36 Drying of Wood

504030Length (cm)2010Moisture content1.00.50.00 1 20Width (cm)50403020Length (cm)100Moisture content1.00.50.00 1 2Width (cm)504030Length (cm)2010150125100755000 12TemperatureWidth (cm)504030Length (cm)2010015012510075500 1 2TemperatureWidth (cm)50(a)4030Length (cm)20100Pressure1.81.61.41.21.00 1 2Width (cm)50(b)4030Length (cm)20100Pressure1.81.61.41.21.00 1 2Width (cm)FIGURE 36.24 High-temperature drying (140/858C). Carpet plot after 5 h of drying. Internal overpressure, resaturation ofthe end piece, thermal conduction along the thickness, and end piece close to the wet-bulb temperature are evident on theseplots. Note the high value of internal pressure and the absence of end-piece resaturation obtained for heartwood (b).actual section, a tensile stress in the surface layers and(because of equilibrium conditions) a counteractingcompressive stress in the core layers (Figure 36.29c).During this period, surface checking is possible.From this point onward, the wood layers dry underload.As the drying proceeds, viscoelastic creep develops,together with mechanosorptive creep. The outer slicesappear similar in configuration to that exhibited forslice n83inFigure36.28,whiletheinternalslicesresembleslice n 81. Consequently, in spite of the flat moisturecontent profile, slicing the section at the end of thedrying wouldgivepictureFigure36.30b; the coreslices,dried under compression, are smaller than the outerones, dried under tension. In the actual section, compressivestress exists in the inner part (Figure 36.30c).This phenomenon is known as stress reversal or casehardening.The residual stress level depends on manyparameters (growth history, sawing pattern, dryingconditions, species, thickness, etc.), which provide mostof the problems of drying optimization. In addition,one must keep in mind that gradients of moisturecontent, strain, and stress exist along the thickness.This explains the curvature of the slices observed inprong test or cup method commonly used in industryto assess stress levels (Figure 36.31). When the innertensile stress is too high, internal checking occurs(Figure 36.25). An interesting simulation of this testcan be found in Dahlblom et al. (1994).36.2.4 .2 Dry ing Stres s Fo rmulationDuring drying, shrinkage appears in all parts of theboard for which the moisture content X is within thehygroscopic range. The shrinkage strain is proportionalto the difference between the local moisture content andthelocalvalueofthemoisturecontentatfibersaturationat the same temperature. A deformation field noted, « sh ,is defined in the material’s axes by Equation S1.If this deformation field does not fulfill the geometricalcompatibility, a strain tensor « mec related tostress is generated. The constitutive equation, whichrepresents the mechanical behavior of the material,relates this strain tensor « mec and the stress tensor.Due to the memory effect of wood, this tensor « mechas to be divided into two parts: (1) an elastic strain,« elas , connected to the actual stress tensor and (2) amemory strain, « mem , which includes all the strain dueß 2006 by Taylor & Francis Group, LLC.