Glass ring cracks

Following the above analytical conclusions, because the maximum tensile stress occurs at the periphery of the indentation, circle BB, it would be expected that at some critical load a crack would nucleate and propagate around the circumference BB. On a surface of glass or another amorphous material a ring crack is observed, but on highly anisotropic crystalline ceramics a less well traced crack is produced as the ring has a definite... 

There is no agreed explanation for the variable radius of the ring crack on ceramic and glass surfaces. A common belief is that a pre-existing surface flaw will reach critical proportions and nucleate the crack in which case a > Oo. To investigate this proposition the relationship between the load needed to cause cracking and the radius of the indenter has been examined by using the relationship between the radius a of the contact circle and sphere radius R... 

F ure 7. Indentations in the 7740 borosilicate glass. Minor cone or ring cracks form along the sides of the 4.9 N indentations, which have a noticeable pinching in of the sides. 

Figure 8.7 Top view of ring cracks generated by a sphere (0.2 mm diameter) on the atmospheric side of a float glass.

At the request of an international petroleum company, a major manufacturer and supplier of down-hole equipment performed tests of the various elastomers commonly used in the construction of packers and other oil field tools. Seven of the nine most commonly used thermoplastic materials were found to be completely inert to TKPP solutions. The test included continual immersion in saturated TKPP for 21 days at 280°F. Only two elastomers, Vi-ton and Fluorel, showed any adverse reaction. O-rings made from these two elastomers showed minor cracking at the termination of the test. A listing of the elastomers that tested inert to TKPP solutions include nitrile, saturated nitrile (HNBR), Aflas, Kalrez, PEEK, Glass-filled Teflon, and Ryton. Several of these elastomers are attacked or degraded by conventional clear completion fluids containing calcium and zinc halides. The inertness of commonly employed elastomers to TKPP is an important advantage for TKPP fluids in normal operations. 

Water environment promotes crack initiation in silica, Tempax, and soda-lime glasses (Table 1). This is because the crack initiation process is assisted by the adsorption of water molecule. In the conceptual framework of this idea, " a water molecule is adsorbed at the strained bonds formed by a scratch process. After that, the attacked Si-O-Si bond and the water molecule break one Si-0 bond to leave new two Si-OH bonds. West and Hench reported that the energy barrier of hydrolysis of strained 3-fold rings is 97% smaller than that of fracture by water-free dilation. This water-assisted bond-breakage process is the origin of lower crack initiation load obtained in water. 

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