made of a wooden support and painted layers

Support: poplar wood (Populus alba L.)Painted layers: “cheese” or hot-melt animal glues, gypsum, tempera, varnish

Painted panels are heterogeneous structures

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Aim of work and methods

Conservation is significantly influenced by environmental condition variations (RH% and T):

damages on the wooden support

Cupping and cracks are caused by mechanical stresses related to the support structural features and moisture gradients along the panel thickness

Compression set shrinkage (shown according to Hoadley, 1995)

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Aim of work and methods

Conservation is significantly influenced by environmental condition variations (RH% and T):

damages on painted layers

Fractures, buckling and detachments can be caused by the interaction between wooden support and paint layers

Buck, 1963

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Aim of work and methods

Characterization of Poplar (Populus alba L.) wood behaviour across the grain

Physical: density, swelling/shrinkage values, diffusion coefficients, moisture gradient distributions Mechanical: MOE, strength, creep and mechano-sorptive deformations, relaxation, compression set shrinkage, swelling pressure

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Aim of work and methods

Wooden material

Fig. 1: specimens

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Poplar wood from one same board

10x20x40 mm (long term test)

30x30x30 mm (short term tests)

ρ12%=0,37 g/cm3

EMC= 6%, 12% or 15%

Fig. 1: specimens

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Aim of work and methods

Environmental test conditionsConstant climate:

dry (30% RH, 30°C, 6% EMC)normalized (65% RH, 20°C, 12% EMC) humid (85% RH, 30°C, 15% EMC)

Variable climate: cyclic humidity variations (30% 80% 30%)

cyclic EMC (6% 15% 6%)constant temperature (30°C)

Fig. 2: variable climate conditions

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Aim of work and methods distaf

Loading test conditions

Short term loading (constant climate conditions): strengthMOE

Long term loading (variable climate conditions): swelling pressure relaxationcompression set shrinkage

Compression

Tension

(Bending)

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Constraint test conditions (variable climate)

B: Free to shrink and prevented from swelling (measured both free shrinkage and restraining force)

Specimens oriented along tangential direction

C: Prevented from deforming (measured: restraining force)

A: Free to shrink and swell (measured: shrinkage/swelling)

AB

C

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STRENGTH according to UNI EN 408 Min. 3 MPa Max. 5 MPa Minor variations with EMC and

load direction

Fig.3: Strength as function of loading direction and EMC

Short term loading tests: compression

0,00

200,00

400,00

600,00

800,00

0 45 90

MO

E [

MP

a]

0,00

2,00

4,00

6,00

0 45 90

Stre

ngth

[M

Pa]

6% EMC 12% EMC 15% EMC

Fig.4: MOE as function of loading direction and EMC

MOE Min. 150 MPa Max. 720 MPa Minor variations between

45° and 90° load directions Significantly larger (and

spread) along 0° (=radial) load direction

Direction between load and growth rings: 0°= RADIAL 45°= INTERMEDIATE 90°= TANGENTIAL

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OE

[M

Pa]

Stre

ngth

[M

Pa]

Fig.6: Graph of MOE as function of anatomical direction and EMC

Short term loading tests: tension

0,00

2,00

4,00

6,00

0 45 90

0,00

200,00

400,00

600,00

800,00

0 45 90

Anatomical direction

6% EMC 12% EMC 15% EMC

Fig.5: Graph of strength as function of anatomical direction and EMC

STRENGTH Min. 2 MPa Max. 6 MPa Homogeneous values for 45°

and tangential directions At 12% EMC specimens

show higher strength

MOE Min. 150 MPa Max. 600 MPa Homogeneous values for 45°

and tangential directions Variable values for radial

specimens

Direction between load and growth rings: 0°= RADIAL 45°= INTERMEDIATE 90°= TANGENTIAL

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Fracture edge

Fracture edge

Crack propagation along middle lamella in fibers

Fracture of vessel

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Fig.9: deformation against time

Long term loading tests:deformation induced by sorption/desorption cycles

Free to shrink and prevented from swelling Free to swell/shrink Shrinkage of specimen

prevented from swelling is about one half of the shrinkage of specimen free to deform

Fig.10: deformation of specimen B (partially prevented from deforming) in successive cycles

Free to shrink and prevented from swelling

Shrinkage of specimen prevented from swelling increases at each cycle

0

0,1

0,2

0,3

0,4

0,5

0,6

30 80RH [%]

Def

orm

atio

n [m

m]

.

I cycle II cycle III cycle

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Fig.11: Evolution of stress in successive cycles

Long term loading tests:stress induced by sorption/desorption cycles

Stre

ss [

MP

a]

Time [min]

The curves are practically overlapping: constraints are different, but the two specimens behave equally

Relaxation behaviour shows up during the first cycle only

Compression stress is larger than tension stress

Compression stress decreases as cycles repeat

Tension stress increases as cycles repeat

B (free to shrink and prevented from swelling)

C (prevented from swelling/shrinking)

compression

tension

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Short term loading tests:

Strength (on average 4,4 MPa), and MOE (on average 350 MPa) are basically independent from:

compression/tension

direction between load and growth rings

EMC (in the examined EMC range

However, both for strength and MOE, in 0° direction, slightly larger values appear for variability (due to earlywood/latewood) and stiffness (due to the stiffening action of rays?)

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Compression set amounts to approximately 1,7% Swelling pressure is larger than shrinking tension Relaxation, both in compression and in tension, appears only

during the first cycle Relaxation is more obvious in compression than in tension The maximum compression stress is definitely smaller than the

elastic limit

Creep and mechano-sorptive deformation measurements are in progress

Conclusions

Long term loading tests

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Thank you for attention

paola.mazzanti@unifi.it