Modelling the mechanical response of Japanese lacquer (urushi) to varying environmental conditions
2012-01-24T13:15:50Z (GMT) by
Urushi is a complex natural polymer that has been used to protect and decorate objects for many hundreds of years. Urushi is an important material as decorated objects can obtain great value and historical worth. A star item of urushi lacquerwares, the Mazarin Chest, property of Victoria & Albert (V&A) Museum in London, is famous as one of the finest pieces of Japanese lacquerware in the world, dating back to as early as the late 1630s. These urushi lacquerwares are often exposed to environments that are detrimental to both their aesthetic appeal and structural performance, and restoration and conservation procedures are needed to preserve these objects over long periods of time. However, the precise behaviour of urushi lacquers is not sufficiently understood to allow accurate prediction of the material response to environmental effects or of the effectiveness of any proposed conservation procedure. Thus a need exists for a comprehensive understanding of this material and a finite element (FE) model to predict the mechanical response to varying environmental conditions. The aim of this research was to model the hygro-mechanical effects during the environmental ageing process of urushi films by means of FE modelling, which will help to make decisions about the environmental conditions required for storage and exhibition. This was achieved by a combination of experimental and computational methods. A synthetic thermosetting (crosslinked) lacquer, polyurethane (PU), was initially studied to develop the methodology of the experimental and numerical studies and to provide a comparison to the natural lacquer. Experimental work was carried out to characterise the material behaviour of both materials, such as hygroscopic expansion, moisture diffusion kinetics and mechanical behaviour under various environmental conditions. Moisture diffusion in the two lacquer films was well described by Fick s law. However, different sorption isotherm models, Flory-Huggins model for PU and Guggenheim-Anderson-de-Boer (GAB) model for urushi, were required to model the equilibrium sorption of these two materials. The mechanical properties of the lacquer films were found to have a complex dependence on environmental conditions. The tensile properties of both lacquer films were shown to change significantly after UV ageing. With increasing time of ultraviolet (UV) irradiation, Young s Modulus and tensile strength increased dramatically, but the maximum strain decreased. With water absorption, both Young s modulus and tensile strength decreased, and maximum strain increased. The two lacquer films were found to behave with a non-linear viscoelasticity, which was highly dependent on environmental conditions. A modified Burger s model was found to provide a good fit to experimental creep data for the PU lacquer well at different stress levels, suggesting this is a satisfactory method for characterising PU rheological behaviour. A novel modified generalized Kelvin fluid (MGKF) model was found to be a powerful non-linear viscoelastic model capable of representing the rheological behaviour of the urushi below the yield stress. However, in order to include the post yield behaviour a visco-elastic-plastic model is required. A hygro-mechanical model of the urushi behaviour based on the MGKF viscoelastic model was developed and tested. Through careful determination of the mechanical behaviour the constitutive properties of a thin layer of lacquer were determined and used as an input to a FE based model of the deformation and stresses that develop in response to changes in the environmental conditions. The model was validated using experimental results that showed the depth averaged stress in a thin layer deposited on a glass substrate, which allowed insight into the time dependent and spatially varying stresses within the layer. It was shown that the regions of highest stress were found in areas of highest moisture ingress, emphasising the need for control of the environment in which urushi covered artefacts are stored. In order to perform a predictive study on the bending behaviour of urushi films with cracks, as can be seen in aged lacquers, a model with a grooved urushi film on an aluminium substrate was created and subjected to bending loads. The time-dependency of the urushi material properties seems to hardly affect the bending behaviour of the model, however, the profile of the displacement field around the groove was found to be considerably affected by the geometry of the groove. To evaluate the effectiveness of a traditional Japanese consolidation method for lacquerware objects, known as urushi-gatame, a strain-based progressive damage failure model was used to model the continuum failure in the bi-material strip under an increasing bending load. The behaviour of damage initiation and evolution was modelled for an unfilled groove, a filled groove with fresh filler and filled groove with UV aged filler. From the finite element analysis (FEA) results, the introduction of the filler, as a simple mimic of the consolidation method, does enable the strip to sustain a higher bending load. However, this effectiveness is weakened as the material is aged, with it behaving similar to a groove without any filler.