15:40   Session 19: Full-scale fatigue testing II Chair: Zlatan Kapidzic 15:40 20 mins Experimental strength and fatigue assessment of a disbonded F/A-18A/B/C/D inner wing step lap joint Eric Dionne, David Backman, C. André Beltempo, Stéphane Brunet, Benjamin B. Bolduc Abstract: This paper describes the test campaign performed by the National Research Council Canada (NRC) on a disbonded F/A-18A/B/C/D Inner Wing Step Lap Joint (IWSLJ). Many disbonds have been reported by the F/A-18A/B/C/D fleet operator. The majority of the disbonds found are small relative to the size of the IWSLJ. The immediate effects of those disbonds and their changes in a long term are not well understood. The NRC is conducting a component test aimed at better understanding the effects of a partially disbonded IWSLJ on the F/A-18 wing structural integrity. This paper will summarize the objectives of the component test, as well as the approach used to address the test objectives, which combines several innovative techniques in the area of data sensing. This paper will summarize the test results to date as well as lessons learned. Residual Strength One of the primary purposes of this test is to evaluate the residual strength of a partially disbonded Stepped Lap Joint (SLJ) of the lower wing skin. For this purpose the United States Navy donated a retired wing with a pre-existing disbond similar to those found in the F/A-18A/B/C/D fleet. Static loads will be applied to the test article to demonstrate sufficient residual strength in presence of the small disbond. The test is performed at Room Temperature (RT) and loads are scaled up for temperature effects; this allows for a simpler test setup. Effect of Constant Amplitude Loading The second aim is to try to generate damage growth data for a common size of disbond in the SLJ. Repeated constant amplitude loads will be applied in an attempt to grow the disbond. The loads will be progressively increased and the disbond will be frequently inspected using customized non-destructive inspection techniques able to detect and measure disbond size and quantify the resulting growth. The data from the numerous sensors will also be monitored to detect if the disbond growth can be measured in a test environment. Load Redistribution The third aim is to evaluate the internal load distribution as a result of the disbond growth. The test article is equipped with over 250 strain gauges on the SLJ as well as on the surrounding structure as it is believed that new load paths will be created as the disbond grows. Digital Image Correlation (DIC) is also used on both the upper and lower SLJ with the anticipation it would detect a perturbation of the strain field at the tip of the disbond. Fiber optic sensors were also installed along the spars to detect load bypassing from the skin to spar caps. The sensor measurements will also be correlated to a Finite Element (FE) model of the wing in an effort to enable the development of FE models to replicate and predict more complex disbond scenarios. 16:00 20 mins Rigid loading accelerates full-scale aircraft fatigue test Jianjun Zheng, Mengmeng Wang Wang, Lei Zhang Zhang, Wei Zhang Zhang Abstract: The fatigue test of full-scale aircraft structures is a time-consuming and laborious long-term work, the purpose of which is mainly to verify whether the aircraft structure meets the requirements of WFD, to verify the fatigue characteristics of the airframe structure, and to provide a test basis for the development of structural damage detection methods and repair plans. It is the eternal goal of the test team to improve the fatigue test efficiency under the premise of ensuring test safety and loading accuracy. In this paper, we summarize the technical advances of using tension pad-lever system in full-scale aircraft structural fatigue test, and integrate these technologies into C919 full-scale aircraft structural fatigue test and C919 aircraft vertical tail/rear fuselage full-scale static, fatigue and damage tolerance test. The application verification shows that the rigid loading system runs normally, is stable and reliable in long-term use, has good control accuracy, and the test efficiency is significantly improved compared with previous. The successful application of these technologies provides an important reference and support for subsequent fatigue tests of other aircrafts. 16:20 20 mins Crack bridging effect in hybrid reinforced fuselage structure Mathias Renner, Ömer Namdar, Derk Daverschot Abstract: The A400M fuselage main frames at the wing-to-fuselage interface have the function of introducing the wing loads into the fuselage. The loads that are introduced by the wing are pre-dominantly tension, which results in an in-plane bending moment in the frame due to their curved shape, especially on the rear wing attachment. These fuselage frames are primarily sized by fatigue and damage tolerance requirements. In order to avoid heavy weight impact these frames had been reinforced by adhesively bonded GLARE straps. The development of the design principles, using bonded reinforcements have been presented in the ICAF conference in 2009. This paper now continues and closes this circle by presenting the outcome of the detailed analysis and test campaign that was done in frame of the development, comprising of component testing and full-scale fatigue testing. It outlines the different aspects of assessing a full-scale fatigue test result. A correlation is provided of this design with main focus on the crack bridging effect of the bonded GLARE strap. Finally the initial assumptions are compared to the latest outcome confirming the crack bridging ability of the GLARE reinforced frame. 16:40 20 mins Bell 525 Vertical Fin / Aft Fuselage and Tailboom Composite/Metallic Hybrid Certification Fatigue Testing André Beltempo, Patrick Langlois, Alain Colle, John Rogers Abstract: This paper describes the certification fatigue test approach of the vertical fin, aft fuselage, and tail boom of the Bell 525, a super medium commercial transport helicopter. Specialized test methods, data collection and analysis are used for these composite/metallic hybrid structures. The approach for testing these full-scale components was developed in partnership with the National Research Council Canada (NRC). Testing for initial structural certification was completed at the full scale structural test lab M-14 in Ottawa, Canada. Test Goals Structural tests were conducted as part of the overall Bell 525 test program and provide structural data in support of the aircraft certification. The goals of the tests include: • Evaluation of damage tolerance for composite Principal Structural Elements (PSE’s) • Determination of fatigue strength of metallic PSE’s to assess retirement times • Validation of the airframe Finite Element Model (FEM) • Demonstration of residual strength of the structure post-fatigue test • Providing substantiating data to show compliance to the applicable CFR Part 29 requirements Test Approach The empennage structures under consideration are located downstream of the turbine engine exhaust and are thus subject to high temperature (hot/wet) operating conditions. The traditional method for addressing this environment effect on composite materials is to determine the effect of the operating conditions on the material properties via coupon testing. The result yields an environmental factor on the material strength. This approach normally results in load factors that are applied to the test specimen which are appropriate for composites but may overstress the metallic structure of the test specimen. As a hybrid structure, the Bell 525 empennage is not amenable to the traditional approach as it may result in unrealistic premature failure of metallic structure, non-representative damage of joints and fasteners, or in extreme cases could result in local yielding which may invalidate the conclusion on fatigue strength. To validate the hybrid structure while accounting for the impact of a hot/dry environment on composites, the test articles were encased in custom NRC designed heated enclosures. As shown in Figure 1 and Figure 2, the enclosures maintain a hot/dry environment throughout fatigue and residual strength testing. The use of heated enclosures allowed a significant reduction in the required environmental load factor, with the remaining difference between a hot/dry and hot/wet condition requiring only a small load correction. As a result, the test loads permitted both the composite and metallic structures to be appropriately evaluated simultaneously. The full paper will describe some of the challenges and findings of conducting these elevated temperature tests. Significance The Bell 525 structural test articles successfully completed a comprehensive durability and damage tolerance test campaign, where the tested structural components incorporated embedded flaws and impact damage. Testing included repeated residual strength testing and additional load cycling in the presence of additional larger structural impact damage. Successful completion of these tests demonstrated the validity of the approach and provided Bell with sufficient information to show compliance for FAA certification.

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