Image correlation to evaluate the influence of hygrothermal loading on wood

Abstract

Many wooden objects from cultural heritage consist in wooden panels, painted on one face. Some of these panels show permanent cupping, micro-cracks of the painted layer, and cracks of the painted support itself. Different physical and mechanical phenomena are at the origin of these damages: wood is a hygroscopic material (its dimensions vary with humidity), it is highly anisotropic, the paint layer on one face has properties of permeability different from those of raw wood of the back face, and a rigid frame possibly restrained the deformation of the panel. Experimentations on our mock-up panels combined with numerical simulations of these panels in real situations of hygrothermal fluctuations will allow us to test specific situations and eventually to make suggestions to conservators and restorers and guide them in their interventions. Hygrothermal treatments are often used to improve wood durability thanks a reduction of its hygroscopicity. They have been considered as means to reproduce the physical properties of ancient wood. We intend to model the mechanisms involved in mechanical and chemical effects of wooden painted panels exposed to climatic variations. To develop such conservation tool, we need to work on mock-up, which replicate panel painting. So we will investigate the process of hygroscopic ageing and compression set generation of the back of painted panels to explain their permanent cupping and replicate their ageing state. For this purpose, digital image correlation is used to evaluate the strain field of the section of a wood piece submitted to variations of relative humidity.