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Session: Session 13: Full-scale fatigue testing I
Session starts: Wednesday 28 June, 13:30
Presentation starts: 14:30
Room: Theatre room: plenary

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Jan Erik Lindbäck (Saab AB)
Zlatan Kapidzic (Saab AB)
Allan Gustavsson (Saab AB)
Risto Laakso (VTT, Helsinki)

Abstract:
The paper presents a combined fatigue and damage tolerance test of the Gripen rudder, which has been performed in a close cooperation with Saab, VTT, Eurofins, and Arecap. Although rudder design is the same for the single and the twin seater versions, E and F, the design load sequence for E is less severe. To cover both versions, an obvious choice would be to test the most critical load sequence, F, however the preceding DT-analyses indicated low or negative margins, so this option was judged too much of a risk. Running two separate tests would be preferable from a technical point of view but drives cost and lead-time. Instead, we adopted a test strategy where the primary goal is to validate the less severe E load sequence and a secondary aim is to find the number of flights, with F load sequence, which the test validates. The following test activities were performed:  The E sequence was tested followed by a residual strength test to 120% Limit Load. A successful test validates the full life of aircraft configuration E.  Thereafter, the same sequence E, but with the increased loads by a factor of 1.2 was run. Inspections were performed with tight intervals to monitor the fatigue crack growth from the artificial defects. The sequence loading was stopped and a residual strength test to 144% Limit Load was performed when the most critical cracks had grown close to the critical length. The challenge is to validate the rudder for sequence F based on the test results. Can it be validated for a full design life or can it only be validated for, say half a life? The paper describes the steps to combine the test results with the fatigue and DT-analyses to determine the number of flights validated by the test for sequence and aircraft configuration F. Conclusively, this approach shows a cost efficient way to utilize the test results as much as possible, without jeopardizing the primary goal of validating the configuration E of the aircraft.