Abstract
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.
Highlights
A thorough understanding of the thermo-hygro-mechanical behaviour of oak wood materials under indoor climate fluctuations is essential for the safe preservation of decorated door panels in historical oak wood cabinets
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
Summary
A thorough understanding of the thermo-hygro-mechanical behaviour of oak wood materials under indoor climate fluctuations is essential for the safe preservation of decorated door panels in historical oak wood cabinets. The fracture behaviour of oak wood is simulated in accordance with the approach applied in a previous work (Luimes et al, 2018a), whereby discrete cracks are simulated by surrounding the continuum elements in the finite element model with interface elements equipped with the mixed-mode interface damage model This modelling strategy was originally proposed in Xu and Needleman (1994), and allows for the robust simulation of crack patterns at arbitrary locations and in arbitrary directions.
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