36 Drying of Wood

Second drying periodSecond drying periodEnd of dryingFSPTensilestressCompressivestressProngtest(a)(b)FIGURE 36.29 Appearance of drying stresses followingshrinkage. (Adapted from Perré , P., The numerical modelingof physical and mechanical phenomena involved inwood drying: an excellent tool for assisting with the studyof new processes, Tutorial, Proceedings of the Fifth InternationalIUFRO Wood Drying Conference, Qué bec, Canada,1996, 9–38.)where DW n is the activation energy associated toelement n.The mechanosorptive effect occurs as soon as themoisture content changes during load. The simplerway to express this effect consists in assuming thatthe strain rate depends linearly on both the stress fieldand the time derivative of the moisture content ḣ :_« msij ¼ m ijkl s kl (sign ḣ)ḣ (36:21)The mechanosorptive strain rate is always in the directionof the stress field, hence the factor sign _u inEquation 36.21.A three-dimensional resolution is very costly interms of calculation time and computer memoryspace. The need for such a cost is justified wheneverthe objective of the calculation lies in evaluating theoverall deformation of the board. In this way, theeffect of reaction wood, fiber angle, and propertyFSPX eq(a)End of drying(b)(c)(c)TensilestressCompressivestressFIGURE 36.30 Stress reversal due to the memory effect ofwood. (Adapted from Perré, P., The numerical modeling ofphysical and mechanical phenomena involved in wooddrying: an excellent tool for assisting with the studyof new processes, Tutorial, Proceedings of the Fifth InternationalIUFRO Wood Drying Conference, Québec,Canada, 1996, 9–38.)CupmethodFIGURE 36.31 Two experimental methods used to assessdrying stresses.variations can be analyzed. Good examples of thesepossibilities can be found in the literature (Dahlblomet al., 1994, 1996; Ormarsson, 1999). Nevertheless, tostudy the stress development within a section farfrom the ends of the board, a two-dimensional simulationis sufficient. A ‘‘planar displacement’’ formulationhas to be used in this case, assuming smalldisplacements (Perré and Passard, 1995; Chen et al.,1997a)orlargedisplacements(MaugetandPerré ,1999).36.2.4 .4 Stres s Deve lopm ent dur ing Dry ing:So me Exampl esAll examples presented in this section, except the nonsymmetriccase, refer to a flatsawn board of heartwood,20-mm thick and 40-mm wide. They have beencomputed using the computer code Transpore (Perréand Turner, 1996b and c, 1999; Mauget and Perré ,1999; Perré and Passard, 2002;). Figure 36.33 depictsthe moisture and stress fields calculated at differentdrying stages for quite severe conditions at a mediumtemperature level (T d ¼ 80 8C, T w ¼ 60 8C). After 6 hof drying, the external part of the section is within thehygroscopicrange,whichgivesrisetotensilestressduetoshrinkageinthezonesclosetotheexchangesurface.As a consequence of the mechanical equilibrium, internalzones undergo compressive stress. A negativeshear stress exists close to the edge (the right angleshavedecreased).Attheendofthedryingprocess(60h)the moisture content is almost equal to the EMC (7%)throughout the section. The stress reversal due to thememory effect of wood is clearly exhibited. It can benoted that the shear stress also changes in sign.When a face of a board is insulated, the dryingconditions are not symmetrical anymore andß 2006 by Taylor & Francis Group, LLC.