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distaf. Paola MAZZANTI. IUFRO Division 5 Conference 5.02.00 Physiomechanical properties of wood and wood based materials and their applications. distaf. Paola MAZZANTI* Luca UZIELLI. Mechanical characteristics of Poplar wood (Populus alba L.) across the grain. University of Florence

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Paola mazzanti

distaf

Paola MAZZANTI

IUFRO Division 5 Conference

5.02.00 Physiomechanical properties of wood and wood based materials and their applications


Paola mazzanti

distaf

Paola MAZZANTI*Luca UZIELLI

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

University of Florence

DISTAF

Wood Technology Section

Via S. Bonaventura, 13

50145 Florence

Italy

Italy

European Union

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Paola mazzanti

Aim of work and methods

distaf

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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Paola mazzanti

Aim of work and methods

distaf

DISTAF activities

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.orgwww.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

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Paola mazzanti

Aim of work and methods

distaf

  • 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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Paola mazzanti

Aim of work and methods

distaf

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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Paola mazzanti

Aim of work and methods

distaf

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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Paola mazzanti

Aim of work andmethods

distaf

  • 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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Paola mazzanti

Aim of work and methods

distaf

Wooden material

  • 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

Fig. 1: specimens

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Paola mazzanti

Aim of work and methods

distaf

Environmental test conditions

Constant 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)

1 week

Fig. 2: variable climate conditions

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Paola mazzanti

Aim of work and methods

distaf

Loading test conditions

Short term loading (constant climate conditions):

strength

MOE

Long term loading (variable climate conditions):

swelling pressure

relaxation

compression set shrinkage

Compression

Tension

(Bending)

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Paola mazzanti

Aim of work and methods

distaf

Constraint test conditions (variable climate)

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

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

C

B

A

C: Prevented from deforming (measured: restraining force)

Specimens oriented along tangential direction

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Paola mazzanti

Results

distaf

6% EMC

12% EMC

15% EMC

Short term loading tests: compression

Direction between load and growth rings:

0°= RADIAL 45°= INTERMEDIATE 90°= TANGENTIAL

STRENGTH according to UNI EN 408

  • Min. 3 MPa

  • Max. 5 MPa

  • Minor variations with EMC and load direction

Strength [MPa]

Fig.3: Strength 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

MOE [MPa]

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

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Paola mazzanti

Results

distaf

6% EMC

12% EMC

15% EMC

Short term loading tests: tension

Direction between load and growth rings:

0°= RADIAL 45°= INTERMEDIATE 90°= TANGENTIAL

STRENGTH

  • Min. 2 MPa

  • Max. 6 MPa

  • Homogeneous values for 45° and tangential directions

  • At 12% EMC specimens show higher strength

Strength [MPa]

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

MOE

  • Min. 150 MPa

  • Max. 600 MPa

  • Homogeneous values for 45° and tangential directions

  • Variable values for radial specimens

MOE [MPa]

Anatomical direction

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

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Paola mazzanti

Results

distaf

Fracture edge

Fracture of vessel

Crack propagation along middle lamella in fibers

Fracture edge

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Paola mazzanti

Results

distaf

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

Free to shrink and prevented from swelling

Fig.9: deformation against time

Shrinkage of specimen prevented from swelling increases at each cycle

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

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Paola mazzanti

Results

distaf

Long term loading tests:

stress induced by sorption/desorption cycles

B (free to shrink and prevented from swelling)

C (prevented from swelling/shrinking)

  • 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

compression

Stress [MPa]

tension

Time [min]

Fig.11: Evolution of stress in successive cycles

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Paola mazzanti

Conclusions

distaf

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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Paola mazzanti

Conclusions

distaf

Long term loading tests

  • 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

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Paola mazzanti

distaf

[email protected]

Thank you for attention

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