In aerospace composite assemblies, panels and components have to be fixed before the final joint is made. This is due to necessary adjustments and application of sealer between panels and structure components. This preliminary fixation is done by means of temporary clamping fasteners, which are removed when the sealant is cured. For the handling of composite structures and components, the avoidance of delaminating is one of the most important topics.
Compared with conventional temporary fasteners, a new type designed by Alcoa Fastening Systems reduces specific loads applied to composite structures. The result is a reduced risk of delamination, especially where sloped surfaces are involved. For disassembly, a controlled screwing process ensures that structure and panel will not be damaged.
As shown in the photos, the sealant applied between the components of a wing box is squeezed out through the holes around the needles of the conventional temporary clamping fasteners and onto the top surface. This causes fouling of the fasteners and necessitates a cleaning process after each application. Sealant may also cause the fasteners to get stuck in the holes, complicating the disassembly process. This can be time-consuming and increase the risk of damage to the needles and/or the panels.
Alcoa's new temporary fastener concept fulfills several requirements, as it:
• Applies a defined preload to clamp the parts together and to squeeze out sealant
• Compensates for variations in the thickness of the structure stack-up
• Is operable from one side of the structure
• Accommodates various grip thicknesses
• Accommodates angularity of the structural components
• Can be handled with standard tools of manufacturing
• Is easy and safe to disassemble.
Composites have very good in-plane strength capabilities but are vulnerable at edges, angles, and holes. The anisotropic structure of composites offers high specific strength in the longitudinal direction but renders the structure susceptible to bearing deformation in the transverse direction. Thus, it’s desirable to distribute the transverse bearing forces to minimize bearing deformation near the fastener installation hole.
Compared to the sickle-shaped footprint of the conventional temporary fasteners, the new design provides an optimized circular area to distribute the required clamping loads. This feature reduces the specific stress by a factor of about 10. For larger diameters, the related circular area increases accordingly so that the specific stress applied to the panel and structure is limited to 220 N/mm², even for the largest hole diameter of 25.4 mm with a clamping load of 30,000 N .
The advantage is even higher when a sloped surface is considered. The new system provides an even load distribution at sloped surfaces up to 5°.
For installation, standard pneumatic nut runners can be used.
This article is based on SAE technical paper 2008-01-2290 by Dieter Jüling, Marcus Scheinberger, and Luke Haylock, all of Alcoa Fastening Systems.