Cracks pressure gradient

A may be modelled as being proportional to 6 so that in crack growth it is constant ance 5 = 8. Assuming an approximately constant pressure gradient, arising... 

Cracks can also appear during the pressure release in the autoclave. In the supercritical drying process, the gel is subjected to high temperature and high pressure. When the critical point is reached, the pressure of the autoclave is decreased while the temperature is kept constant. At this instant, the pressure applied to the supercritical fluid is equal to that within the pores. The supercritical fluid has a very low density and viscosity compared with that of the liquid at room temperature however, the low permeability of the gel resists the flow of the supercritical fluid out of the gel. In other words, if the supercritical fluid release is performed too fast a pressure gradient appears. In this case the supercritical fluid within the gel, which is in compression, suddenly expands and the solid part suffers tensile stress. Experiments show that cracking depends on the pressure release rate, on the nature of the gel (basic or neutral), and on its geometrical dimensions. 

Because it is the pressure gradient that causes differential strain and cracking, it is evident that fast evaporation and low permeability are detrimental. Cracks usually appear at the end of the constant rate period when the shrinkage stops and the meniscus recedes into the pore. At this point, the radius of the meniscus is minimal and depends on the pore radius tp and contact angle 6 ... 

Samples prepared in this way were studied by x-ray diffraction, infrared absorption and optical polarizing microscopy and errors in the accuracy of the results were always present because of the large pressure gradient. In this ungasketed configuration, sample pressures not much greater than 30 GPa were reached before cracks developed in the diamond anvils ultimately leading to failure. 

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