Thermomechanical surface instability at the origin of surface fissure patterns onheated circular MDF samples

When a flat sample of medium density fibreboard (MDF) is exposed to radiant heat in an inert atmosphere, primarycrack patterns suddenly start to appear over the entire surface before pyrolysis and any charring occurs. Contrary tocommon belief that crack formation is due to drying and shrinkage, it was demonstrated for square samples that thisresults from thermomechanical instability.In the present paper, new experimental data are presented for circular samples of the same MDF material. Thesample was exposed to radiant heating at 20 or 50 kW/m2, and completely different crack patterns with independentEigenmodes were observed at the two heat fluxes. We show that the two patterns can be reproduced with a full 3-Dthermomechanical surface instability model of a hot layer adhered to an elastic colder foundation in an axisymmetricdomain. Analytical and numerical solutions of a simplified 2-D formulation of the same problem provide excellentqualitative agreement between observed and calculated patterns.Previous data for square samples together with the results reported in the present paper for circular samples confirmthe validity of the model for qualitative predictions, and indicate that further refinements can be made to improve itsquantitative predictive capability.