Crack formation during solid pyrolysis: evolution, pattern formation and statistical behaviour.

As solids pyrolyse during combustion, they lose chemical and structural integrity by gradually degrading into residual char and forming defects such as voids, fissures and cracks. The material degradation process, which is coupled to the crack formation process, is described using a theoretical model and is numerically simulated using the finite-element method for a generic, charring, rubber-like material. In this model, a slab of material is subjected to an external, localized heat flux and, as the material degrades, cracks form when the local principal stress exceeds a defined cracking threshold. The magnitude of the cracking threshold sigma(c) is systematically varied in order to examine its influences on crack initiation, evolution, distribution and behaviour over time. When sc exceeds the maximum principal stress for the entire process, sigma(m), then no cracks are generated. We quantify how the average crack spacing, total crack length and crack initiation time depend upon the ratio sigma(c)/sigma(m). Two characteristic domains of crack formation behaviour are identified from the crack initiation behaviour. Correlations are produced for the crack length evolution and final crack length values as functions of sigma(c)/sigma(m). Crack intersection patterns and behaviour are described and characterized.