Model fitting for visco-elastic creep of Pinus radiata during kiln drying

This work examines the applicability of mathematical models for correlating the visco-elastic strains during kiln drying of radiata pine (Pinus radiata D. Don) sapwood at various temperatures and moisture contents. The eventual aim is to use a mathematical model incorporating these strains to optimise the drying schedules and minimise the degradation. Data sets from previous experiments (Keep 1998) obtained at temperatures from 20 to 140 degreesC for sapwood at 5, 15 and 20% moisture contents (dry basis) were analysed. The data were fitted for various theoretical models, namely the Maxwell, Kelvin and Burgers models, and the Bailey-Norton equation. The parameter values and standard errors for the above models over the range of experimental data have been determined. The results indicate that the Maxwell model did not fit the experimental data well, having only one parameter. In most cases, the Bailey-Norton equation was inadequate, as it is a power-law model with a predicted continuous increase in creep with time and does not predict a plateau in the creep strain, as has been observed for most of Keep's (1998) data. The Kelvin model gave a better fit than the Bailey-Norton equation for most of the data sets. From visual inspection of the plots for the experimental data and the model predictions with time, it was found that both the Kelvin and Burgers models fitted the data satisfactorily. However, the three-parameter Burgers model was not a significant improvement over the two-parameter Kelvin model at the 0.01 level of significance, as shown by an analysis of variance.