COST Action FP0802 Workshop Thermo-Hygro-Mechanically Modified Wood Thematic Session in Cooperation with COST Action FP0904

Transcription

1 COST Action FP0802 Workshop Thermo-Hygro-Mechanically Modified Wood Thematic Session in Cooperation with COST Action FP0904 August 24-25, 2011 Helsinki, Finland A computational framework for stress calculation in wood products under humidity and temperature variations Stefania Fortino

2 2 Thermo-Hygro-Mechanical Modelling of wood behaviour and processes What can we learn from the research on Moisture Induced Stresses? Background Improved Moisture project (WoodWisdom-Net, ) Partners: VTT, TUWien/Austria, KTH/Sweden, MPA/Germany, Harbin University/China, coordination: VTT (Tomi Toratti). Topics: computational and experimental wood mechanics, multiphysics and multiscale methods for wood (from micro- to macroscale). COST Action E55: "Modelling of the performance of Timber Structures" ( , closed on May 2011). COST Action FP0802 COST Action FP0904

3 Improved Moisture 23/08/ Microscale Cell wall layer Unit Cell Method, FEM (TUW, Austria, Karin Hofstetter s group) 50 µ 50 µm Stiffness properties for softwood (spruce and pine) with dependency on density and moisture content Hygroexpansion coefficients Viscoelastic parameters Macroscale Constitutive models VTT latewood earlywood l T l R =20-40 m Unit Cell Method l SW =2-4 mm multilayered composite laminate theory 20 mm Influence of microfibril angle and orientation (based on nano-indentation tests) growth rings

4 Improved Moisture 23/08/ D Orthotropic-viscoelastic-mechanosorptive constitutive model Stefania Fortino, Tomi Toratti et al. viscoelastic creep hygroexpansion elastic strain Thermomechanics approach Helmholtz free energy Recoverable mechanosorptive strain + irrecoverable mechanosorption (simple dashpot scheme) Fortino S., Mirianon F., Toratti T. (2009). A 3D moistur stress FEM analysis for time dependent problems in timber structures. Mech. of Time-Dep. Mater. 13, The model is a specialization of the Hanhijärvi and Mackenzie-Helwnein model for drying (2003).

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6 6 Model matrices On-going work: new matrices based on recent micromechanics results Future work: new matrices for treated wood products

7 7 Some implementation details moisture content history from Fickian or multi-fickian analysis NOTE: general computational algorithm suitable also for treated wood products

8 Structural scale - Some applications to civil engineering problems Glulam beams subjected to wetting or drying in laboratory (Jönsson, 2004) moisture flow Improved Moisture Timber bridge under natural environmental conditions (city of Lisbon) 8 Stresses perpendicular to grain after a wetting process S1 sensor, moisture content in 3 different points (surface, 1 cm depth, 2 cm depth)

9 Improved Moisture 23/08/ Moisture transfer in wood moisture transfer at the microscale: multi-fickian approach (water vapor and bound water phases) macroscale model

10 10 Multi-Fickian model by Frandsen et al. Multi-Fickian (or multi-phase) model (Frandsen et al., 2007) a) water vapor diffusion in the cell lumens c v = water vapor concentration b) sorption of bound-water (coupling) c) bound-water diffusion in the cell walls c b = bound water concentration n J b 0 n J v v s v k p p a v c t c t v b ( D c ) Frandsen H.L., Damkilde L. and Svensson S. A revised multi-fickian moiture transport model to describe non- Fickian effects in wood. Holzforschung 2007; 61(5), Dvinskikh S.V., Henriksson, M., Mendicino A.L., Fortino S. Toratti T. NMR imaging study and multi-fickian numerical simulation of moisture transfer in Norway spruce samples. Engineering Structurers (2011) doi: /j.engstruct v v ( D c ) b b c c Sorption rate

11 11 State-of-art: new research results on transport properties for wood based on micromechanics research (Eitelberger and Hofstetter, 2011) Future work: further research needed for diffusion parameters in treated wood products

12 Improved Moisture 23/08/ Some results from implementation of Frandsen s method in Abaqus NMR (nuclear magnetic resonance) experiments on small cylindrical samples of Norway spruce (KTH, Sweden) Comparisons between experimental data and numerical results - Tangential uncoated case geometry boundary conditions - temperature (23 C) - relative humidity

13 13 Fickian vs multi-fickian models and influence of the sorption curve on the numerical results Glulam beams: wetting to RH 80% after 4 months at RH 40% (Jönsson, 2005) moisture flow Different sorption curves used for Fickian and multi-fickian simulations

14 14 Moisture induced stresses in timber structures exposed to different climates The variations of MC in a point P of a timber member during its service life depend on: wood species environmental variations of relative humidity RH=RH(t); environmental variations of temperature T=T(t) initial moisture content type of exposure size of the timber cross-section location of the point P in the cross-section application of coating on the surface of the member

15 15 Applications of the VTT model for wood Timber stuctures under European environmental conditions Fragiacomo M., Fortino S., Tononi D., Usardi I., Toratti T. Moisture-induced stresses perpendicular to grain in cross-sections of timber members exposed to different climates. Engineering Structures, (2011), doi: /j.engstruct

16 16 Narrow and medium section - Influence of daily variation of RH on moisture content (4 measurements in 1 day, Fickian simulations, no hysteresis implementation) Uncoated Narrow Section Coated Narrow Section Uncoated Medium Glulam Section CITY OF HELSINKI ΔRH = 0.45 ΔRH = 0.45 ΔRH = 0.45 ΔMC maximum = 0.5 ΔMC maximum = 0.05 ΔMC maximum = 0.5 High gradient Low gradient High gradient Moisture Content Moisture Content Moisture Content True distance along section (m) True distance along section (m) True distance along section (m)

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18 18 THM wood processes and products What can we learn from the research on Moisture Induced Stresses? During the high temperature heat treatment of wood, it is important to know how the temperature and moisture distribution change with time. This information can be used: to adjust the treatment parameters to control the quality of final product more effectively. Effects to be studied the influence of initial moisture content the influence of the heating rate the influence of initial density the maximum treatment temperature ohter effects Importance of the Hygro-Thermal modelling! Suggestions from existing literature on High-Temperature Heat-Treatment of Wood : Research performed by the Group on the Thermotransformation of Wood (GRTB), Canada from 2006 to present (Kokaefe et al.). Mainly Canadian hardwood was studied.

19 19 Future work: Thermo-Hygro-Mechanical modelling at high temperatures Starting point: Drying modelling by Hanhijärvi and Mackenzie-Helnwein (2003) max T= C Mechanosorptiveplastic element (including isotropic and kinematic hardening) Hanhijärvi A. and Mackenzie-Helnwein P.: Computational analysis of quality reduction during drying of lumber due to irrecoverable deformation. i: Orthotropic viscoelastic-mechanosorptive-plastic material model for the tranverse plane of wood. Journal of Engineering Mechanics, 129(9): , Mackenzie-Helnwein P. and Hanhijärvi A.: Computational Analysis of Quality Reduction during Drying of Lumber due to irrecoverable Deformation. II: Algorithmic Aspects and Practical Application. J. Eng. Mech. 129(9): , 2003.

20 20 Future work: Thermo-Hygro-Chemo-Mechanical modelling at high temperatures Models to be built in a thermodynamics framework: + contribution of molar volume fractions ξ i ξ i Experiments needed. Example: B. Esteves, I. J. Domingos, H. M. Pereira. Pine Wood Modification by Heat Treatment in Air. BioResources, 3: , 2008

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