EXPERIMENTAL INVESTIGATION OF MOISTURE DRIVEN FRACTURE IN SOLID WOOD

Transcription

1 EXPERIMENTL INVESTIGTION OF MOISTURE DRIVEN FRCTURE IN SOLID WOOD Finn Larsen 1, Sigurdur Ormarsson 2, John Forbes Olesen 3 BSTRCT: This paper describes experiments made to find methods to reduce cracks in ood. Experiments ere performed ith Noray spruce by drying the ood from green moisture content don to equilibrium moisture content (EMC). The moisture related strains and crack development on solid ood discs ere measured ith ramis [1] during drying at 23 C and RH of 64%. The moisture gradient in the fibre direction had a major influence on the crack behaviour. With discs thicker than 30 mm, this influence as pronounced, hile it as limited ith thinner discs of 15 mm. The results indicate that sealing of timber log ends in the green moisture state could significantly reduce the development of end-cracks. lso the initial moisture content and the shrinkage properties varied pronouncedly from pith to bark. The conclusion is that modelling of crack propagation in solid ood must take into account the material inhomogeneities. KEYWORDS: Wood, moisture, drying, cracks, ramis 1 INTRODUCTION 123 Solid timber products, containing both heartood and sapood, often have a high tendency to crack during the drying process. This can cause severe loss of material for the sa-mills, especially for products ith large cross sectional dimensions. Cracks caused by kiln drying are extremely difficult to predict. The cracks (e.g. endcracks) arise, in some cases, early in the drying process and close again later in the process. It is difficult and in some cases impossible to see the closed cracks ith visual grading. This can result into high grading of the damaged material hich can cause problems for customers such as building and furniture industries. The main aim here is to accumulate experimental results and data for the development of a model that can evaluate the various couplings in the hygro-mechanical problem that govern moisture driven cracking in ood. The material model ill be an orthotropic model including effects of shrinkage, mechano-sorption and fracture opening as ell as material inhomogeneity and fibre orientations. The fracture model ill be based on a cohesive fracture approach for 2D-crack propagation in the RL- and TL-planes. The cohesive crack model ill take into consideration a mixed mode softening 1 Finn Larsen, Department of Civil Engineering, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark. finla@byg.dtu.dk 2 Sigurdur Ormarsson, Department of Civil Engineering, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark. sor@byg.dtu.dk 3 John Forbes Olesen, Department of Civil Engineering, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark. jfo@byg.dtu.dk relationship beteen crack opening and sliding. The future model ork ill be a further development of the 3D distortion model presented in [1,2]. Moisture content (MC) in green ood varies ithin the cross section of a timber log. The MC of heartood, for example, is significantly loer than the MC of sapood. The fibre saturation point (FSP) in heartood and sapood is reached at different times. Shrinkage starts at different times ithin different parts of the cross section, hich results in a complex state of strains and stresses. Experiments ith moisture loss above FSP have been described by [3]. Their results sho a significant change in the rate of drying (evaporation) from a state above FSP, to a state belo FSP. Moisture transport in ood, both above and belo the FSP, has been modelled and studied numerically, and compared ith experimental results by [4]. This study shoed that drying of ood above FSP is strongly dependent on evaporation from the surface of the ood, hile drying belo FSP is dependent on the moisture transport inside the ood. These results sho that drying above FSP is much faster than drying belo FSP. Mechano-sorptive strains can occur in ood because of drying in areas here significant tensile or compressive stresses build up. Many experiments have been conducted to identify mechano-sorption due to changes in the ambient climate. Several papers, such as [5,6,7] have reported studies of mechano-sorptive behaviour in ood material under controlled climatic conditions and constant loading. Elastic strains and mechano-sorption ere identified in these experiments and their magnitude as also determined. Ho mechano-sorption parameters affect distortion of solid timber has been investigated numerically by [8]. The difference beteen the

2 shrinkage coefficient in radial and tangential directions has a significant influence on the internal stress generation during the drying process. Variation in shrinkage properties has e.g. been studied experimentally by [9]. Investigations conducted by [10] sho furthermore that the radial shrinkage coefficient varies over the cross section. Experiments ith Noray spruce have shon that the radial shrinkage coefficient for juvenile ood is significantly smaller than for mature ood. The juvenile part of the studied ood samples as the part containing around 6 to 8 annual rings. The moisture related crack pattern in ood often becomes quite complex because of the annual ring structure and the different MC levels ithin heartood and sapood. Crack directions can be divided into 6 different orientations: (TL, RL LR TR and RT, LT) here L=longitudinal, R=radial and T=tangential. Three different fracture modes (Mode I, II and III) may occur for each cracking orientation. For a more detailed description of fracture models, see [11]. Hoever, for Mode I directions TL and TR are the most relevant for moisture related crack propagation. Therefore the focus of this ork represents the cross sectional behaviour of a timber log. One hypothesis to be investigated is that the moisture gradient has a significant influence on the cracking behaviour. The rate of ood drying is usually more than 5-10 times faster in the fibre direction than in any other direction. To accommodate this aspect of time, tests ere conducted on thin discs of a timber log. In this ay the experiments ere carried out ithin a reasonable time and thereby a larger number of verifying experiments could be performed. The variation in the transversal strain fields across the ood cross sections ere measured by an optical photogrammetric measuring system, [12]. Using ramis it is possible to observe the strain field and the early crack initiation folloed by the crack propagation (and possible closing) during the drying process. 2 MTERILS ND METHODS 2.1 TEST SPECIMENS Wood specimens ere selected from a 20 year old Noray spruce tree from North Zealand, Denmark. The tree as felled in the morning on the 20 th of January timber log (1.5 m long and 200 mm in diameter) as selected from the tree and delivered at DTU.byg. The log as cut into 4 parts marked sequentially from 1 (top) to 4 (bottom), and placed in a freezer for conservation. Specimens cut from Part 1 ere numbered 1.x, and specimens from the other parts ere numbered 2.x and so on. Experiments ere conducted to analyze ood cracking behaviour using thin discs from timber logs hich ere dried from green condition don to approximately 12% MC. 2.2 EXPERIMNTL SETUP The experimental setup consisted of several components hich together provided the necessary information during drying. C E Figure 1: Experimental arrangement Figure 1 shos one of the setups for strain and eight measurements during changes in MC. In the figure the components are marked as follos: () Wood disc specimen (15 mm); (B) ramis digital cameras mounted on a tripod; (C) ramis computer; (D) Cold spotlight; (E) Load cell mounted on a stand; (F) Load cell logger Online eight registration. The eight of the test specimen as recorded online during the experiment. The eight history as used to calculate and define the MC history. The load cell had a maximum capacity of 10 kg (+1 g). The eight as recorded by a logger at intervals of 15 minutes Temperature and relative humidity Temperature and RH ere measured ith a hygrometer throughout the experiments. Both temperature and RH ere kept constant during all experiments. The temperature, hoever, varied beteen C, hile the RH varied beteen 62-64%. This climate condition resulted in EMC=12% for all the test specimens Spatial MC variation of green ood To study the spatial MC variation over the cross section in a green condition a 50 mm thick disc as cut into small sticks of 13x13 mm in cross section. Initial MC as determined for all sticks. The sticks ere dried at 103 C until they became in eight equilibrium (MC=0%). The eight of each stick as determined ith a precision of g. Every stick eighted in the dry condition about g Strain measurements The equipment used for strain measurements as ramis, hich is a non-contact optical 3D deformation measuring system, see [12]. ramis recognizes the surface structure of specimens to be measured in digital images and allocates coordinates to the image pixels. ramis determines deformation of the specimen from one image to another by means of various square image details (facets), typically 15x15 pixel facets ith a 2 pixel overlapping area. The changes of position of different facets define the deformations. Displacements are measured and strains are calculated at the specimen surface. Because every facet has to be unique and easy to B F D

3 recognise, it is very important that the surface has an arbitrary pattern ith striking contrasts. Normally it is necessary to apply a black speckled pattern on the surface. Displacements and deformations developed on the specimen surface during the experimental process are visualized and calculated using the ramis softare. Paint could hinder evaporation from the surface, so initial experiments ere carried out to investigate ho the surface could be kept as permeable as possible, and these led to the folloing procedure: The light ood colour itself served as the background, and the black dots of the speckled pattern ere applied as sparingly as possible. Examples of good pattern are shon in [12, 13]. This pattern allos the moisture to evaporate almost unhindered from the surface. ramis measures strains at the specimen surface. If evaporation occurs through this surface and a moisture gradient develops, then there might be a difference beteen strains measured and strains in depth underneath the surface hen MC gets beneath FSP. Every ramis stage (picture) consists of 2-3 MB data, so it is important to reduce the amount of stages to an acceptable level. t the same time there has to be enough stages to illustrate the strain history of the experiment. Pictures ere taken ith 15 minutes interval for all the experiments hich as found to be an adequate time step. 3 RESULTS ND DISCUSSIONS 3.1 MOISTURE CONTENT IN GREEN WOOD Initial MC in green ood varied significantly over the cross section of a timber log. MC as lo in heartood (around FSP); hile in sapood it could exceed 200%, see Figure 2. heartood had a radius of about mm. The results shoed cracks in the beginning of the drying process that radiate mm out from the pith. 3.2 DRYING HISTORY OF TEST SPECIMENS Figure 2 clearly shos ho the initial MC varied strongly over the cross section of a log. This means that the shrinkage of the ood material started at different times in different areas. Online eight measurements ere used to clarify average moisture changes of the heart- and sapood, respectively. Results from three discs (4.4, 4.7 and 4.12) ere studied. Heartood, ith a radius of 40 mm from the pith, as cut out from the discs. Figure 3: verage MC history for 15mm thick discs. The average MC history as calculated for the heartood and the sapood. Figure 3 shos that the rate of change of MC change above FSP (MC~30%) as high and fairly constant until FSP as reached. The rate changed nonlinearly belo FSP until EMC as reached. The same trend of moisture loss as obtained for both the heartood and the sapood parts, except for the difference in initial MC value. With the knoledge from initial MC and the drying course from this point to EMC a MC content history for the hole cross section can be generated, see Figure 4. Figure 2: Variation of initial (green) MC from pith to bark (Specimen 4.8). The experimental results shoed that the transition as not distinct beteen heartood and sapood. The moisture content varied almost linearly from 20 mm (MC=50%) to 50 mm (MC=200%) from the pith. MC as a little higher than FSP close to the pith (MC=40-45%), hile MC in sapood as above 200%. The transition zone from lo to high MC varied over 3-4 groth rings. The first ramis investigations on strain development and on crack propagation indicated that Figure 4: MC history for 15mm thick disc. 3.3 SHRINKGE COEFFICIENT Free shrinkage is the main driver of distortion in ood. Therefore good shrinkage data is needed for simulation of moisture related deformation in ood. The small experimental study presented here deals ith the estimation of the radial and the tangential shrinkage coefficients (α r and α t ) for heartood and sapood,

4 respectively. To ensure shrinkage ithout stresses and mechano-sorptive effects all the specimens ere slit from the pith to the periphery. In this ay the shrinkage could develop quite freely, especially ithin samples consisting solely of heartood or sapood. Tangential distance (perimeter of the sample) and radial distances ere measured at different MC levels. Geometry as determined at green condition (MC > FSP), hich determined the geometry at FSP and at oven dry condition (MC=0%). Geometry and MC as determined hen EMC as reached. The initial length in the radial direction at Stage 1 is denoted l r1, hile the initial length in tangential direction as perimeter length denoted l t1. Corresponding total eight of the specimens is defined as m tot1 =m ater1 +m 0 here m 0 is the dry eight of ood at MC = 0%. Ne lengths l r2, l t2 and eight m tot2 at Stage 2 ere found after the moisture loss. The shrinkage coefficient is found on the basis of measurements at EMC (stage 1) and MC=0% (Stage 2). See Equations (1)-(3) belo: 2 1 t r m m m tot 2 tot1 (1) 0 lt 2 l lt1 t1 2 1 lr 2 l lr1 r1 2 1 (2) (3) The data found at stage 1 and 2 and used in Equation (1)- (3) ere also used to find MC at FSP according to Equation (4). The lengths l r3, l t3 ere found at green state, hich as the same as at FSP. The shrinkage as expected to be linear in relation to MC from FSP to MC=0%. l3 l2 FSP % (4) l l 1 The calculated shrinkage coefficients and FSP for four representative specimens are shon in Table 1. Table 1: Shrinkage coefficients and FSP 2 had a smaller radial shrinkage coefficient than sapood (α r ~0.15). The calculated results for α t ~0.3 and FSP=MC~30-32% ere as expected just as α r ~0.15 for sapood. The large difference in the radial coefficient from heartood to sapood is confirmed by [10]. 3.4 STRIN RESULTS The ramis-system as used to measure strain development on the surface of the studied specimens during drying from green condition don to EMC. ramis calculates strains in x, y and z-directions as ell as major and minor strains and direction of these strains. Results from some of the measurements are presented belo. The strain measurement experiments ere performed ith discs of different thicknesses and ith or ithout a 5 mm slit from pith to bark. The slit as made to enable free shrinkage in the disc. Other disc experiments ere performed ith sapood only. Examples of strain development during drying of four specimens are shon in the folloing figures. Specimen 3.1 as a 30 mm thick disc ith a 5 mm ide slit from bark to pith. Three strain situations from different times during the drying are shon. Specimen 4.10 as a 15 mm thick disc ith a 5 mm ide slit from bark to pith. Specimen 4.6 as a 15 mm thick disc ithout a slit. Specimen 4.7 as a 15 mm thick disc of sapood. The heartood as removed ithin a radius of 43 mm from the pith. Figure 5 shos a major strain plot for Specimen 3.1 after 37 hours of drying. Based on Figures 2 and 3 heartood as beneath the FSP (MC varied beteen 12-20%) hile sapood still as above FSP (MC~60-70%). The result shos negative major strains over the hole cross section. The major strains are mainly radial oriented except for zones in the heartood here the strains are tangentially oriented. If the shrinkage strains in the heartood zone are subtracted from the total strains, positive elastic (tensile) strains occur in the heartood zone. The strains indicate crack propagation radiating from pith to a limiting line (in the transition zone) here compression strains began. The 5 mm slit is at this state completely closed in the sapood area because of the heartood shrinkage deformation. Specimen nr. α t,inner α t,outer α r FSP [MC] % % 4.4 heartood % 4.4 sapood % Specimen 3.1 and 4.10 ere discs of a hole cross section of the timber log, hile 4.4 heartood as just the heartood part ith a radius of 40 mm and 4.4 sapood as the sapood part of the same disc. The tangential shrinkage coefficient (α t ) as quite similar for all the specimens hile the radial coefficient (α r ) had significant variation. The heartood (α r ~0.1) Figure 5: Major strain distribution ithin Specimen 3.1 after 37 hours drying (MC heartood ~ 12-20% and MC sapood ~ 60-70%). Figure 6 shos a plot of major strains for Specimen 3.1 after 55 hours drying hen heartood and sapood are beneath and around the FSP. The average MC for

5 heartood as 12-15% and for sapood 25-35%. The figure shos a clear crack pattern in the radial direction over the hole sapood area. This crack pattern is quite similar to the crack patterns observed in log-ends. This indicates that moisture gradients in the longitudinal direction have a significant influence on the crack pattern. t this state the 5 mm slit starts to open again; heartood cracks in Figure 6 are mainly closed and heartood is exposed to radial compressive strains. plots after approximately 25 hours of drying for discs ith and ithout a slit. Drying of a 15 mm thick disc for 25 hours corresponds to the same drying condition as for a 30 mm thick disc after 37 hours of drying. The results sho significant (radial oriented) major strain variation over the transition zone but no fracture of the specimens. This supports that the moisture gradient in the fibre direction has been significantly reduced. Figure 6: Major strain distribution ithin Specimen 3.1 after 55 hours drying (MC heartood ~ 12-15% and MC sapood ~ 25-35%). Figure 7 illustrates an ramis plot of the major strains after drying of the hole cross section don to EMC=12%. The hite stripes illustrate major strain direction. Every crack observed earlier in Figures 5 and 6 are closed and invisible. The slit had at this stage a quite large opening, and relatively large negative (radial oriented) major strains over the entire cross sectional area. Note that major strains in the heartood area are significantly smaller than in the sapood area. This corresponds very ell to the difference in radial shrinkage coefficient found in Section 3.3. Figure 8: Major strain distribution ithin Specimen 4.10 (ith a slit) after 25 hours drying (MC heart ~12% and MC sap.~ Figure 9: Major strain distribution ithin Specimen 4.6 (ithout a slit) after 25 hours drying (MC heart ~12% and MC sap.~ Elastic and mechano-sorptive strains The free shrinkage strains are generally to times larger in the tangential direction than in the radial direction, see Section 3.3. Specimens ith a slit, for example Figure 7: Major strain distribution ithin Specimen 3.1 after 75 hours drying (MC heart=mc sap = EMC). The crack patterns shon in Figures 5 and 6 supports the hypothesis that the moisture gradient in longitudinal direction has a significant influence on the crack pattern i.e. the influence of the slit is limited because the moisture gradient is the main reason for the crack propagation. To reduce the influence of the longitudinal moisture gradient on the crack pattern 15 mm thick specimens ere also studied. Figures 8 and 9 sho major strain Figure 10: Minor strain distribution ithin Specimen 4.10 after drying to EMC=12%.

6 Specimens 3.1 and 4.10, ere supposed to allo relatively free shrinkage during drying and therefore only cause limited elastic and mechano-sorptive strains. Figure 10 shos the minor strain in Specimen 4.10 after drying to EMC. White stripes in this figure illustrate minor strain direction. The result is similar to the result in Figure 7 since minor strains are perpendicular to major strains. The measured strains are total strains, ε total, hich consist of shrinkage, elastic and mechano-sorptive strains as total sh el meso 0 / E m (5) The shrinkage strain ε sh is calculated as ε sh =α Δ, here α is the shrinkage coefficient in the actual direction and Δ is the MC change. The elastic strain ε el is calculated as ε el =σ/e, here σ is the stress and E is the modulus of elasticity, both in the actual direction. m 0 is the mechanosorptive parameter. From Equation (5) the stress can be eliminated as t total t (1/ E ) t t m t0 (6) Mechano-sorption has a significant reductive influence on the stress development. Figure 11 shos a sum of elastic and mechano-sorptive strains (ε total-t -α t Δ) in tangential direction at Specimen 4.10 after EMC=12% is reached Strains in cross sections ithout heartood To get more homogenous test samples, experiments ere performed ith discs ithout the heartood part. In this ay the influence of different shrinkage coefficients from heartood to sapood, as ell as the difference beteen initial MC in the different areas could be eliminated. Figure 12 shos Specimen 4.7 after EMC = 12% as reached. The hole in the middle as circular in green condition, ith a radius of 43 mm. Note that the hole no longer is circular after drying. Furthermore, Figure 12 shos also some dimensions arros. The distance above as 41 mm hile the other distances varied from 50 to 54 mm. B Figure 12: Cross section of Specimen 4.7 after 12% MC is reached. Figure 11: Distribution of tangential elastic plus mechano-sorption strains (ε el-t+ε meso-t= ε total-t-ε sh-t) ithin specimen (4.10) after drying (EMC=12%). White areas in Figure 11 represent strains from 0.5 to 1% hile black areas represent strains from -0.5 to -1%. The tangential stress can be calculated according to Equation (6) using tangential shrinkage coefficient α t =0.3, MC changes, Δ=-18%, tangential modulus of elasticity E t =220MPa, and the mechano-sorption parameter m t0 =0.2. The positive strains in the heartood (and in some sapood areas) ill result in tensile stress variation beteen σ t =0.12 to 0.24 MPa. These stresses are belo the characteristic strength value given in Eurocode as f t,90,k =0.5 MPa. The negative strains in the sapood areas ill result in compressive stresses about σ c =0.12 to 0.24 MPa, hich also is belo the strength value. Development of stresses in this cross section arose because of the difference in initial moisture content and material properties over the cross section. The effect of mechano-sorption reduced the stresses significantly. Figure 13 shos a sum of elastic and mechano sorption strain in the tangential direction for Specimen 4.7. Zone and B are marked in Figures 12 and 13. Zone has large negative strains (approximately -4%) in an area close to the hole surface. The strains in the rest of the cross section are much smaller. The development of relatively large negative strains in area arise primarily because this part had smaller height than the other parts of the specimen, hich lead to less stiffness in this part than in the rest of the cross section. Zone B had unusual local positive strains (approximately 1%). B Figure 13: Elastic and mechano sorption strain in tangential direction ithin Specimen 4.7 (only sapood) after EMC~ 12% is reached.

7 The annual rings sho some defects in this zone hich might have caused this change in strains compared to the rest of the disc. The result from Specimen 4.7 shos that geometry has large influence on the strain development during drying. 3.5 STRESS ND MECHNO SORPTION IN CLOSED CROSS SECTION Extensions of some of the strain experiments ere performed to analyse the effect of mechano-sorption. The purpose as to examine the reversibility of the mechano-sorptive effect. Figure 14 shos pictures of Specimen 4.6 at different time stages during the drying experiment. In Figure 14 the pictures are marked as follos: () Specimen in green condition; (B) Specimen after drying from green condition to EMC =12%; (C) Specimen at MC=12%, after a crack has propagated from a short san slit; (D) Specimen after it has been etted, from MC=12% to FSP=30%; (E) Specimen after drying from et condition to EMC=12%; (F) Specimen at MC=0%. Specimen 4.6 as a 15 mm thick disc ithout a slit. The disc as dried from green condition to EMC=12% ithout any sign of fracture. slit as cut ith a sa to initiate a crack. The crack developed instantly hile the slit as san. The depth of the slit as about 10 mm. The idth of the crack opening as 3 mm just after the crack occurred and 6 mm 1 day later. From this point on there as no further development of the crack idth. The crack idth developed because of elastic stresses in the disc. The disc as then put into ater until FSP as reached 3 days later. The crack closed totally during this stage. From FSP a ne drying period started similar to the previous drying period until EMC=12% as reached. The idth of the crack opening as no 22.5 mm. Finally the disc as dried to MC=0% at 103 C and the opening extended to 34 mm. The tangential length (l perimeter ) as measured along the bark at all the 6 stages and the shrinkage coefficient as calculated as shon in Section 3.3. During the first drying period the strains ere a combination of moisture driven shrinkage, elastic and mechano-sorptive strains because free shrinkage as hindered by the closed annual rings. In the phase from EMC=12% to FSP (Stage C-D) the disc expanded to a geometry like in the green condition. The expansion closed the crack and developed compressive stresses in the cross section. In the second drying period the cracked cross section as dried ith more free shrinkage. The mechano-sorptive effect ould be fully reversible, if the subsequent drying ould develop as a disc ith a slit. Table 2 shos the measured and calculated values: Table 2: Shrinkage development of specimen 4.6 Specimen 4.6 Figure 14 l perimeter [mm] Crack idth[mm] α t [MC] Stage -B % Stage B-C % Stage C-D % Stage D-E % Stage E-F % B Table 2 shos that the average value of the tangential shrinkage coefficient in free shrinkage (Stage D-E and E-F) is α t ~0.3, hich correspond to the values found in Table 1 in Section 3.3. The crack idth also corresponds exactly to crack idth of other specimens ith slit, such as Specimens 3.1 and This shos that mechanosorption can be fully reversible. C D E F Figure 14: Crack development ithin specimen (4.6) 4 CONCLUSIONS The experimental results presented here have increased our knoledge on ho samples of round ood behave during drying. Differences beteen heartood and sapood properties highly influence the strain and fracture behaviour during drying. The properties relate both to different initial green moisture content and different shrinkage properties. Specimen thickness and boundary conditions (samples ith or ithout slits) had a strong influence on the strain development and crack behaviour. It as found that mechano-sorptive strains can be fully reversible and it is much larger than the elastic strains. lso difference in geometry has a large influence on the strain development. The longitudinal moisture content gradient had a significant influence on the crack pattern for round ood disc specimens 30 mm or thicker. Cracks that developed at an early stage in the drying process can visually disappear at the end of the

8 drying process. It can be concluded that sealing of logends in green moisture condition ould significantly reduce the risk of end-cracks during drying of solid timber products. The results indicate that moisture related fracture modelling needs to take geometry and material inhomogeneity into account, including local defects, MC gradient and different variation in the MC history for both heartood and sapood zones. 5 FURURE WORK ramis measurements have given ne possibilities to calibrate and validate distortion and crack models. The results ill be used to compare ho moisture-induced displacements and strain fields develop during the hole drying process. It ill furthermore be possible to compare in detail ho strains and cracks develop in small regions. The aim is to develop numerical models that simulate distortion and fracture in solid ood during ambient climatic conditions corresponding to those of typical kiln drying. Ne models ill be calibrated by experimental verification here the moisture related strains and crack propagation during forced drying are measured by ramis. One of the primary objectives is to find optimal drying schemes regarding elimination of cracks during drying. [6] Lazarescu C., vramidis S.: Drying Related Strain Development in Restrained Wood. Pages Drying Technology 26(5), [7] Svensson S.: Strain and Shrinkage Force in Wood under Kiln Drying Conditions. Holzforschung, 49: , [8] Ormarsson S.: Numerical nalysis of Moisture- Related Distortions in San Timber. PhD thesis, Göteborg, Seden, ISBN [9] Dahlblom O., Petersson H., and Ormarsson S.: Characterization of shrinkage, European project FIR CT , Improved Spruce Timber Utilization, Final report Sub-task B1.5, [10] Rosner S., Karlsson B., and Konnerth J.: Shrinkage processes in standard-size Noray spruce ood specimens ith different vulnerability to cavitation. Tree Physiology, 29, , [11] Valentin G.H., Boström L., Gustafsson P.J., Ranta- Maunus., and Goda S.: pplication of fracture mechanics to timber structures RILIM state-of-theart report. Research Notes UDC :620.17:539.42, [12] ramis v6 User Manual. GOM mbh, GOM Optical Measuring Techniques. (.gom.com), [13] Larsen F., Ormarsson S., and Olesen J.F.: Experimental study of moisture-driven distortion and fracture in solid ood. Proceedings of the 11th International IUFRO Wood Drying Conference, Pages , Luleå University of Technology, Skellefteå, Seden. CKNOWLEDGEMENT We ould like to thank the Danish Forest and Nature gency, Haraldsgade 51, 2100 København Ø for the funding of this project. REFERENCES [1] Ormarsson S., Dahlblom O., and Petersson H.: numerical study of the shape stability of san timber subjected to moisture variation. Part 1: Theory. Wood Science and Technology, 32:, , [2] Ormarsson S., Dahlblom O., and Petersson H.: numerical study of the shape stability of san timber subjected to moisture variation. Part 2: Simulation of drying board. Wood Science and Technology, 33: , [3] Wiberg P., Sehlstedt-Persson S.M.B., and More n T.J.: Heat and Mass Transfer During Sapood Drying bove the Fibre Saturation Point. Pages Drying Technology 18(8), [4] Krabbenhøft K.: Moisture Transport in Wood. Study of Physical-Mathematical Models and their Numerical Implementation. Ph.D. Thesis. ISBN nr Denmark [5] rmstrong L.D., and Kingston R.S.T.: The effect of moisture content changes on the deformation of ood under stress. ustralian Journal of pplied Science, vol. 13. Pages , 1962.