Final answer:
The failure stress is higher in a ring on ring test compared to a four-point bend test because of the more uniform distribution of stress over a larger area, allowing the glass to withstand more stress before failing.
Step-by-step explanation:
The ring on ring test and the four-point bend test are methods used to evaluate the strength of materials such as glass. When testing glass, the failure stress observed in a ring on ring test is often higher than that recorded during a four-point bend test. This difference is due to the distribution of stress across the sample during testing. In a ring on ring test, the load is distributed over a larger area, resulting in a more uniform stress distribution. It allows the glass to withstand higher levels of stress before failing. In contrast, the four-point bend test concentrates stress over a smaller area, particularly at the points of contact, making the glass more susceptible to failure at lower stress levels.
Materials like glass and plastic become optically active when subjected to stress, which can be used for optical stress analysis. By creating plastic models and using crossed filters, one can observe the stress distribution in complicated shapes. This technique also highlights that the optically active behavior depends on the wavelength of light used, which can provide additional details about the stress patterns.
Thermal stresses can become critical for materials such as glass, especially when cooled unevenly. Pyrex, known for its small coefficient of linear expansion, is less susceptible to breaking due to thermal stress as it does not expand or contract as much as regular glass. This property makes it more suitable for glass cookware that experiences rapid temperature changes.