Modelling the creep behaviour of chipboard: The rheological approach

Summary The process of applying the rheological approach to describe creep deformation has been investigated using the 5-parameter model developed by Pierce and Dinwoodie (1977).A method utilising exponential curve fitting techniques was examined by attempting to produce fits to the first 6 months' data from long-term experiments (12 years). The procedure included a means of assessing the variability occurring in the derived model parameters which showed that acceptable fits were obtainable for only 3 out of the 20 data sets available. Further analysis of one of these data sets (that for cement-bonded particleboard, subjected to a stress level of 30% of the short-term ultimate in an environment of 25 °C and 90% relative humidity) revealed that the model attributed most of the creep deformation to the viscous component; resulting in an over-estimate of the deformation at 12 years when the 6-month fit was projected.An alternative approach to deriving the value of the model parameters is proposed, based on the theory underlying the model. The applicability of this approach was investigated using the above mentioned data set. It was found that the model can be constrained to give a more realistic ratio between the time-dependent components of deformation (ie. viscoelastic and viscous creep) for the material considered, thereby improving the accuracy of the projected creep deformation over the 12-year period.