De Sitter:
Thanks for the lesson (didn't really need it), but I think you misunderstood. A cylindrical fuselage structure - as in the 757 and 777 - will be more easily bent, in a good way (one that dissipates energy) on landing. Any shape that is not precisely cylindrical will be much stiffer to bending moments as a whole structure. That means the energy will be internally absorbed into the structure as heat and lattice dislocations in the metal structure of the fuselage. That is what leads to cracks.
Now, I'm getting up in age, but I can still tell a cylinder from a football.
-drl
I'm afraid you misunderstand. The Boeing and Airbus engineers who design Aircraft cross sections are perfectly aware of the insignificant issue you raise.
What determines aircraft cross section is the internal dimensions necessary to provide the required seating layout and passenger space requirements. plus cargo containment, consistent with the available engine power to make the thing move. That is a function of the complex economic modelling that is required to maximise the profitability in service (and hence the saleability) of the finished product.
The fuselage shape has everything to do with economics. Third or fourth order metallurgical concerns such as those you mention are immaterial.
To put it another way, if the marketing departments and economic modellers decided a triangular fuselage cross section was optimum from a profitability in service point of view, then that is what the engineers will produce.
To put it yet another way, the 757 and 777 profiles are a result of economics, it has nothing to do with strength or fatigue. Those aircraft hulls will be designed to exactly the same limits (barring technology or regulatory change) as the B737 fuselage. This was all sorted out by about 1962.