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

Phase solubility diagrams.

<p>Gallic acid (GA) in aqueous solutions of βCD (A) and HPβCD (B). CE is the complexation efficiency and D:CD is the molar ratio between GA and HPβCD.</p

Analysis of hypha counting.

<p>Median with range of the attributed scores for each group after hypha counting.</p

SEM micrographs.

<p>Gallic acid (GA) 190X magnification (A), HPβCD 190X magnification (B), physical mixture of GA and HPβCD 150X magnification (C) and GA/HPβCD spray-dried microparticles, 2,000X magnification (D). In Fig C the upper arrow points out to the GA morphology and the lower arrow points out to the HPβCD one.</p

DSC thermograms.

<p>Gallic acid (GA) raw material (A), HPβCD (B), physical mixture between GA and HPβCD (C) and GA/HPβCD spray-dried particle (D).</p

Antifungal activity on oral candidiasis in rats.

<p>Control Group (A), GA/HPβCD spray-dried microparticles group (B) and Nystatin group (C). (A1) Histologic sections stained by Hematoxylin and eosin (HE) method, 100X magnification, exhibiting hyperkeratosis, and micro-abscesses formation (circle) in the epithelium that is also acanthotic. The fibrous connective tissue has wide vessels surrounded by a moderate inflammatory infiltrate. (A2) PAS staining, 100X magnification, where <i>C</i>. <i>albicans</i> yeast and mostly hyphae invade the epithelium reaching the connective tissue (arrowheads). (A3) HE staining, 200X magnification. Inflammatory epithelial alterations as hyperkeratosis, epithelial stratification loss (square), and basal layer disorganization (arrows) are evident. (B1) Histologic sections stained by hematoxylin and eosin (HE) method, 100X magnification, exhibiting hyperkeratosis, and acanthosis of the epithelium that covers a hyper-vascularized (arrows) connective tissue. (B2) HE staining, 200X magnification. Reactive hyperkeratosis of the epithelium (arrowheads) was the only remarkable inflammatory induced alteration for the GA/HPβCD group. Some congest vessels can be seen in the connective tissue (arrows). (B3) PAS staining, 100X magnification, where <i>C</i>. <i>albicans</i> yeast and mostly hyphae invade the epithelium, some of them reaching the connective tissue (arrows). (C1) Histologic sections stained by hematoxylin and eosin (HE) method, 100X magnification, hyperkeratosis (line), micro-abscess formation (circle), and acanthosis of the epithelium that covers a hyper-vascularized (arrowheads) connective tissue can be seen. (C2) PAS staining, 100X magnification where large groups of <i>Candida albicans</i> yeast and hyphae can be noted. Though most of them are limited to the keratin layer, some invade the epithelium (arrows). (C3) HE staining, 200X magnification, where epithelial hyperkeratosis (line) is evident. We can see poor inflammatory alteration (arrows), as hydropic degeneration and duplication of the basal layer. Some congest vessels can be seen in the connective tissue (arrowheads) surrounded by moderate inflammatory infiltrate.</p