Title: The Effect of Machining–Induced Micro Texture on Lightning Current Arcing between Fasteners and Composite Structure
Technology: Manufacturing systems & technology,Surface technology,Aerospace design,Fastening Systems
Year: 2009
Abstract: Drilling fastener holes in composite is much more difficult than in aluminum or other metallic materials since individual carbon fibers fracture at irregular angles resulting in numerous microscopic voids. These voids can trap excess sealant inhibiting the intimate electrical contact between the fastener and the composite structure. As the cutting tool wears there is an increase of surface chipping and an increase in the amount of uncut fibers or resin. This condition is referred to as machining–induced micro texture. Machining–induced micro texture has been shown to be associated with the presence of arcing between the fastener and the composite structure during lightning strike tests. Lightning protection of composite structure is more complex due to the intrinsic high resistance of carbon fibers and epoxy, the multi-layer construction and the anisotropic nature of the structure. The inherent conductivity of metallic fasteners and the large number of fasteners used in aircraft construction combine to create a condition of a high probability of lightning attachment to fasteners. Intimate contact between bare metallic fastener and the hole in the composite structure has been found to be the best condition for the electrical current dissipation Improved machining techniques could improve machining-induced micro texture and reduce the development of microscopic voids. Fasteners developed in this study which conform to the inherent machine–induced micro texture offer another approach for reducing arcing between the fastener and the composite hole. The test results presented in this paper indicated that conforming fasteners decrease the voltage drop across the interface and reduce the dielectric effect caused by the sealant thus minimizing the possibility of arcing between the sleeve and the composite panel.

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Title: A New Generation Cordless Electric Tooling
Technology: Aerospace design,Automotive design,Fastening Systems
Year: 2009
Abstract: Following the trend in automotive manufacturing, electric cordless tooling platforms are gradually being adopted for aerospace assembly tool systems. This paper introduces a new portable aerospace fastener assembly tool system based on the cordless electric technology platform. These systems are significantly more accurate than traditional pneumatic–based assembly tool systems and offer a range of process monitoring options. Cordless assembly tool systems make the assembly process easier, faster, safer and more accurate. These systems have the ability to provide traceability with time & date stamp for each installed fastener and provide wireless communication to enable process monitoring in real time. A comparison with traditional aerospace tool systems is made, in terms of historical evolution and working performance. Lessons learned from automotive applications are brought to aerospace industry. Key benefits include better ergonomics, improved accessibility, productivity and cost. The adaptability of these new assembly tools to different families of fastening systems is also presented.

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Title: Evaluation of Alternatives to Electrodeposited Cadmium for Threaded Fasteners Applications
Technology: Manufacturing systems & technology,Surface technology,Fastening Systems
Year: 2009
Abstract: Since 2005, Alcoa Fastening Systems (AFS) and Lockheed Martin have been partnering to identify a Cadmium (Cd) plating replacement for threaded fasteners. Previously reported Phase I, II and III studies resulted in alternative coatings that indicated promise as suitable plating materials. Phase IV and V studies continued the program by testing two different fasteners (NAS1580 and NAS4452) manufactured in AFS facilities. Testing included plating material characterization such as coating thickness, torque-tension relationships, locking and breakaway torque measurements, salt spray (fog) corrosion, stress corrosion, and push-in and interference properties. Additionally, mechanical properties of the plated fasteners were tested (tensile, double shear, durability, and fatigue). Candidates included two electroplated zinc-nickel coating systems (Zn-Ni and Zn-Ni2) and an electrodeposited aluminum coating (Al). Testing results consistently showed that all three candidates were quite similar in the overall characteristics of fasteners coated with cadmium. Fatigue tests showed very large variations among different kinds of fasteners, even within the same group. Multi-cycle run-on and break-away tests showed a continuous dropping of torque values for Al as the cycle numbers increase. On the other hand, the Zn-Ni coating demonstrated increased values, especially for the NAS1580 fasteners. The Zn-Ni2 coating showed fairly stable values for both NAS1580 and NAS4452 fasteners, including consistency from cycle to cycle and within each cycle. Due to surface lubrication, all Cd alternatives have similar torque to achieve the required tension for both NAS1580 and NAS4452 parts. Furthermore, fastener tensile and double shear tests revealed all coated fasteners meet product specifications.

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