Experimental investigation and material modelling of fresh and UV aged Japanese lacquer (Urushi)

Urushi is a complex natural polymer that has been used to protect and decorate objects for many hundreds of years. It is an important material as decorated objects can obtain great value and historical worth. These objects 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. The conservation work requires a detailed understanding of the material properties of the Urushi lacquer film. However, Urushi exhibits complex viscoelastic behaviour under load that has not been fully characterised to date. This paper presents the sample preparation technique and experimental data from a comprehensive mechanical testing programme for Urushi film. The viscoelastic response was investigated by tests at various loading speeds and creep and recovery tests. A number of constitutive models were fitted to the creep and recovery data and a good fit was seen with a number of these, most notably a modified generalized Kelvin fluid (MGKF) model. Some samples of Urushi were artificially aged by laboratory exposure to ultraviolet (UV) irradiation before mechanical testing. The ageing increased strength and reduced the ductility of the Urushi and it was shown that the MGKF model was capable of modelling the ageing behaviour by using ageing-time dependent material parameters. The models were implemented in the commercial FEA software ABAQUS, offering the potential to accurately model the Urushi behaviour in a complex structure.