A theoretical and numerical model for the degradation of solid materials in combustion is developed. As solid materials are heated by the flame, they undergo an internal thermo- chemical breakdown process known as 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 mass. The crack front may precede the pyrolysis front into the sample. Crack patterns and fracture behaviors vary depending on material properties and heating level and distribution. Cracks cause loss of material integrity by forming isolated loops or fragments. Cracks concentrate the stresses and reduce material ability to withstand external loads. Cracks expose uncharred materials to flame, accelerating combustion. The process is highly nonlinear: crack patterns display fractal behavior. Dimensionless groups that define the model are examined: each yields different crack patterns.