Relaxation of stresses in glass surface regions by subcritical crack growth

In previous reports, we dealt with the SIF computations of semielliptical surface cracks introduced in glass specimens by grinding. It can simply be shown that subcritical extension of such cracks must result in a reduction of stresses applied to the specimen.
We apply this effect on tensile tests and on torsion tests by Aaldenberg et al. carried out on cylindrical specimens at temperatures between 550°C and 700°C in normal lab air environment. The torsion moments Mt measured by Aaldenberg et al. decreased with time t for a constant drill angle. This effect was interpreted by Aaldenberg et al. as a proof for a relaxation effect by the presence of molecular water.
In the present report, we discuss the possibility that the reduction of the torsion moment is caused by subcritical crack growth that results in comparable effects.