# Compressive Strength of Glass – a highly underestimated material property

## Glass tubing for robust pressure vessels with high burst pressure stability

Nowadays, glass is no longer considered as a weak and easy to break material that is only  for decoration or serving drinks in extravagant glasses. Quite contrary to its historical use, glass now gathers worldwide attention for large-scale industrial applications even under harsh environmental and high-pressure conditions. Explosion proof lamp housings, large-scale gauge pressure engineered systems, concentrated solar power in the desert zone of Nevada, are just a few examples of the broad field of application for todays’ tubular glass.

Therefore, it is more than proven that even brittle materials such as glass can provide extraordinary performance even in heavy industry that require robust and pressure-stable materials. However,  glass can only be relied on as long as  correct handling is ensured, the right dimensions are used and the appropriate glass type with the applicable physical and chemical properties is selected. The main criteria that have to be met to challenge tubular glass to its maximum performance for pressure vessels will be explained in the following paragraphs.

In general, by applying a pressure to glass, a characteristic stress pattern is evolving (see Figure 1). In the region close to where the pressure is applied, all glass elements are moving closer to each other, generating local compressive stress within the material. At the opposite region, tensile stress is generated within the glass material because the glass elements are moving apart from each other. As a consequence, any type of surface flaws within the tensile region becomes wider and eventually this can lead to glass breakage. Therefore,  as already discussed in the previous article “The Mechanical Strength of Glass – still an ongoing mystery”, the risk of breakage increases with decreasing the quality of the glass surface in terms of surface flaws. Accordingly, the quality of the glass surface has to be very good to allow the material to withstand high pressure.
Figure 1: Schematic drawing of the cross section of a glass wall with micro cracks on the surface without pressure applied (a) or under applied pressure (b-d). The time that is required for glass breakage depends on many factors, such as the applied pressure, the quality of the surface etc. See “The Mechanical Strength of Glass – still an ongoing mystery” for more detailed information.
Stress within the glass matrix is not necessarily caused by applying external pressure only, but can also be already in-built into the glass matrix during glass production or glass converting. Usually, such kind of internal stress is provoked either  by cooling a very hot glass too quickly or by applying inhomogeneous thermal loads on the glass. Similar to the generation of stress via applying mechanical pressure, here the glass elements are non-stress-free arranged in the glass matrix. Especially, in the case when the stress profile is not homogenously distributed along the tubing, in-built stress can be a risk. Hence, it is recommended to keep also the in-built stress level of glass to a minimum value to assure the fabrication of high mechanical stability tubing for high-pressure containers.1

In addition, to minimise the risk of crack-growth caused by thermal expansion upon heating or cooling and in order to keep glass surface corrosion due to interaction with surrounding material at the lowest, only borosilicate glass 3.3 is an officially approved material for overpressure glass containers.1 Boroslicate glass 3.3 is characterised by its very low thermal expansion (Coefficient of Thermal Expansion (CTE) for borosilicate 3.3=3.3*10-6K-1; CTE of soda lime glass~9-10*10-6K-1) and by its high chemical stability. SCHOTT DURAN® glass matches all of those compulsory criteria.
Besides the general material characteristics: internal stress, glass surface quality and glass type. The way  the load is applied on the final glass product has to be considered. Is the load applied only at one particular small area along the whole glass tubing or is the load homogenously distributed over the entire tubing dimension? Considering overpressure tubular glass containers, the second scenario is most common. Here, a properly sealed, stress-free glass tubing holds gas or liquid at a certain internal overpressure level. The maximum pressure, p in bar, that can be applied to such a container under room temperature conditions can be estimated acc. to 1 by

$\phantom{\rule[-1ex]{2ex}{0.5ex}}p=\frac{\mathrm{WT}*140\mathrm{bar}}{\mathrm{OD}-\mathrm{WT}}\phantom{\rule[-1ex]{2ex}{0.5ex}}{\text{}}$
with WT being the wall thickness, and OD being the outer diameter of this hollow cylindrical body. The formula is derived from the Barlow’s formula (correlates the dimension of a pipe with its internal pressure resistance), while considering the properties of a borosilicate 3.3 glass. If the glass tubing is utilised in an appropriate manner, not risking the generation of new surface flaws or high internal stress (e.g. thermal shock), no additional safety factor has to be added to this formula. Furthermore, all unit conversions are included, giving the resulting pressure in bar right away by typing in the dimensional characteristics of the glass tubing.

Eventually, it is even possible to run overpressure containers of tubular glass at elevated temperature conditions. By doing so, thermal shocks by e.g. sudden cooling of the outer tubing surface while the inner surface is still exposed to a hot solution, should be strongly avoided. Cooling or heating of the tubing should always proceed in a slow and controlled manner in order to lower the risk of generating internal stress within the glass matrix, especially, when operating temperatures larger than 200 °C, special precautions have to be considered.1

In summary, if you

-    Treat the glass gently to ensure high surface quality
-    Avoid internal stress caused by e.g. thermal shocks
-    Use the high quality glass type 3.3 borosilicate (e.g. SCHOTT DURAN®)
-    Use official formula to calculate the required tubing dimensions
-    Do not exceed operating temperatures larger than 200 °C without taking special precautions
-    Avoid non-homogenously applied mechanical load on the tubing by e.g. incorrect installation,

then glass tubing can be a great opportunity for highly stable pressure vessels with outstanding burst pressure stability that can sustain high temperatures and chemically aggressive materials.

[1] AD 2000 specification N4, Issue 2000-10: Pressure vessels for glass with Annex 1, Issue 2000-10: Assessment of errors in pressure vessel walls of glass, and B1 Issue 2000-10: Cylinder and spherical shells under excess interior pressure. The approved strain of 7N/mm2 is given under DIN EN 1595: Pressure equipment made from borosilicate glass 3.3. The formula is only valid for OD/ID ≤1.2 or OD/ID ≤1.7 if OD≤200 mm

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