Glass is commonly used in many industries such as automotive and consumer electronics. Predicting glass cracking becomes a crucial part of CAE analysis. To predict the failure accurately, we need to characterize the strength in a repeatable way. Theoretically, practical glass strength is,
Where K_IC is the fracture toughness (material property) and c is the flaw size. Determination of the glass strength requires measuring the fracture toughness using a notched double cantilever beam and statical measurement of flaw size.
Glass edge and surface strength can also be characterized through 4 points bend and ring-on-ring tests. Typically, edge strength is smaller than the surface strength due to the likelihood of creating large flaws in dicing process. 4 points bend is commonly used to characterize edge strength. They create uniform stress within the load span. The probability of failure depends on the statistics of edge flaw size within that region. To achieve statistical significance, often a relatively large number of samples (e.g. 20+) are needed. Results of this test typically follow Weibull distribution and a steep curve indicates good consistency of processing. It worth noting that the sample processing such as chemical strengthening and edge polishing need to be representative of actual form factor, otherwise the test results won’t be meaningful.
Ring-on-ring test is commonly used to characterize surface strength. Similarly, uniform stress is obtained inside the smaller ring. The probability of failure depends on the statistics of surface flaw size within that region. As the surface typically has smaller flaws than edge, larger strength is expected from this test. In fact, the surface strength is usually the upper limit the edge polishing capability.
As a brittle material, glass is typically not strain rate dependent. Therefore, results from quasi-static 4 points bend and ring on ring tests can be used to interpret high strain rate load cases such as drop and impact. As a test closer to actual impact type of load, ball drop test is also used to characterize the surface strength. Through simulating the same test condition, failure strength can be interpreted.
