A model discussion of stress and avoiding cracking

From the preceding sections it has become clear that tensile stresses developing during the drying of the lyogel and subsequent calcination are important causes for defect formation by cracking. A tentative scheme to account for a number of data emerges but many details are unknown. Nevertheless some trends are qualitatively predictable. 

The relation between dr)dng process and stress formation is shown in Fig. 8.21. In accordance with drying theory in porous membrane layers a constant drying rate period (CRP) and a falling rate period (FRP) can be distinguished. The transition between them is sharper with increasing thickness. A clear explanation has not been presented but is probably related to the width of the drying zone (see Fig. 8.7) which increases with width w of the pore size distribution. 

During the CRP2 a considerable reorganisation of the gel network takes place and so drying is also a rearrangement process which determines the pore size distribution. 

This capillary stress (see Eq. (8.3)) is related to, and usually smaller than, the developing drying stress (see below). The larger the capillary stress the larger the drying stress which reaches its maximum at or just after the critical point (see Eq. (8.7). 

The gel network in the unsaturated dry part expands slightly when the drying front proceeds because capillary forces no longer act here and the stress 

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