Crack channelling mechanisms in brittle coating systems under moisture or temperature gradients

Abstract

Crack channelling is predicted in a brittle coating-substrate system that is subjected to a moisture or temperature gradient in the thickness direction. Competing failure scenarios are identified, and are distinguished by the degree to which the coating-substrate interface delaminates, and whether this delamination is finite or unlimited in nature. Failure mechanism maps are constructed, and illustrate the sensitivity of the active crack channelling mechanism and associated channelling stress to the ratio of coating toughness to interfacial toughness, to the mismatch in elastic modulus and to the mismatch in coefficient of hygral or thermal expansion. The effect of the ratio of coating to substrate thickness upon the failure mechanism and channelling stress is also explored. Closed-form expressions for the steady-state delamination stress are derived, and are used to determine the transition value of moisture state that leads to unlimited delamination. Although the results are applicable to coating-substrate systems in a wide range of applications, the study focusses on the prediction of cracking in historical paintings due to indoor climate fluctuations, with the objective of helping museums developing strategies for the preservation of art objects. For this specific application, crack channelling with delamination needs to be avoided under all circumstances, as it may induce flaking of paint material. In historical paintings, the substrate thickness is typically more than ten times larger than the thickness of the paint layer; for such a system, the failure maps constructed from the numerical simulations indicate that paint delamination is absent if the delamination toughness is larger than approximately half of the mode I toughness of the paint layer. Further, the transition between crack channelling with and without delamination appears to be relatively insensitive to the mismatch in the elastic modulus of the substrate and paint layer. The failure maps developed in this work may provide a useful tool for museum conservators to identify the allowable indoor humidity and temperature fluctuations for which crack channelling with delamination is prevented in historical paintings.

Chemo-mechanical model for degradation of oil paintings by amorphous and crystalline metal soaps

“…11(d) that the delaminating cracks are close to coalescence once the mode I surface crack has grown through the entire thickness of the upper, pictorial layer, thereby forming a T-shape crack with the mode I surface crack. T-shape cracks are regularly observed in historical paintings, and may originate from metal soap formation and/or indoor climate fluctuations, often leading to local paint flaking and paint loss (Bosco et al, 2020(Bosco et al, , 2021.…”

After the paint has dried: a review of testing techniques for studying the mechanical properties of artists’ paint

“…The FEA simulation used to create this stress field is based on the simulations used in Bratasz et al [116] dePolo et al Herit Sci (2021) 9:68 growth behavior of metal soaps combined the chemical information related to the diffusion of metal soaps into aggregates with estimated mechanical properties for a paint layer to both understand the directionality of metal soap formation and the most likely regions for cracks to form within the model from the stresses caused by the aggregates [120]. More recent investigations from Bosco et al have been focused on understanding the underlying deformation mechanisms that can lead to crack opening or delamination when a paint layer is exposed to flexural stresses as a result of increased moisture [121,122].…”

Hygro-mechanical short-term behaviour of selected coatings: experiments and material modelling on vapour permeability and mechanical properties

“…Hygric properties are investigated in [3,4,[6][7][8][9][10][11]. Simulation approaches are presented and used in [1,3,4,8,9,12,13]. In [1,2], the focus is on dimensional changes of coated wood causing stresses by differences in hygroexpansion and stiffness between wood and coating and, therefore, only mechanical and swelling/shrinking properties are investigated for glue, gesso, oil paints, canvas and wood.…”