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After passing 9,000 metres (30,000 ft), one of the outer Plexiglas window panes cracked, shaking the entire vessel. (Wikipedia)

Why did the first layer of plexiglass Window panes crack on the Bathyscaphe Trieste while descending down to the Mariana Trench? Weren't they tested under such amounts of pressure? As far as I know, plexiglass can hold pressure of up to 20,000 PSI before it can actually crack.

Related cross-post: https://physics.stackexchange.com/questions/353497/why-did-the-plexiglass-on-the-bathyscaphe-trieste-crack

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  • $\begingroup$ @Coto For future reference, if you wan to move your post to a different stack, make a flag to the moderators, and they will migrate it. Please avoid cross-posts the same question to different stacks. StackExchange policy is against cross-posting. $\endgroup$ – Nick Alexeev Aug 26 '17 at 15:36
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According to this datasheet (Table 12, page 10), modern Plexiglass has the following properties:

Tensile strength (0.25" specimen-0.2"/min)
--------------------------+---------+------------
Maximum psi               | 10,500  | 10,200
Rupture psi               | 10,500  | 10,200
Elongation, maximum %     | 4.9     | 4.5
Elongation, rupture %     | 4.9     | 4.5
Modulus of elasticity psi | 450,000 | 450,000
Poisson’s ratio           | 0.35    | —
--------------------------+---------+------------
Flexural strength (span depth ratio 16, 0.1"/min)
--------------------------+---------+------------
Maximum psi               | 16,000  | 15,000
Rupture psi               | 16,000  | 15,000
Deflection, maximum in    | 0.6     | 0.5
Deflection, rupture in    | 0.6     | 0.5
Modulus of elasticity psi | 450,000 | 450,000

The datasheet also has the following statement (page 5):

Design stresses

Plexiglas sheet has good tensile and flexural strength properties. However, stress considerably below the values shown in Table 12 (page 10) will produce light surface cracks known as crazing.To avoid stress-crazing, design limits for continuously imposed loads should not exceed 1,500 psi for Plexiglas G sheet and 750 psi for Plexiglas MC sheet. Stresses of greater magnitude but short duration will not generally cause stress-crazing.

Whether the four or so hours it took to reach 9000 meters is enough to cause stress-crazing, I can't say, but maybe that was it. Given that the cracking was apparently violent enough to shake the craft, I doubt it, though.

So, a square Plexiglass sheet with a span/depth ratio of 16 will fail at 16000 psi. What isn't clear to me, however, is what precisely is meant by that. Is that the ultimate applied load or the maximum internal stress it can withstand? I'd guess it's the applied load, but it isn't clear (to me, at least) from the datasheet.

However, we (or at least I) have no idea what the span/depth ratio was in the Trieste, nor do we know if perhaps Plexiglass was weaker back then.

As to how it wasn't tested for such loads, it almost certainly was. One possible explanation would be a manufacturing or testing failure, where the conditions in the tests were different in some slight but significant way, or where the tests actually unknowingly weakened the bathyscaphe.

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  • $\begingroup$ Or it might have just been bad luck. $\endgroup$ – joojaa Aug 25 '17 at 13:37
  • $\begingroup$ Taking today's resources into consideration, what would be the best transparent material (and thickness) to use if your goal is to reach the bottom of the Mariana's Trench (~16,000 PSI)? $\endgroup$ – Coto Aug 25 '17 at 15:57
  • $\begingroup$ @Coto: "Best" is a meaningless word. $\endgroup$ – Wasabi Aug 25 '17 at 19:47
  • $\begingroup$ @Wasabi I'm looking for a material that can hold up to such pressure and be transparent (Camera could see through) $\endgroup$ – Coto Aug 25 '17 at 19:55

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