Experimental and simulation studies on the mechanism of moisture-induced warping of traditional Japanese sliding doors

As a traditional Japanese fixture, fusuma is sliding doors, often displaying paintings on paper. However, because fusuma is also building components, it is often preserved and exhibited inside cultural heritage buildings, such as temples, shrines, and castles, which have low thermal insulation and airtightness; therefore, fluctuations in temperature and humidity may cause deformation of fusuma and tears in painted paper. This study focused on the deformation mechanisms of fusuma paintings preserved and exhibited inside cultural property buildings. Specifically, the study investigated the warping behavior of tobusuma—a type of fusuma made by attaching a support such as plywood or carbon fiber reinforced plastic (CFRP) board to one side to increase strength—when temperature and humidity differences occur on the front and back, clarified the warping mechanism, and evaluated the effect of different support types on warpage. In the experiment, an actual-sized tobusuma was prepared, and warpage was measured by subjecting its front and back surfaces to different temperatures and relative humidity. Additionally, we reproduced our experimental results using a finite element method (FEM) simulation and analyzed the effect of each tobusuma component on the warpage. The results showed that tobusuma tends to warp toward higher relative humidity if temperatures on both sides are the same, and that the direction and magnitude of warpage depend on the support type. For the plywood support, greater warpage occurred when the interior side was humid, whereas for the CFRP support, greater warpage occurred when the exterior side was humid because of its low moisture permeability; however, when the exterior side was humid, the amout of warpage was similar for both supports. In typical field environments, where the interior is generally more stable and less humid, the type of support may have a limited impact on warping. In the FEM simulation, the reproducibility of the simulation model was lower for the CFRP support than for the plywood support, which is considered to be due to uncertainties in the mechanical boundary conditions and moisture content input of the components. In particular, when the hygrothermal properties of components differ significantly, like CFRP, it is more important to perform coupling between moisture transfer and moisture-induced mechanical deformation simulation. The study results contribute to the selection of suitable support for preserving fusuma paintings and to investigation of environments in which they are installed. For a deeper understanding of fusuma paintings’ deterioration mechanism, it is necessary to investigate not only the warping of tobusuma but also the deformation behavior of painted paper and the discoloration/fading of the paintings. Since these phenomena are closely related to environmental factors such as temperature, humidity, illumination, and ultraviolet light levels, we consider that clarifying these deterioration mechanisms step by step will lead to the suggestion for optimal preservation and exhibition methods inside cultural property buildings with environmental fluctuations.