Maximum shielding stress intensity factors in silica

When water penetrates into silica surfaces near a crack tip, it reacts with the
SiO${}_2$ network and generates hydroxyl $\equiv$SiOH. Due to the hydroxyl generation,
silica expands and gives rise for swelling stresses. The consequence is a
fracture mechanics shielding stress intensity factor K${}_{sh}$<0. The fracture mechanical
treatment of swelling in the crack-tip region is significantly complicated
by the occurrence of damage due to the production of hydroxyls.
A maximum possible shielding effect is obtained by using an asymmetry parameter
h=1 in the crack wake ($\delta$${}_h$<0) and neglecting the tension region ahead
the crack tip, where strong damage is present under hydrostatic tension
($\delta$${}_h$<0)
.