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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1 week
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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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Aim of work and methods distaf
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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distafResultsM
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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distafConclusions
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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