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!

“It’s a bird, it’s a plane …it’s a Coke Can!” - Airframes

I wonder how many people think of airliners they fly in, very much like the can in the picture above! In fact it started out much like that in the early 1950’s with the De Havilland Comet aircraft. The Comet became quite (in)famous for tragic in-flight structural failures, almost complete and rapid decompression at 35000ft above sea level.

There were a total of 3 such disasters with the Comet (and a loss of many lives), all of them within a short time after entry with the airlines at the time. This indicated to the designers that there was a flaw in the basic structural design of the aircraft-A ‘stressed skin’ fuselage construction design that imposed or transferred, almost the entire structural load on the aircraft skin. That is, the aircraft skin became the primary load carrying structure. Though that might sound absurd, it wasn’t really a case of structural overloading on a relatively slight aircraft skin that caused those catastrophes. In fact, we may be quite tempted to ask “What were the designers thinking?! How can the aircraft skin be asked to withstand structural loads of the magnitude relevant to an airliner??”

To be sure, the aircraft skin was built many times stronger, heavier, and thicker than today’s airliners. The flaw lay in the fact that repeated pressurization and depressurization (called pressurization cycles*) of the fuselage, induced fatigue in the structure (in this case, the outer skin) of the aircraft. This in turn created a mini fatigue fracture or a fault line on the aircraft skin resulting in localized stress concentration. The rest of the account lies in the coke can theory! A further diagnosis of this theory in future posts.

*Pressurization cycles- One pressurization cycle consists of one complete pressurization and one complete de-pressurization of the fuselage. In plain terms that means one complete sequence of the events involving,
1. A Take-off
2. Climb to cruise altitude
3. Descent from cruise altitude, and
4. Landing
Pressurization and Air conditioning