Determination of residual stresses in medium density fibreboard

Due to the moisture and temperature gradients developed during hot pressing of medium density fibreboard (MDF), residual stresses occur within the boa rd as it equilibrates to room conditions. It would be extremely useful to m easure these residual stresses and to determine their effects on board prop erties such as moduli of elasticity and rupture in bending, internal bond s trength and dimensional stability. In this article two methods, namely diss ection and hole drilling, have been adapted to measure residual internal st ress distributions in six different samples of industry produced MDF. The d issection method involves cutting several pieces of MDF perpendicular to th e thickness direction at different depths. The residual stresses released b y the dissection can be determined by measuring the curvatures of cut piece s 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 MD F and drilling a hole to release residual stresses in close proximity. The released stresses are manifested as strains in the forms of which can he me asured in three directions on the surface of the board. A theoretical model for predicting residual stresses involving various para meters has been developed and an excellent agreement with the experimental results from both the dissection and hole drilling methods has been achieve d. Linear moisture expansion coefficient appears to have the greatest influ ence on residual stress. When compared against each other, the residual str esses measured by the hole drilling method show some shortcomings towards t he centre of the board. While all six of the MDF boards exhibited similar t rends in their residual stress distributions, significant differences were identified in the magnitudes of residual stress measured. Finally, some pre liminary results linking the residual stress with the thickness swell of th e samples and their surface densities have been presented.