Bell 525 Vertical Fin / Aft Fuselage and Tailboom Composite/Metallic Hybrid Certification Fatigue Testing

icaf2023 Tracking Number 120

icaf2023 presentation

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.