Experimental observation of crack formation on surface of charring timber

Crack formation on the charring surface of burning wood is an important factor increasing the burning rate by offering a passage for heat and oxygen, but it remains a poorly understood process. This work considers crack formation on pyrolyzing Norway spruce, Scots pine and birch timbers. Timber specimens of different sizes were tested under various radiative heat fluxes in nitrogen atmosphere. The cracking process was followed with an infrared camera mounted above the specimen. The obtained recordings were used to determine the formation times and lengths of cracks and to estimate the validity of an existing thermomechanical model for crack formation. The results show that the crack formation time has no significant dependence on the specimen geometry. Further, the inverse of the square root of crack formation time follows grows linearly with external heat flux, which is a similar dependence as with time for ignition, according to the thermal model of ignition. The analytical model predictions were of correct order of magnitude, but not consistently accurate at all experimental conditions. This could be accounted for the simplifying assumptions within the analytical model, and therefore creating a more detailed three-dimensional numerical model for crack formation is suggested as future research.