The Mechanical Strength of Glass – still an ongoing mystery

Extraordinary material characteristics that even in the 21st cannot be fully predicted

When it comes to the definition of mechanical strength, brittle materials such as glass have an exceptional position among all material classes. Theoretical considerations, which are only based on the material characteristics of the glass composites, conclude that glass is much more stable than common metals. However, when thinking about our own everyday life experiences, we expect it to be the other way round. Why the difference?

In general, the mechanical strength defines how much force per area can be applied to a material until breakage occurs. For glass, it is not a well predictable material characteristic that strictly depends on the intermolecular forces which keep the glass composites together, but rather is a number of statistical nature. Its magnitude and its statistical variance is mainly a function of the quality of the glass surface.
During the entire glass production-cycle, flaws cannot be fully avoided. Any type of post-processing or handling can especially have a major influence on the quality of the glass. The larger the amount of flaws or the bigger the size of the flaws, the worse the quality of the glass surface, and therefore a higher probability of glass breakage under applied mechanical force. Making it even more complex, the temperature and the products’ shape, may have an impact on the mechanical strength of the glass as well. Under elevated temperatures, close to the glass transition temperature, the glass becomes less stiff, and therefore can withstand lower mechanical stress. In terms of shape, a randomly pre-damaged glass tubing of poor surface quality can be a good example.
Figure 1 shows the breakage energy of identical segments cut from the same glass tubing. It was measured according to DIN 52295 and can serve as an indicator of the mechanical stability of glass. Its magnitude is determined by the surface quality of the individual segments. Considering a complete glass tubing, with the same breakage energy profile, most probable, breakage under applied mechanical force would occur at 45 cm, 57 cm, or 63 cm.  In general, the longer or the larger the glass tubing, the higher the probability of flaws within the material. Accordingly, the larger is the probability for breakage. Since the breakage will always occur at the weakest point of a product, even regions of high mechanical strength cannot prevent the product against breakage at its weakest point. 

Eventually, glass has the extraordinary characteristic of statistical fatigue. Under constant applied force in combination with a certain level of humidity, it may happen that the glass does not break immediately, but breaks after a certain period. This is because chemical reactions on the glass surface enhance crack growth, finally leading to macrocracks and breakage.
Figure 1: Deviation in the mechanical stability as a function of the length of a randomly pre-damaged glass tubing. The tubing was cut into pieces, while the breakage energy of each piece was measured acc. to DIN 52295. Most probable, under applied mechanical force, the breakage of the long glass tubing would have occurred at the weakest point, which is located at 45 cm, 57 cm or 63 cm. 
To summarize, considering the fabrication of a glass product along the complete value chain, the glass is exposed to a huge variety of different environmental conditions. Therefore, every glass product has to be considered as an individual, even when it is fabricated via mass-production. Theoretical equations in combination with empirical studies can help to estimate the mechanical strength of glass, but its exact mechanical strength can never be fully predicted. 

Therefore, one of the most important things is to keep the production cycle stable and avoid as many flaws as possible by treating the glass in the correct way. Only high quality glass production will result in high quality glass products. With an experience of more than 130 years, SCHOTT is able to meet the high requirements of today’s premium glass production. Long tubing, with high quality are not a myth but SCHOTT standard.
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