Paola MAZZANTI

1. distaf Paola MAZZANTI IUFRO Division 5 Conference 5.02.00 Physiomechanical properties of wood and wood based materials and their applications

2. 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 2

3. 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) 3

4. 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 4

5. 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 5

6. 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) 6

7. 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 7

8. 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 8

9. 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 9

10. 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 10

11. 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) 11

12. 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 12

13. 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 13

14. 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 14

15. Results distaf Fracture edge Fracture of vessel Crack propagation along middle lamella in fibers Fracture edge 15

16. 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 16

17. 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 17

18. 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?) 18

19. 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 19

20. distaf paola.mazzanti@unifi.it Thank you for attention 20