Crack morphology evolution due to pyrolysis and combustion in solids

"A model is presented for processes that couple thermal degradation to cracking, with a focus on crack formation and propagation during pyrolysis. As the pyrolysis front propagates into the sample, a charring layer is left behind which contains voids, fractures and defects. Cracks propagate to release tensile stresses accumulated when the sample is losing its mass. They can intersect each other, forming loops, which are isolated fragments, unable to provide structural support. The pyrolysis cracking process is simulated using FEM (Finite Element Method). The FEM code is parallelized using MPI (Message Passing Interface) in order to accelerate and capture the damage on a meso scale. Various dimensionless groups characterizing the problem are determined. Parameter groups are varied to investigate their effects on the morphology of the crack patterns. The crack patterns obtained from the numerical simulations are quantified using image analysis algorithms and functions that were developed and implemented in MATLAB. The crack patterns share similar morphological features with other patterns found in nature or in laboratories, such as the hierarchy of the cracking arrays of quenched plates, the polygonal mud cracks, the tree-like structures of river network, and leaf veins. The expression of the tree-like or loop-like behavior is dependent upon the choices of the parameters. In particular, as the ratio of tensile strength to Young's modulus increases, the crack behavior shifts from intersecting toward branching. The behavior is also influenced by possible anisotropy in the thermal diffusivity: behaviors that range from cracks that spread out to cracks that cluster together. Furthermore, other quantities, such as crack spacing, crack length, crack propagation rate, loop directions, junction angles and their distributions, crack initiation time, as well as their dependence on material properties, are computed as well, which provides additional understanding of the governing mechanisms."--Pages ii-iii.