Improving seasoned hardwood timber quality : with particular reference to collapse

This thesis principally concentrates on the prevention of degrade due to collapse during the controlled kiln drying of timber. Collapse is an abnormal shrinkage which occurs in wood during drying to fibre saturation point (FSP). Formation of internal checking is often associated with it. Collapse is particularly common in native Australian eucalypt timbers. Specific work was also performed on the drying of karri (Eucalyptus diversicolor F.Muell) and brushbox (Tristania conferta R.Br.) as part of the commercialisation process of the Clever Kiln Controller¬¨vÜ (CKC). The CKC is a personal computer based kiln control package which uses sensed acoustic emissions (AE) and on-line modelling of drying to optimise the seasoning process. Collapse shrinkage occurs as a result of the physical flattening of water filled fibre cells due to the action of internal tension. The work on collapse presented in this thesis begins with stress-strain modelling of a single fibre under such conditions. This model predicted that collapse of fibres is precipitated at the inner fibre wall, and that fibre collapse is strongly dependant on temperature. Following the predictions of the single fibre model and observations of drying trials, the concept of a \collapse threshold temperature\" was introduced. If wood at moisture content (MC) above FSP is dried at temperatures above the collapse threshold temperature then it will collapse. If it is dried at temperatures below the collapse threshold temperature then it will not collapse. This concept was first tested on a 20m3 kiln load of Eucalyptus regnans F.Muell. The timber was successfully dried to 40% MC free of collapse. An existing model of the timber seasoning process was modified to model wood as a heterogeneous material. This model showed that if the dry bulb temperature exceeded the collapse threshold temperature during drying to FSP (that is if collapse shrinkage occurred) then checking was very difficult to avoid even if the humidity of the modelled schedule was very high and the drying thus very slow. It is common practice in industry to increase the dry bulb temperature during drying to FSP. This does not always induce collapse. That is the collapse threshold temperature varies during drying. Mathematical modelling predicted that the effect was not due to the lowering of water surface tension with increase in temperature or strain relaxation by creep or mechano-sorptive deformation. The most likely cause for the variation of collapse threshold temperature during drying was shown to be the effect of bulk stress in the board due to drying. Drying trials controlled by the CKC were conducted on batches of karri and brushbox. The drying trials yielded information on the two species which allowed accurate modelling of their behaviour. The work on karri revealed a stress raiser effect from opened vessels lying along the surfaces of boards; modelling of the effect produced results consistent with those from drying trials and acoustic emission monitoring. This study has established the link between collapse in Australian hardwoods and temperature. Mathematical models developed to quantify the effect predicted that collapse induced checking could best be avoided by using drying conditions which did not cause collapse shrinkage; collapse was shown to be very sensitive to drying temperature. Collapse checking was shown to be due to stresses resulting from localised collapse shrinkage. Optimisation of dry bulb temperature during drying has not yet been achieved by mathematical modelling. Suitable non-collapsing dry bulb temperature schedules can be determined by small-scale drying trials."