Mechanical deformation of a wooden panel due to a varying climate – Numerical simulations

Abstract

ABSTRACT Wood has been used for thousands of years as interior decoration. It is therefore logical that many objects made of this material are found in a cultural heritage context. A large amount of historical art pieces, such as decorated wooden panels, have been stored in historical buildings. Heating systems are not common in historical buildings, not even after renovations. In some occasions it is not allowed to apply an HVAC-system because of the cultural heritage value of the building itself. Since these buildings are often not climate controlled it is important to know how the indoor climate is affecting the historical art. The aim of this study is to understand the cause-effect relationship between a fluctuating climate and wooden panels. Wood is a hygroscopic material, meaning that it will absorb and desorb moisture during changes in relative humidity and temperature of the ambient air. Changing moisture content in the wood comes with expansion or contraction of the wood cells, which could cause deformations of the panel. A simulation model is developed to compute the state of deformation, which is expressed in curvatures. Two types of simulations are made; simulations of moisture transport in wooden panels subjected to a fluctuating climate during a year; and isothermal simulations for mechanical deformation of the panel due to changes in the vapour concentration at the surface. Before running the simulations, a literature study was done to get a greater view of the possible outcome of the simulations. With the numerical tool, moisture transport was analysed first and afterwards the curvature of the panel was computed. Six indoor climates were taken as an input, which gave six simulations of the curvature for one panel. The model confirms that deformation of the panel is in line with fluctuations of relative humidity of the ambient air. By increasing the surface vapour resistance factor of the panel, the range of the curvature decreases. The calculations show many repetitive and alternating cycles of curvature. However, it is not known how many and how big these cycles need to be before cracks occur. Therefore a proposal for future research is to develop a scale that represents damage against deformation. Keywords: Wood, relative humidity, moisture content, temperature, moisture transport, moisture transport coefficients, expansion coefficients, deformation, pores, paintings, historical buildings