In situ measurement of wood stress during drying process

Innovative in situ measurement of internal stress of wood during the drying process is presented. In order to quantify the internal stress and subsequent strain generated in wood during the process(1,2), different indirect and semi-direct techniques can be adopted such as the asymmetrical drying test, and other standard destructive tests(3), these semi-direct techniques provide measurement of the overall effects of stress as the combination of internal gradients. In order to specifically measure the internal stress at a given depth in the material, an in-situ measurement is required, by inserting a probe which can measure the local pressure of the wood along a specific axis. This system needs to be non destructive and minimize the interference of the measurement specimen with the material. Moreover, it has to be chemically inert towards the environment of the drying oven. A custom sensor was designed based on a silicon micromachined pressure gauge inserted in a Teflon nail: Teflon provides protection, it works as a medium between the gage and wood, and it makes it possible to handle the sensor. In this configuration the pressure gauge has directional sensitivity, and therefore axis-selective measurement is possible. An easy to use sensing device was realized, reducing the package dimension to the minimum (6mm diameter). A silicon pressure gauge by Entran (TM) was used. The influence of the Teflon package on the gauge sensitivity was studied by finite element simulation using ANSYS (TM) software. Measurements of internal stress during drying process were carried out and compared with results obtained by different measurement techniques. A first empirical confirmation of the theoretical models for wood stress was obtained. The research have been carried out in the framework of "SILE 2" project, with the financial support of the Autonomous Province of Trento.