The Coke-Can Theory.

If you’re looking to find this theory (as named) in a text book, or the internet, it’s likely you won’t find it. Simply put, it’s ‘made-up’!...the name, that is..!!

Let’s take a look at that can again. Assume it’s an empty can with the outer skin smooth, seamless and unbroken through it’s entire length. Now, I make a little dent on the surface, with a finger nail, at any point along it’s length. Next I place my palms on the two ends of the can, and compress….
At some point, the pressure is sufficient to crush the can; and, it’s very likely the ‘fault’ line, along which it caves in, lies on the point at which the can was dented.

It’s logical to assume that when pressure was applied at the two ends, all that pressure was concentrated along that fault line - localized stress concentration. When the pressure exceeded the threshold, the can gave in along that fault line and crumpled. If you try the same experiment without denting the can outer surface, the compression required to crush the can will be greater, and, if you keep the pressure applied at the two ends ‘square’, (i.e., perpendicular to the length of the can) the can will likely crush ‘straight-in’.

Based on this simple experiment, one can imagine the effect of a fatigue induced ‘fault line’ in a pressurized fuselage of an aircraft.
In a stressed skin fuselage construction, fatigue, due to constant flexing of the fuselage structure at every pressurization cycle (As mentioned in the previous post) is the major factor that induces a fault line on the aircraft skin; usually at the point of maximum load bearing, or at points of changes in fuselage cross-section etc. A fatigue induced fault line could start out at the microscopic level, and with continuous load reversal (flexing), grow until it causes the structure to fail entirely.

Why didn’t they think about such a seemingly simple phenomenon before they put people in them? To be fair to the designers of the De Havilland comet aircraft, they did conduct tests for structural integrity. However, fatigue related failures are those that occur over a period of time. Also, speculatively speaking, simulating the effects of fatigue in the hostile environment that an aircraft flies in (at altitudes of 35000ft above sea level), may not have been appropriate. Maybe due to a lack of understanding of conditions at that level, maybe due to the lack of availability of appropriate testing equipment…?

Irrespective, the fatigue factor, and the ‘coke-can theory’ got the stressed skin construction fuselage beat a hasty retreat. Empty cans…they made their noise!