The simulation of stresses and strains in the drying of Pinus radiata sapwood: the effects of board geometry

Abstract One- and two-dimensional structural analyses have been combined with a drying model to predict the stress levels in sapwood boards of softwood timber as a function of time and position within the boards during the high-temperature drying of this timber. As examples, stress levels in boards with a thickness of 50 mm and widths of 50 mm, 100 mm and 200 mm, respectively, have been estimated during a typical high-temperature drying schedule for Pinus radiata timber, considering only shrinkage and instantaneous elastic strains. The two-dimensional analysis suggests that the stress patterns only become significantly two dimensional within 25 mm of the edges of the timber. For the purposes of predicting the maximum stress levels in boards of timber, the one-dimensional analysis yields predictions of stress levels which are not significantly different from those of the more complex two-dimensional model for non-square boards. The one-dimensional approach is therefore useful for predicting crack formation inside timber boards. The two-dimensional approach has applications when it is necessary to predict collapse in a board or when the grain angle varies significantly across the board. The over-prediction of the stress levels inside the timber compared with typical measured rupture stresses (up to 4 MPa predicted, 3 MPa measured) is consistent with the neglect of the visco-elastic and mechano-sorptive elements of strain in the model, both of which would be expected to decrease the predicted stress levels inside the timber. The inclusion of visco-elastic and mechano-sorptive strains would decrease the predicted stresses for both one- and two-dimensional models by the same proportion, so the comparison between the two models would not be greatly affected.