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

When a flat sample of medium density fibreboard (MDF) is exposed to radiant heat in an inert atmosphere, primary crack patterns suddenly start to appear over the entire surface before pyrolysis and any charring occurs. Contrary to common belief that crack formation is due to drying and shrinkage, it was demonstrated for square samples that this results from thermomechanical instability. In the present paper, new experimental data are presented for circular samples of the same MDF material. The sample was exposed to radiant heating at 20 or 50 kW/m2, and completely different crack patterns with independent Eigenmodes were observed at the two heat fluxes. We show that the two patterns can be reproduced with a full 3-D thermomechanical surface instability model of a hot layer adhered to an elastic colder foundation in an axisymmetric domain. Analytical and numerical solutions of a simplified 2-D formulation of the same problem provide excellent qualitative agreement between observed and calculated patterns. Previous data for square samples together with the results reported in the present paper for circular samples confirm the validity of the model for qualitative predictions, and indicate that further refinements can be made to improve its quantitative predictive capability.Comment: 9 pages, 13 figures. New title and abstract, added experimental and simulation details and figures, conclusions unchanged. Matches the version published in Fire And Material

Microscopic images of dorsal surface of <i>C</i>. <i>albicans</i>-infected tongues 48 hours after AM-CAPPJ exposure.

<p>(1) HE-stained; (2) PAS-stained; (A) non-treated group; (B) plasma-treated; (C) Nystatin-treated; (D) Scores (median values and ranges) attributed to the inflammatory alterations and tissue invasion 48 hours after exposure to AM-CAPPJ.</p

Microscopic images of dorsal surface of <i>C</i>. <i>albicans</i>-infected tongues 24 hours after AM-CAPPJ exposure.

<p>(1) HE-stained; (2) PAS-stained; (A) candidiasis induced and non-treated group; (B) plasma jet-treated; (C) nystatin-treated; (D) Scores (median values and ranges) attributed to the inflammatory alterations (column 1) and tissue invasion (column 2) 24 hours after exposure to AM-CAPPJ.</p

Scores for epithelial alterations.

<p>Scores for epithelial alterations.</p

Number of viable cells in <i>C</i>. <i>albicans</i> ATCC 18804 biofilms (median and range) exposed or not to AM-CAPPJ (n = 9).

<p><b>* p<0.05</b>.</p

Typical current and voltage waveforms for 2.0 slm of He flow and 1.5 cm distance to the target.

<p>Typical current and voltage waveforms for 2.0 slm of He flow and 1.5 cm distance to the target.</p

Counts of colony forming units in mice’ tongues 24 hours after the treatment with nystatin or AM-CAPPJ.

<p>Counts of colony forming units in mice’ tongues 24 hours after the treatment with nystatin or AM-CAPPJ.</p

Experimental setup.

<p>Experimental setup.</p

Q-V Lissajous figure for one voltage cycle.

<p>The figure’s area is 151.3 μJ.</p

Timeline of the experiment.

<p>Timeline of the experiment.</p