Vacuum collapse — why do strong metals implode but glass doesn't? ము506iJGwప Kk Aaల JuhSsZB jI ck vONAి
This question has been puzzling me lately.
I'm sure you've seen demonstrations of metal containers imploding when evacuated. Here, for example, are two videos of vacuum collapse: experiment 1, experiment 2.
However, when the same experiment is conducted with a material as fragile as glass, nothing implodes or shatters. Two videos of the same experiment conducted with glass: experiment 3, experiment 4.
Nothing is special about the quality of the glass used in experiments 3 and 4. The glass is also not very thick. Yet, evacuating the glass, almost to 100% vacuum, doesn't so much as put a scratch on it, but the metal containers implode with great force. What is the reason for this anomaly?
My guesses are:
- The total surface area of the glass used in experiments 3 and 4 are much smaller compared to the surface area of the metal in experiments 1 and 2. Greater surface area equates to greater force absorbed by the entire structure, even though the force per unit area remains the same.
- Ductile deformation (metal) is fundamentally different from brittle fracture (glass), involving different mechanisms of atomic displacement.
(Blaise Pascal famously conducted several experiments with vacuum in a glass test tube in the 17th century.)
Please share your thoughts on this. Thank you!
1 Answer
For a cylindrical pressure vessel loaded in compression (that is, vacuum inside), failure occurs by buckling instability in which a random and small inward perturbation of the stressed wall grows without bound at and beyond a certain critical load value. This is analogous to buckling instability in a thin column loaded in compression.
The characteristic which resists buckling instability is not the yield strength but the stiffness of the wall, which depends on its thickness and on its elastic modulus. The thicker the wall and the higher the modulus, the more resistant to buckling the cylinder will be.
The elastic modulus of common glass is about 48 x 10^6 psi compared to that of steel at 29 x 10^6 psi. The glass cylinder will hence be more resistant to implosion than a steel cylinder of identical size and wall thickness.
Note also that putting a scratch or microcrack in glass renders it weak in tension. In compression, however, the applied stress tends to press microcracks shut, so scratching a glass vacuum vessel does not cause the sort of catastrophic failure you expect to get when the glass is in tension.
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1$\\begingroup$ The elastic modulus of glass at room temperature is around 72 GPa (en.wikipedia.org/wiki/List_of_physical_properties_of_glass) while that of steel is 200 GPa (engineeringtoolbox.com/young-modulus-d_417.html) $\\endgroup$ – Akash Chandra 7 hours ago