October 27, 2023, ©. Leeham News: We are discussing the different design phases of an airliner development program. After covering Conceptual, Preliminary, and Detailed design (Figure 1), we now discuss prototype manufacturing and testing, where we today go deeper into structural testing.
An essential part of the testing for a new airliner program is the stress testing of the airplane structure. The manufacturing of prototype structures has the static load test structures as one of the first items produced.
The airplane structure must pass the Limit load test before it’s allowed to fly the first test flight. Limit load is the highest load the structure is foreseen to be subject to during its operational lifetime. During these tests, there can be no deformations or changes in the structure’s integrity.
The static test is done similarly to the most spectacular static load test, the Ultimate load test (Figure 2), where the structural parts are subject to 1.5 times the Limit load. Here, damage to the structure is allowed, such as wrinkles, etc., but there can be no structural failures before the limit load is reached.
There have been several cases of structural failures just before 150% of the Ultimate load is reached for the wing. If the failure mode is according to predictions but happens a few percent before 150%, the OEM can suggest structural reinforcement and, via simulations, convince the regulator that the final certified structure will meet the certification criteria for structural static load.
The Ultimate load test does not have to take place before the first flight but shall be completed satisfactorily before the aircraft can be certified.
The Limit and Ultimate load tests are static test cases; that is, it does not simulate long-time operation which subjects the aircraft to repeated stress cycles. This is done in the Fatigue tests, where the parts of the aircraft that see a continuously varying load are long-time tested.
The goal is to test the structure to high material fatigue cycles long before an operational aircraft reaches such cycle counts. For the A350, the fatigue tests simulated 86,000 flights, which was three times the planned cycle life of the aircraft.
The fatigue tests can be done separately on different aircraft parts, as in Figure 3, as long as the loads are representative of the loads in the complete airplane. Airbus gave iABG in Munich the contract to expose the fuselage middle section and wing to the 86,000 flight cycles.
An essential part for cycle tests is the pressurized section of the fuselage. It gets exposed to about 8.6PSI/0.6 Bar higher pressure than the surrounding air at cruise altitude. The pressure difference cycles the fuselage pressure tube, creating hoop stresses in the fuselage skins during flight.
To avoid fatigue problems, designers avoid non-round fuselage sections, as these create bending forces on top of the hoop stresses in the skins.
The cabin floor can be used as a stress-transferring member, which allows the fuselage to be designed with a “double-bubble” cross-section without causing troubling bending moments, Figure 4.
Metal fatigue from cabin pressurization was poorly understood when jet flights to high cruise altitudes started in the 1950s and caused the tragic De Haviland Comet accidents. The ensuing fatigue research involved complete Comet fuselages pressurized underwater in tanks (to dampen ruptures). The tests taught the aeronautical community a lot about metal fatigue when subjected to a high number of repeated stress cycles.