Sub-critical crack growth in sodium germanate glasses

Sub-critical crack growth in binary sodium germanate glass was investigated over a wide range of the crack velocities, 10 7 -10 2 m s 1 , by using small-size specimens with double cleavage drilled compression configuration. For evaluating the intrinsic sub-critical crack growth, crack initiation and subsequent propagation of the crack were performed in heptane. With increasing Na 2 O content in sodium germanate glass, sub-critical crack growth curve shifted toward higher stress intensity factors first up to 10 mol% Na 2 O, but more addition of Na 2 O caused the curve to shift to lower stress intensity factor regions. In other words, fracture toughness shows a maximum at the composition of 10Na 2 O.90GeO 2 , whose value is 1.07 M Pa m 1/2 . This compositional dependence of fracture toughness originates from the so-called germanate anomaly. On the other hand, the slope of sub-critical crack growth curve for the glass containing >10 mol% Na 2 O was much shallower than that for soda-lime glass. These glasses are very fatigable even in inert condition. It is considered that this fatigue behavior can be caused by the microscopic structural variation, which is the presence of GeO 6 units in GeO 2 glass network, and that these units can be the fatigue crack path.