Modeling Slow Crack Growth Behavior of Glass Strengthened by a Subcritical Tensile Stress Using Surface Stress Relaxation
Glasses exhibit slow crack growth under stress intensities below the fracture toughness in the presence of water vapor or liquid water. It has been observed by several authors that when an oxide glass with a large crack is held under a subcritical stress intensity (where no slow crack growth occurs) in room-temperature water vapor or liquid water, upon reloading to a higher stress intensity, a finite restart time is observed prior to measurable crack extension. This phenomenon of apparent strengthening, or crack arrest, has been attributed to concepts such as corrosive dissolution of the crack tip, crack tip blunting, or water diffusion, and subsequent swelling of the material around the crack tip. Recently, a newly observed surface stress relaxation process that is aided by molecular water diffusion was used to improve the mechanical strength of glass fibers and to explain the subsurface compressive stress peak observed in ion-exchange strengthened glasses. The same process is employed here to explain these delayed slow crack growth data. A simple mathematical model has been developed utilizing water-assisted surface stress relaxation and fracture mechanics. Predictions of restart times using the model agreed well with published experimental data, indicating that surface stress relaxation is responsible for the anomalous delayed slow crack growth behavior.