Investigation of Fracture in Polymeric Coatings

The objective of this study is to quantify the effect of low amplitude cyclic stresses, such as those induced by environmental condition fluctuations and transportation, on multilayer paint systems found in works of art. A model was developed to establish criteria for damage, which take into account viscoelastic fatigue, and to establish safe rates of change for environmental parameters. To establish the methodology, the investigation focused on modern paintings executed in mixed media. In particular, acrylic gesso grounds with superimposed alkyd paint layers on canvas were investigated, which have been found to be vulnerable to stresses and delamination. Data from uniaxial testing of free-standing paint films were used to determine the constitutive properties of the paint. The effects of temperature, strain rate and age on the tensile properties were investigated. Results from peel tests, performed to determine the energy release rate of the interface between the paint and gesso layers, are reported. The peel tests were modelled using Finite Element Analysis with cohesive zone elements at the interface in a commercial finite element software Abaqus. The value of the maximum traction in the traction-separation law was determined by comparing numerical and experimental peel loads and the cohesive energy was determined using an established analytical method. The cohesive zone properties determined from the peel tests, and the calibrated constitutive model for the alkyd paint, were used in a separate finite element model of a coating on a primed canvas substrate subjected to combined cyclic hygrothermal and static mechanical loadings typically experienced by fine art paintings; interface separation was controlled by an irreversible cohesive zone model that includes damage accumulation due to cyclic loading. Fatigue crack initiation times in years, and crack propagation rates, are predicted under various conditions including ordinary and extreme histories that paintings may experience in museum and conservation settings