论文和技术资料

Wooden sculptures are an important part of European cultural heritage, but they are also among the most vulnerable objects to variations of temperature and relative humidity (RH). Fluctuations in the ambient environment can lead to irreversible structural and aesthetical damage. The differences between the dimensional responses of wood in its various anatomical directions due to the loss or gain of moisture generates hygric stresses. These stresses are most pronounced in objects with significant curvature of the growth rings i.e. sculptures, which can be approximated as wooden objects with cylindrical symmetry. As the moisture diffusion process in wood is not instantaneous, tensile and compressive stresses can develop in the outer and internal part of the object respectively. If the stress reaches the critical value of material strength, it can lead to damage development, usually in the form of radial cracks. In the present work, COMSOL Multiphysics® was used to estimate the risk of wooden sculpture cracking due to environmental variations. Thermal Stress Multiphysics (part of Structural Mechanics Module) was applied to simulate the influence of dynamic climate changes on a sculpture. As the mathematical laws describing heat and moisture diffusion are the same, we exploited the capabilities of this Module to describe the development of hygric stresses. Although a sculpture was approximated by a cylindrical geometry, the model was created in 2D due to the negligible dimensional response of wood in the third – longitudinal – direction. The Heat Transfer interface was used to simulate the uptake or loss of water in wood due to the increase or decrease of RH respectively, while the Solid Mechanics interface facilitated the study of the mechanical implications of these processes such as stress field development. A J-integral calculated in the vicinity of a crack tip introduced into the geometry was compared with the critical value of the energy release rate of wood to determine whether the crack will propagate in the simulated conditions. The simulation was run for a range of crack lengths and various RH change scenarios which allowed for the determination of time windows during which the object was at risk of cracking. Experimental work to validate model predictions is underway in both a laboratory and cultural heritage setting. Ultimately, this model will be used to assist cultural heritage management professionals to develop policies for the safe management of building environments in order to preserve and protect precious wooden objects.