2

It doesn't happen with every egg. It happens with only some of them.

It doesn't have anything to do with fridge. Almost always I take the eggs out of the fridge and put them in water for boiling. Many times they don't crack. Some times they do.

What is cause of eggs getting cracked while boiling?

4

Well, sometimes eggs have non-visible (or non-visible without candling - shining a very bright light into the egg and looking at the light coming out of the egg) cracks.

That would be one possible cause - the crack pre-existed. Even if it was not cracked when graded, cracks both visible and invisible are possible in the distribution process.

Getting cracked from moving around in the pot is possible, depending on how hard you boil them. If actually boiling them I'd suggest a simmer, but in fact I don't suggest that at all - just boil the pot of water, lower eggs in with a spoon, turn the heat off and cover - yank when they suit your concept of "done" (if hard boiled, 20-30 minutes by this method.) "Coddling" is sometimes used to describe this, though it's also used to describe cooking (without shell, and possibly with added ingredients) in a specialized ceramic container (an egg coddler.) Anyway, it greatly reduces the odds of cracking via "rattling around in the pot" (though not why I do it that way; I do it because it appears, observationally, to reduce my incidence of overcooking the yolk and making it nasty.)

4

The thickness of the egg shell varies. The cooking procedure results in a pressure of about 1 bar / 14 psi inside the egg, 0.3 bar / 4 psi due to the expanding air and 0.7 bar / 10 psi due to the expanding water of the egg white - at a temperature inside of 90°C/194°F, which is a reasonable maximum temperature during cooking.

Some egg shells can bear a pressure of up to 3.5 bar / 50 psi (yes, more than a tire!), but some already fail below 1 bar / 14 psi.

(The answer assumes chicken eggs.)

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