Numerical modelling of climate-induced fracture and deformation in wood: Application to historical museum objects

Damage in oak wood museum objects under indoor climate variations (relative humidity, temperature) is studied using a thermo-hygro-mechanical model. The model incorporates the effects of moisture sorption hysteresis and discrete cracking, and is implemented within a finite element framework using a staggered update procedure. Sorption experiments were performed in order to calibrate the moisture sorption hysteresis model. The basic features of the coupled formulation are stepwisely demonstrated by solving the response of a basic oak wood specimen under a sequence of thermo-hygro-mechanical loading conditions of increasing complexity. These simulations show that the deformation and fracture generated by thermal variations representative of indoor museum conditions are minor compared to the contributions caused by relative humidity variations. Subsequently, the hygro-mechanical response of an oak wood cabinet door panel is analysed under a drop in relative humidity from 60% to 20%, and the results are compared to those obtained by experiments. The numerical and experimental results are in good agreement, and indicate that at the connection between the cleated end and the vertical boards the restrained hygric shrinkage is maximal, which stimulates local crack development. The susceptibility to fracture only becomes activated after the oak wood has reached a certain age, as the effect of aging induces a decrease of the oak wood tensile strength perpendicular to the grain direction with time. Further, when the initial moisture content lies on the desorption boundary curve, the amount of fracture generated is larger compared to when it lies on the adsorption boundary curve. Also, fracture only nucleates after a specific drop in relative humidity, whereby the crack growth under a continuous decrease in relative humidity initially develops relatively fast, but at a certain stage decreases substantially and becomes (almost) zero when reaching a fully developed failure crack. The location and orientation of this failure crack are in accordance with in situ observations on historical oak wood cabinets.