Damage of Porous Materials During Drying

摘要

The shrinkage stresses during drying can exceed the strength of the material. In such a case the fracturing of the layers near the surface of the material is expected. The fracture does not happen in the middle of the plate because of the compressional stresses in this region. The stresses caused by two various drying processes are shown. The stresses arising during the first proces exceed the strength of the material, but those arising during the second one do not. This is the consequence of increase of moisture flow coefficient Λ_X together with the temperature (see (5). The greater is this coefficient the greater rate of moisture transfer inside of material takes place and consequently the smaller moisture gradient inside of material occurs. Smaller gradient involves smaller stresses. The shrinkage of the clay ends at some critical value of moisure content and at this moment the second drying period for this material begins. When the moisture content inside of the plate becomes smaller than X_r (the moisture content at which the shrinkage ends, see (8)), the stresses decreases very quickly. So the damage of the material is not expected during this period of drying.