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Adhesive Bond Strength of Bonded Structures in Confined Locations


OBJECTIVE: Develop an inspection system to measure the adhesive bond strength for bonded composite structures that are contained near edges or in confined spaces. DESCRIPTION: Bonded composite materials offer considerable opportunity to reduce manufacturing cost, improve structural performance, and improve fuel efficiency of aircraft. However, bonded composite aircraft structures continue to be a challenge to manufacture due to the certification requirement to determine the strength of the bonds in the structures - before they are placed into service. Current testing techniques involve statically loading the bonded structure to some specified load level to place the bond line under load. If the bond does not fail, it is determined to be acceptable and the structure is placed into service. This test method is costly and time consuming to undertake. There is a need to be able to"proof"test these bonds to quantify their strength with an efficient nondestructive method. A reliable and repeatable system for inspecting bonds would eliminate the need for full scale load testing resulting in a savings of $20 million in the aerospace industry alone. Recent developments with the use of well controlled stress waves have been demonstrated to be able to locally proof test the bond line. This bond inspection method has been matured for the inspection of non-confined structures. Non-confined bonded structures include wing skins bonded to the main spar or other internal structures. Challenges to implementation of an inspection process include the identification of data and inspection criteria/requirements (to address confined bonded structures and to define the system requirements for the inspection of these confined structures). The laser bond inspection process may provide verification of a successful bond and obviate the need for expensive global proof load tests. Discussions with Original Equipment Manufacturer (OEM)/Tier 1 designers and nondestructive inspection (NDI) personnel have yielded insufficient definition of these inspection requirements, as the designers often make design choices based on available NDI capability. NDI personnel would like the design community to provide requirements on what NDI resolution is needed for the structure. This leaves NDI equipment providers at a disadvantage and impedes further development of technology needed for the equipment/method to inspect primary bonded structures. The nondestructive inspection method shall be capable of proof testing the strength of confined adhesively-bonded composite structures. The identification of the confined structures with an OEM is to be used to determine the equipment requirements for the system to inspect confined structures. The research should identify approaches for measuring the strength and then demonstrating the ability to actually quantify the strength of a confined structure. PHASE I: Define the requirements for inspection system hardware that can quantify the strength of adhesive bond joints in composite structures, such as Pi joints that are contained within confined spaces. Define the dimensions of confined space with a vehicle OEM and then identify and evaluate hardware concepts with the potential to inspect bonds located in realistic vehicle confined spaces. PHASE II: Design the inspection hardware based upon one of the concepts defined in Phase I that addresses the inspection needs of bonded composite structures. Construct a functional breadboard inspection head and demonstrate its ability to conduct inspections within a confined space structure. PHASE III DUAL USE APPLICATIONS: The compact inspection head would apply to inspection of commercial aircraft and automotive bondments, as well as high end recreational marine structures. DoD applications consist of bonded manned and unmanned vehicle systems to include air, ground, and sea platforms. REFERENCES: 1) R. Bossi, K. Housen, and C. Walters, and D Sokol,"Laser Bond Testing"Materials Evaluation July 2009 pp 819-827. 2) Baker A.A., Jones R. Bonded Repair of Aircraft Structures, Martinus Nijhoff 1988. 3) Tenney, Darrel R.; Davis, John G., Jr.; Pipes, R. Byron; Johnston, Norman,"NASA Composite Materials Development: Lessons Learned and Future Challenges,"NATO RTO AVT-164 Workshop on Support of Composite Systems; 19-22 Oct. 2009; Bonn; Germany.
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