Method for improving the accuracy and safety of pneumatic loading of fuselages during fatigue test

Fatigue and survivability tests of full-scale structures play a decisive role in the airframe support system. The main purpose of these tests, usually carried out on one of the first production aircraft is the design certification and development of the methods and procedures of the integrity control for subsequent reproduction of the elaborated regulations and methods of in-operation control. Tests are the main requirement of the airworthiness standards for aircraft and helicopters. The results of full-scale tests determine the quality of the airframe design and the operation safety. The endurance of pressurized compartments of aircraft and, first, the fuselages of high-altitude passenger aircraft, occupies a special place in the problem of fatigue strength of aircraft structures. When flying at high altitude, the pressure difference between the cabin and the external atmosphere is maintained at 0.63 × 10⁵ Pa, which results in large radial loads acting on the aircraft fuselage from the inside. The fuselages are loaded with compressed air (pressurization, pneumatic loading) in laboratory conditions. High requirements for the accuracy of pneumatic loading are determined by the fact that in a number of elements of the fuselage structure, e.g., in the area of window cutouts, the stresses created by external loads are significantly lower than those attributed to the pressurization loads. Hence, the accuracy of loading by excess pressure should be higher than the accuracy of the external loading system not to distort the stress state pattern of the structure. Apart from ensuring high accuracy of pneumatic loading of aircraft fuselages, it is necessary to comply the safety conditions of the tests, since the energy reserve of compressed air accumulated inside the fuselage is equivalent to the energy of several kilograms of TNT and in this regard a sudden explosive depressurization can lead to high consequences: the unsuitability of the damaged object for further testing; the failure of structures surrounding the tested object, etc. We present a method of cyclic loading of aircraft fuselages with excess compressed air pressure during fatigue tests which provides (due to the automatic control of the joint operation of high-precision and low-flow valves) an increase in the accuracy of pneumatic loading, expansion of the range of application of the pressurization system and monitoring of the tightness of the tested products in each loading cycle to prevent sudden destruction of the structure (depressurization).

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