Modellierung und numerische Simulation des Feuchte- und Temperaturverhaltens von Holz-Aluminium-Verbundträgern

Abstract

Knowledge about the wood moisture condition in a timber component is essential to predict its mechanical behaviour. Not only stiffness and strength properties are highly dependent on the wood moisture content but also diffusion coefficients, density, specific heat capacity and the thermal conductivity. Therefore, modern prediction tools, which are able to describe these effects, can be of great benefit for the development of new wood-based products. Especially if these products exhibit complex geometries and are made of materials with different moisture characteristics, as it is the case with the new formwork beam I tec pro developed by Doka, which is made up of three different materials: spruce in the flange, an aluminium rail also in the flange, and a special particleboard as web. Different and direction-dependent coefficients of expansion of these materials may lead to critical stresses due to deformation. The stress levels depend on geometric and climatic conditions. These stresses can, under unfavourable circumstances, exceed the adhesive strength of the glue between materials or lead to cracks within wood itself. For this reason, the aim of this thesis has been to better understand these geometry, moisture- and temperature-induced processes and to make predictions based on a newlydeveloped numerical simulation tool. Thereby, the moisture transport processes below the fibre saturation point have been modelled by a multi-Fickian approach, consisting of two moisture transport equations. For a description of heat and moisture transport, this approach has been expanded by an energy conservation equation according to Eitelberger [2011]. The resulting three coupled differential equations were implemented with user subroutines into Abaqus and solved iteratively in a sequential manner. The simulation results were compared with results from experiments in Frandsen et al. [2007a] and Dvinskikh et al. [2011]. Essentially, a very good agreement between the simulated and measured moisture fields was obtained. As additional validation, a climate change experiment on a formwork beam, performed by Holzforschung-Austria [2010], was simulated. Again, a good predictive quality of the developed simulation tool was obtained. Finally, an alternating climate test cycle, as defined in DIN EN 321 [2002], of the above described formwork beam I tec pro was fully simulated. This consists of four phases: water storage, freezing, drying and cooling. Important trends, like the stress level and deformation at lower humidities than the initial state, could be well predicted. Moreover, it was found that the position of the stiffener of the aluminium profile has a significant influence on the results. With the numerical simulation tool presented in this thesis, it is possible to calculate at any time the values of the relative humidity, the wood moisture content and temperature in any point of a timber cross-section. Important information, unidentifiable from experiments, on critical stress and moisture states as well as good deformation predictions can be obtained therefrom.