Sonic fatigue

Fig. 4. B-52 landing gcai" door damage from sonic fatigue. Reproduced by permission of the Boeing Company.

Stiffened panel designs. Stiffened panels are exterior fairing panels that are attached on three or four sides. They carry little to no structural loads they are designed primarily to take air loads. Early designs using simple riveted concepts were heavy and prone to rapid sonic fatigue. 

Sorenson, A., Sonic fatigue testing and development of aircraft panels. Symposium on Structural Adhesives and Sandwich Construction. Wright Air Development Center, Wright Patterson Air Force Base, Ohio, 1957. 

For the mathematical modelling of the sonic fatigue process of PAN monofilaments, the following equation was considered ... 

The mechano-chemical nature of the MELANA fibres sonic fatigue was proved by measuring the average molecular weight, which diminished in time. The formed free macroradicals were reacted with an aromatic diamine, benzidine. By free primary amino groups diazotizing, the resorcinol chemical bounding was possible, which irreversible coloured the fibres in red. 

In conclusion, the mechano-chemical act of chemical bonds splitting on the ternary copolymer main chains stays on the basis of the mechanical resistance decay in the case of MELANA fibres supposed to sonic fatigue. The generated mechano-radicals initiate o chain of reactions that in time affects the supramolecular-morpho-logical structure, reflected by the decrease of mechanical properties [830-951]. 

It was proved that the equation (3.178) adequately describes the sonic fatigue of other polymers [952, 953],... 

The design of the Buccaneer was such that the tail brakes opened into the jet stream. This meant that they not only were exposed to high temperatures but also suffered from sonic fatigue in the e-flux. The problem was solved by bonding the components with Hidux 1197A and C. 

Stiffened Panels These designs refer to exterior fairing panels, which are attached on several sides but, essentially, only carry air loads. The initial riveted design was heavy and suffered badly from sonic fatigue. The design of the first bonded replacement used beaded double panels but these proved to be difficult to bond accurately into place and, perversely, they reduced the overall stiffness of the panel. 

The second approach, and the one still being employed, was to use bonded honeycomb sandwich structures, which not only virtually eliminated the eifect of sonic fatigue but also enabled very thin skins to he used. This latter led to a heneficial weight saving. 

---