Predicting craquelure and delamination mechanisms in canvas and panel paintings

This study aims at predicting cracking
mechanisms in historical paintings due to
indoor climate fluctuations, with the objective
of helping museums developing appropriate
strategies for the preservation of
art objects. Painting systems are modelled
as bilayers consisting of a brittle coating
(the paint layer) adhering to a substrate of
finite thickness, and subjected to uniform
moisture or temperature variations. Different
failure scenarios may develop, which
consist of a channelling crack located in the
paint layer with or without delamination at
the interface between the paint and the support.
The problem is studied based on linear
elastic fracture mechanics principles, combined
with two-dimensional plane-strain
finite element analyses. Remote stresses for
steady-state channelling and plane-strain
delamination are computed, as a function
of the stiffness and hygroscopic coefficient
mismatches of the layers, and the fracture
toughness ratio between the paint layer
and the interface between the paint and the
wooden or canvas support. Based on this,
failure maps are constructed that show the
competition between the identified failure
scenarios. The influence of the framing system
on the fracture response of the painting
is also investigated. From a conservation
perspective, in historical paintings, crack
channelling with delamination needs to be
avoided under all circumstances, as it may
induce flaking of paint material. Considering
the substrate thickness to be typically
more than ten times larger than the thickness
of the paint layer, the failure maps
constructed from the numerical simulations
indicate that paint delamination is absent if
the delamination toughness is largerthan
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.