A phase-field approach to pneumatic fracture with anisotropic crack resistance

Phase-field models of fracture allow the prediction of crack propagation and crack patterns. In this contribution, externally driven fracture processes in linear and finite elasticity are investigated. Different approaches to consider pneumatic pressure and materials with non-isotropic crack resistance are studied, combined, and examined in detail. The versatility of the proposed models is proven by a series of numerical simulations in two and three dimensions.

Continuously Reinforced Concrete Pavement Cracking Patterns and Properties with Internal Curing and Active Cracking

Transverse cracking in continuously reinforced concrete pavement (CRCP) develops over time and may exhibit irregular patterns such as cluster cracks, Y-shaped cracks, and divided cracks. These undesirable cracking patterns can increase the probability of premature spalling and punchouts. Ideally, a uniform transverse cracking pattern with small crack widths leads to favorable long-term performance in CRCP. An experimental field project was constructed with internally cured concrete and active cracking to control crack initiation timing, patterns, and properties. Prewetted fine lightweight aggregate was used for internal curing, and edge notches of 2 in. depth and 2 ft length every 4 ft along the test section were sawcut for active crack control. The three experimental CRCP sections in Illinois were monitored for crack spacing, crack width, and formation of undesirable cracks over a 4-year period. Internally cured concrete significantly reduced the undesirable crack patterns relative to the control section. Internally cured concrete coupled with active crack concrete produced superior crack patterns and properties, that is, uniform crack spacing of 3.6 ft without cluster cracks, 0.2 mm surface crack width, and only a small number of undesirable cracks. Active crack control produced a higher number of transverse cracks near the terminal joint (last 150 ft) compared with the control section. Moisture and temperature management with active crack control during construction of the CRCP provided a desirable crack pattern and properties that should increase the overall service life of a pavement.

Three-dimensional numerical and experimental study of fracture saturation in panel paintings

Panel paintings—complex multi-layer structures consisting of wood support and a paint layer composed of a preparatory layer of gesso, paints, and varnishes—are among the category of cultural objects most vulnerable to relative humidity fluctuations and frequently found in museum collections. The current environmental specifications in museums have been derived using the criterion of crack initiation in an undamaged, usually new gesso layer laid on wood. In reality, historical paintings exhibit complex crack patterns called craquelures. The present paper analyses the structural response of a paint layer with a virtual network of rectangular cracks under environmental loadings using a three-dimensional model of a panel painting. Two modes of loading are considered—one induced by one-dimensional moisture response of wood support, termed the tangential loading, and the other isotropic induced by drying shrinkage of the gesso layer. The superposition of the two modes is also analysed. The modelling showed that minimum distances between cracks parallel to the wood grain depended on the gesso stiffness under the tangential loading. Despite a nonzero Poisson’s ratio, gesso cracks perpendicular to the wood grain could not be generated by the moisture response of the wood support. The isotropic drying shrinkage of gesso produced cracks that were almost evenly spaced in both directions. The modelling results were cross-checked with crack patterns obtained on a mock-up of a panel painting exposed to several extreme environmental variations in an environmental chamber.

Anisotropic fragmentation of shrinking thin layers on a substrate

<p>Layers of dense pastes, colloids attached to a substrate often undergo sequential cracking due to shrinkage stresses caused by desiccation. From the spectacular crack patterns of dried out lake beds through the polygonal ground patterns of permafrost regions to the formation of columnar joints in cooling volcanic lava, shrinkage induced cracking is responsible for a large variety of complex crack structures in nature. Under laboratory conditions this phenomenon is usually investigated by desiccating thin layers of dense colloidal suspensions in a container, which typically leads to polygonal crack patterns with a high degree of isotropy.</p><p>It is of great interest how to control the structure of shrinkage induced two-dimensional crack patterns also due to its high importance for technological applications. Recently, it has been demonstrated experimentally for dense calcium carbonate and magnesium carbonate hydroxide pastes that applying mechanical excitation by means of vibration or flow of the paste the emerging desiccation crack pattern remembers the direction of excitation, i.e. main cracks get aligned and their orientation can be tuned by the direction of mechanical excitation.</p><p>In order to understand the mechanism of this memory effect, we investigate a fragmentation process of a brittle, cylindrical sample, where the driving force of the cracking coming from a continous shrinkage, which sooner or later destroys the cohesive forces between the structure’s building blocks. Our study is based on a two dimensional discrete element model, where the material is discretised via a special form of the Voronoi-tesselation, with the so-called randomised vector lattice which allows to fine-tune the initial disorder of the system. We assume that the initial mechanical vibration imprints plastic deformation into the paste, which is captured in the model by assuming that the local cohesive strength of the layer has a directional dependence: the layer is stronger along the direction of vibration. We demonstrate that - based on this simple assumption - the model well reproduces the qualitative features of the anisotropic crack patterns observed in experiments. Gradually increasing the degree of anisotropy the system exhibits a crossover from an isotropic cellular structure to an anisotropic one.</p>