13:30
Session 13: Full-scale fatigue testing I
Chair: Rene Alderliesten
13:30
20 mins
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Review of aircraft landing gear tests as part of structural testing
Zbigniew Skorupka, Min Liao
Abstract: Aircraft Landing Gear is the one of the most important safety systems onboard ensuring the well-being of cargo and passengers on board the aircraft. It is also the system which gives aircraft ability to take off, land, and manoeuvre on the ground while taxiing. Due to the important safety role in aircraft, Landing Gears have their own requirements defined in regulations and are subjected to many tests going beyond the minimal legal and technical requirements. The full set of Landing Gear tests includes the tests of individual parts, full Landing Gear assemblies, and whole Landing Gear systems. The tests are made in laboratories for initial proof of safety and operation and inflight for full operation testing. Also modern fast and portable digital data acquisition systems make real-time data capture and process possible from various sensors. This capability makes real-time health monitoring feasible for improving test efficiency, detecting faults and dangers or just providing real-time data for possible testing process optimization. The tests are done for various operational conditions beginning from static/strength tests, dynamic tests, to fatigue tests. These tests are performed on specialized test stands in dedicated laboratories and in most of the cases are excluded from being defined as structural tests. Although except from the part of dynamic tests most of the Landing Gear tests share similarities with other types of structural tests in means of load introduction, measurement systems. In this paper authors present a comprehensive review of aircraft Landing Gear tests, including the current state of the Landing Gear tests; modern testing and modelling approaches and capabilities of Landing Gear manufacturers and laboratories; and Landing Gear health monitoring as complementary testing methods and continuing airworthiness support. The review provides examples to show that Landing Gear tests should be considered as part of structural testing even when tests are performed for Landing Gears only. In the end, the paper reviews and discusses the certification requirements for Landing Gears of existing aircraft, as well as a brief outlook on the Landing Gears testing methods and requirements for future new aircraft configuration such as eVTOL.
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13:50
20 mins
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Some observations to recent full-scale fatigue tests
Hoi Yiu, Richard Bulmer, Peter Webb
Abstract: Today, full-scale fatigue testing are required as means of compliance for proof of structures to certify both composite and metallic structures. This paper presents the common practice to set up and to monitor the full-scale fatigue testing of aircraft control surfaces done in GKN Aerospace Services Ltd. These tests include Bombardier projects on testing ailerons, rudder and elevator, etc, and recently the Dassault F6X project of testing wing movables consisting of flap, flaperon, aileron and airbrake.
Although full scale fatigue testing have been applied in the past to demonstrate compliance for aircraft certification, some recent full-scale fatigue testing results reveal some issues on structure design philosophy, measurement techniques, and correlations. This paper outlines some findings based on the results observed using standard practice and measuring techniques to validate the structural analysis. Buckling found during fatigue test case has been monitored throughout the test duration. Strains and deflection surveys are used for correlation to the predictions, and are used to check test consistency. In cases of crack growth from saw-cuts, predictions and monitoring to the crack growth are presented.
Finally, from the above mentioned there is a need to review the-state-of-arts measurement techniques to support full scale tests. New techniques such as DIC is applied with some level of success but more reliable and mature measuring techniques are still required to be explored in industries.
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14:10
20 mins
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Test and analysis of fuselage structure to assess Emerging Metallic Structures Technologies
Yongzhe Tian, Dave Stanley, John Bakuckas, Kevin Stonaker, Mike Kulak, Erin Fulton, Walt Sippel, Marcelo Rodrigues, Fabricio Fanton, Carlos Chaves
Abstract: In partnership with Arconic and Embraer, the Federal Aviation Administration (FAA) is assessing emerging metallic structures technologies (EMST) using the FAA’s Full-Scale Aircraft Structural Test Evaluation and Research (FASTER) facility. In this collaborative effort, full-scale fuselage panel test data will be used to assess the effect of EMST fuselage concepts on damage tolerance performance as compared to the current baseline aluminum fuselage structures located on the crown of a typical single aisle aircraft forward of the wing. Several technologies will be considered, including advanced aluminum-lithium (Al-Li) alloys and selective reinforcement using fiber metal laminates. Data from this study will be used to verify improved weight and structural safety performance of the EMST and will also be used to assess the relevance of existing regulations and to inform whether additional safety standards and regulatory guidance should be developed to provide improved safety beyond that afforded by the existing airworthiness standards.
Initial efforts focused on the first baseline panel, consisting of Al-clad 2524-T3 skin and conventional 7000-series aluminum substructure assembled through riveting. Results from baseline panel 1 test were presented at ICAF 2019, and will be compared to future tests on advanced panels containing varying EMST to see if there are improvements in damage tolerance capabilities. In the latest fuselage panel tests, investigations continue in the assessment of advanced alloys. This includes panel 2 and panel 3, consisting of 2060-T8 Al-Li and 2029-T3 clad aluminum skins, respectively. Both of these panels were stiffened with integral frames made of 2099-T83 Al-Li and stringers made of 2055-T84 Al-Li. A predominant challenge throughout the testing program was to develop an approach to account for small differences in final panel-to-panel chem-milled skin thickness, which met manufacturing tolerances but resulted in significant skin stress intensity differences for these thin skin gage panels.
Results from first three panels are presented in this paper, summarizing experimental and analytical procedures and results demonstrating improvements in damage tolerance performance using advanced alloys, the challenge encountered, and lessons learned.
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14:30
20 mins
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Fatigue and damage tolerance testing of Grippen e/f rudder
Jan Erik Lindbäck, Zlatan Kapidzic, Allan Gustavsson, Risto Laakso
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.
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14:50
20 mins
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Swiss Titanium Research Experiments on the Classic Hornet (STRETCH)
Ben Main, Isaac Field, Keith Muller, Ricardo Filipe do Rosario, Mirco Figliolino, Simon Barter
Abstract: The Swiss Titanium Research Experiments on the Classic Hornet (STRETCH) is a collaboration between Swiss and Australian Governments, with support of RMIT University and RUAG Switzerland, to experimentally evaluate small fatigue crack growth in titanium combat aircraft structures, to improve related analytical tools and finally to support with its results the Swiss F/A-18 life extension program. The centrepiece of the collaboration is the full-scale fatigue test (FSFT) of a Swiss F/A-18 C/D centre barrel. Research undertaken includes coupon level small fatigue crack growth (FCG) studies, damage induction studies, and full-scale durability and damage tolerance (DaDT) testing.
The coupon level small FCG tests focused on spectrum truncation and marker band studies for recrystallization annealed Ti-6Al-4V. These studies included a marker band development set of tests and the application of the best result in conjunction with quantitative fractography (QF) to provide the groundwork for the FSFT spectrum design. A novel method of damage induction has also been investigated that utilises plasma arc spot melting to impart precise and localised damage to the Ti-6Al-4V bulkheads of the test article. This damage induction method is shown to impart controllable crack-like damage down to depths less than 0.01 inches (0.254 mm). Finally, DaDT testing of the F/A-18 C/D centre barrel has demonstrated the full-scale application of the derived spectrum and damage induction methods.
This paper shall present an overview of each of these research streams encompassed within the STRETCH project with a focus on the preparatory component of the durability and damage tolerance phases of the titanium full-scale centre barrel fatigue test.
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