Determination of Residual Stresses in Medium Density Fibreboard

Abstract

Summary Due to the moisture and temperature gradients developed during hot pressing of medium density fibre-board (MDF), residual stresses occur within the board as it equilibrates to room conditions. It would be extremely useful to measure these residual stresses and to determine their effects on board properties such as moduli of elasticity and rupture in bending, internal bond strength and dimensional stability. In this article two methods, namely dissection and hole drilling, have been adapted to measure residual internal stress distributions in six different samples of industry produced MDF. The dissection method involves cutting several pieces of MDF perpendicular to the thickness direction at different depths. The residual stresses released by the dissection can be determined by measuring the curvatures of cut pieces and knowing their elastic moduli. The hole drilling method, on the other hand, involves mounting three strain gauges on the surface of a piece of MDF and drilling a hole to release residual stresses in close proximity. The released stresses are manifested as strains in the forms of which can be measured in three directions on the surface of the board. A theoretical model for predicting residual stresses involving various parameters has been developed and an excellent agreement with the experimental results from both the dissection and hole drilling methods has been achieved. Linear moisture expansion coefficient appears to have the greatest influence on residual stress. When compared against each other, the residual stresses measured by the hole drilling method show some shortcomings towards the centre of the board. While all six of the MDF boards exhibited similar trends in their residual stress distributions, significant differences were identified in the magnitudes of residual stress measured. Finally, some preliminary results linking the residual stress with the thickness swell of the samples and their surface densities have been presented.